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Tissue equivalent phantoms for evaluating in-plane tube current modulated CT dose and image quality

University of Florida Institutional Repository

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1 TISSUE EQUIVALENT PHANTOMS FOR EVALUATING IN-PLANE TUBE CURRENT MODULATED CT DOSE AND IMAGE QUALITY By RYAN F. FISHER A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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2 Copyright 2006 by Ryan F. Fisher

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3 ACKNOWLEDGMENTS I thank my supervisory committee chair (Dr. Da vid E. Hintenlang) for his guidance, help, and patience throughout the course of this rese arch. I also thank Dr. Manuel Arreola for his expertise and direction in the cl inical aspects of the research.

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4 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................3 LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES................................................................................................................ .........7 ABSTRACT....................................................................................................................... ..............9 CHAPTER 1 INTRODUCTION AND BACKGROUND...........................................................................11 Introduction................................................................................................................... ..........11 Basic Principles of T ube Current Modulation........................................................................12 Angular Modulation........................................................................................................13 Z-Axis Modulation..........................................................................................................14 Simulation and Phantom Studies wi th Tube Current Modulation...................................15 Clinical Studies in Tube Current Modulation.................................................................17 Current State of the Art...................................................................................................21 Conclusions.................................................................................................................... .23 Purpose of Study............................................................................................................... ......24 Overview....................................................................................................................... ..25 Approach....................................................................................................................... ..25 2 TISSUE EQUIVALENT MATERIAL DEVELOPMENT....................................................36 Silicone-Based Rubber Material Testing................................................................................36 Sample Preparation..........................................................................................................37 Sample Testing................................................................................................................37 Urethane-Based Rubber Material Testing..............................................................................38 Material Testing...............................................................................................................39 Results of Attenuation and Density Testing....................................................................40 Materials Testing with PMC 121/30.......................................................................................41 Further Testing................................................................................................................42 3 ELLIPTICAL PHANTOM CONSTRUCTION.....................................................................51 Materials and Methods.......................................................................................................... .51 Improved Construction Methods............................................................................................52

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5 4 PHANTOM TESTING ..........................................................................................................57 Materials and Methods.......................................................................................................... .57 Phantom Dose Measurement...........................................................................................57 Phantom Image Quality Measurement............................................................................59 Comparison of Modulated and Fixed Tube Current Techniques....................................60 Results and Discussion......................................................................................................... ..60 Image Uniformity Measurements....................................................................................60 Dose Measurements.........................................................................................................62 Overall Trends.................................................................................................................63 Comparison of Modulated and Fixed Tube Current Techniques....................................63 Conclusions.................................................................................................................... .........65 5 FUTURE WORK....................................................................................................................72 APPENDIX ...................................................................................................................... .............74 LIST OF REFERENCES............................................................................................................. ..77 BIOGRAPHICAL SKETCH.........................................................................................................78

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6 LIST OF TABLES Table page 1-1 Results of phantom dose reduction studies........................................................................33 1-2 Comparison of image noise and diagnostic acceptablity...................................................34 1-3 Reductions in tube current time product with Z-axis modulation.....................................34 2-1 Ecoflex samples prepared with various additives..............................................................44 2-2 Material properties of various Smooth-On rubbers...........................................................45 2-3 List of slabs poured with 121/30 as a base........................................................................49

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7 LIST OF FIGURES Figure page 1-1 Phantom demonstration of angul ar tube current modulation............................................28 1-2 Real-time tube current modulation....................................................................................28 1-3 Z-axis modulation........................................................................................................ ......29 1-4 Z-axis modulation....................................................................................................... .......30 1-5 Summary of phantoms a nd associated path-lengths..........................................................31 1-6 Image noise as a function of modulation parameter..........................................................32 1-7 Kidney phantom........................................................................................................... ......35 2-1 Three tissue equivalent slabs curing in epoxy molds.........................................................44 2-2 Relative attenuation of samples.........................................................................................44 2-3 Relative attenuation of EcoflexTM based tissue equivalent materials................................45 2-4 Slabs of PMC 780, 744, 121/30 a nd 121/50 urethane-based rubbers................................46 2-5 Experimental setup for attenuation measurement..............................................................46 2-6 Full experimental setup.................................................................................................. ....47 2-7 Comparison of attenuation coeffi cients of urethane based rubbers...................................47 2-8 Comparison of densities of urethane based rubbers..........................................................48 2-9 Attenuation coefficients of PMC 121/30 based materials.................................................48 2-10 Densities of PMC 121/30 based materials.........................................................................49 2-11 Attenuation values of 121/30 with various additives.........................................................50 3-1 Diagram of proposed phantom design...............................................................................54 3-2 Elliptical phantom mold.................................................................................................. ...54 3-3 CTDI head phantom centered in phantom mold................................................................55 3-4 Phantom mold filled w ith urethane liquid rubber..............................................................55 3-5 Five elliptical tissue equivalent phantoms of increasing major axis..................................56

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8 4-1 Experimental setup for dose measurements.......................................................................66 4-2 Screen capture of a CT scan of an elliptical phantom surrounding a uniform region of an image quality phantom..................................................................................................67 4-3 CT number uniformity as a function of phantom major axis and reference mAs setting........................................................................................................................ .........68 4-4 Percent increase in CT number unifo rmity for increasing reference mAs settings...........68 4-5 Percent increase in CT uniformity w ith error bars for 190 reference mAs setting............69 4-6 Increases in dose as a result of changes in phantom major axis length and reference mAs setting.................................................................................................................... ....69 4-7 Comparison of image quality between m odulated and fixed tube current techniques......70 4-8 Comparison of dose between modulated and fixed tube current techniques.....................71 A-1 CT number uniformity as a function of phantom major axis.............................................74

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9 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science TISSUE EQUIVALENT PHANTOMS FOR EVALUATING IN-PLANE TUBE CURRENT MODULATED CT DOSE AND IMAGE QUALITY By Ryan F. Fisher December 2006 Chair: David Hintenlang Major: Nuclear Engineering Sciences A compressible, flexible, urethane-based tis sue equivalent material was developed and utilized in the production of five ellipsoid-s haped phantoms for evaluating the in-plane tube current modulation performance of multi-slice CT scanners. The created phantoms were designed to be integrated with a computed tomography dose i ndex (CTDI) dose assessment head phantom. Each phantom has a minor axis of 16 cm (corresponding to the diameter of the CTDI head phantom), with major axes ranging from 26 to 36 cm. A Siemens Somatom Sensation 16 CT scanner (Malvern, PA) was used to take centr al axis ion chamber dose measurements of each phantom using six different reference mAs settings with Siemens CARE Dose4DTM tube current modulation system. Image uniformity from each scan was measured as a method of tracking changes in image quality as a result of changes in the reference mAs setting. Tests were also performed comparing dose and image quality for scans using modulated and fixed tube current techniques. Image uniformity was found to be relativel y constant for each reference mAs setting regardless of phantom major axis length, provi ng the proper functionali ty of the in-plane component of the CARE Dose4DTM system. Image uniformity was found to increase as the reference mAs setting was increased, at the expe nse of higher doses. In comparing modulated

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10 versus fixed tube current techniques, dose sa vings of up to 63% were observed, but at the expense of slightly noisier images. Elliptical phantoms of varying major axis lengt h can be easily and effectively used to test the performance of in-plane tube current modul ation systems in multi-slice CT scanners. Such phantoms can also prove useful in comparing image quality and dose measurements between differing commercial CT scanners.

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11 CHAPTER 1 INTRODUCTION AND BACKGROUND Introduction X-ray Computed Tomography exams have become increasingly popular because of recent technological developments (such as multi detector spiral CT and greatly reduced scan times that allow a large amount of diagnostic information to be collected in a short period of time). As volumetric CT becomes more commonpl ace, concerns have arisen over increases in patient dose as a result of wider beams and more frequent exams. Studi es have shown that although CT accounts for only 11% of x-ray based ex aminations in the United States, it delivers over 65% of the total radiation dose associated with medical imaging.1 Effective doses for standard protocols of neck, chest abdomen and lumbar spine examinations can range from 3-15 mSv in adults, and currently there are no limits in place on the amount of radiation delivered per scan in the US.2 Radiation exposure from CT is of pa rticular concern for pediatric studies due to childrens relative increased lifetime cancer risk and higher radiosensitivity compared to adults.2 It should be noted that desp ite these statistics and facts, CT remains a low dose imaging modality. However, in the interest of keepi ng radiation exposure as low as reasonably achievable, methods to reduce CT dose and im prove image quality have been explored. Numerous solutions have been suggeste d in response to dose concerns, including a general lowering of tube curre nt techniques for all exams.2 Although this suggestion would reduce patient doses, such a reduc tion would come at the expense of noisier images, given that image noise is related to the number of photons incident on the detector.3 A decrease in tube current could thus compromise image resoluti on and low contrast delectability in medical images, possibly leading to misdiagnosis.3 As an alternate appro ach, CT scanner manufacturers have developed technologies for lowering patient doses without sacrifici ng image quality. These

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12 technologies are referred to as tube current modul ation, and act to adjust the x-ray tube output during the CT scan in either the x-y plane (angul ar modulation) or in the Z direction (Z-axis modulation) in response to changes in patient anatomy and attenuation in order to reduce dose and maintain a constant im age quality throughout the scan.3 Basic Principles of Tube Current Modulation In Computed Tomography exams, selectable techniques such as tube current and tube potential determine the photon flue nce output of the x-ray tube.1 This fluence, along with the attenuation characteristics of th e patient, determines patient dose as well as the number of photons reaching the detectors, which in tu rn determines reconstructed image noise characteristics.1-3 If all other variables ar e held constant, a reduction in tube current leads to a reduction in patient dose, but an increase in quant um noise or mottle in the reconstructed image. Images with too much quantum noise may obscure low contrast lesions or tumors that would normally be visible in less noisy images.1-3 In conventional CT, a technologi st selects the tube current and tube potential based on patient characteristics such as si ze and weight, as well as base d on the particular exam being performed. These techniques are held constant for each slice throughout the exam. Since patients are not homogeneous in composition, nor ci rcular in exterior body shape, these fixed techniques lead to variable attenuation though the body and as such, a variable number of photons reaching the detectors on th e opposite side of the patient fo r different projection angles.4 Certain anatomical areas, such as the shoulders, ar e problematic in that lateral views have much higher attenuation, up to three orders of magnitude higher,4 than anteroposteri or views of the same area. In these instances, tube current must be increased to allow more photons to reach the detector in order to minimize reconstruction artif acts due to high image noise in those planes.

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13 The result of this increase in tube current is an increased patient dose throughout the entire scan area of the exam if the techniques are held constant for the entire exam area.4 In response to these problems, CT manufactur ers have developed automatic tube current modulation techniques that allow the tube current to be automa tically adjusted during a CT examination in order to provide lower patient do ses and constant image noise characteristics. Tube current in low attenuation projections can be greatly reduced without loss of image quality, thus reducing patient dose for the exam. There are currently two major strategies employed by manufacturers to accomplish this ta sk; angular and Z-axis modulation.1 Angular Modulation Angular modulation was first developed by GE Medical Systems and works to modulate tube current in the x-y plane with in a single rotation of the tube.3 Photon fluence is increased in areas of higher attenuation, such as lateral vi ews though the shoulders, an d decreased in areas with lower attenuation, such as AP views through the chest. The first angular modulation system from GE appeared in 1994 (SmartScan), and us ed two localizer radiographic images, AP and lateral, to determine attenuation values of the patient.3 The tube current was then modulated in a preprogrammed sinusoidal pattern that matched the attenuation characteristics. These systems attained a dose reduction of up to 20% while maintaining a re latively constant level of image noise.3 More recent offerings of the technol ogy by Siemens employ an online real-time anatomy-adapted system (CARE Dose) that automa tically adjusts the tube current for a given projection based on the attenuation cal culated from the previous rotation.5 Thus, tube current is modulated on the fly without the need for localizer radiogra phic images, and is adjusted by attenuation information provided from the previous 180 degree projection as seen in Figures 1-1 and 1-2 The CARE Dose system has been shown to produce dose reductions of up to 90% for the anteroposterior projection in regions such as the shoulders with marked asymmetry.3

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14 Philips Medical Systems also uses an angul ar tube current modulation system (DoseRight Dose Modulation) in their CT scanners. The Philips technology modulates current within a single tube rotation according to the square root of the attenuation measured during the previous rotation. This modulati on technique is based on the fact that image noise is inversely related to the square root of the number of photons captured.3 Z-Axis Modulation The second major type of tube current modul ation technique is Z-axis modulation. This technology also acts to adjust the photon output of the x-ray tube according to patient specific attenuation characteristics, but unlike angular modulation, it does not alter tube current within a single rotation of the x-ray source. Instead, a scout radiographic image is taken of the patient, and the system calculates the photon flux required in each slice in order to maintain a user designated noise level in the reconstructed image.3 The tube current remains constant for each rotation around the patient, but is altered along the length of the pa tient as the table translates through the beam. Lower current va lues are thus used in lower attenuation regions, such as the chest, lowering patient dose in comparison to higher attenuation regions such as the pelvis ( Figures 1-3 and 1-4 ). The user can choose between several noise index values depending on the quality of images required by the exam. The noise index va lue is approximately equal to the standard deviation of pixel values in the central region of an image of a uniform phantom.3 A higher noise index corresponds to a greater standa rd deviation of pixel values fo r similar tissues in an image, but also to a lower overall tube current during the exam and thus lower patient doses. It should be noted that the noise index chosen will not al ways exactly match the noise in a reconstructed patient image since reconstruction para meters also influence image noise.3 Z-axis modulation is

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15 currently offered by GE medical systems (Autom A) as well as Toshiba and Siemens medical systems, and acts to keep image noise th e same in each slice image in an exam.3 Simulation and Phantom Studies with Tube Current Modulation A two-part study was conducted in 1999 by Mich ael Gies et al. at the University of Erlangen-Nuremberg in Germany4 that tested the theory of tu be current modulation with both mathematical simulations as well as phantom studies. In the first part of the study, computational simulations of CT imaging were run on a series of four geometric phantoms including: an elliptical, water-f illed, shoulder phantom, an oval shaped, acrylic, hip phantom, an oval, water-filled, abdominal phantom, a nd a standard water-fille d circular phantom ( Figure 1-5 ). Mathematical simulations were run on all phantoms for a range of modulation factors ranging from 0, corresponding to no modulation (fixed tube current cases), to 1, corresponding to modulation proportional to attenuation. For these va rying parameters, image noise in the central pixel of each phantom was computed mathematic ally to quantify the possible noise reduction and efficiency of tube current modulation.4 The effect of tube current modulation was also evaluated on noise in reconstr ucted images. For all simulations, both a sinusoidal and an attenuation based modulati on function were used. The results of the mathematical simulations showed that a sinusoidal modulation function provides much less noise reduction than attenua tion-based methods for all noncircular shaped objects.4 It was also shown that image noise is minimized when an atte nuation based modulation factor of 0.5 is used, corresponding to current co ntrol proportional to the square root of object attenuation. In reconstructed images without tu be current modulation, anisotropic noise patterns were visible in the direction of highest attenuation in the obje ct (along the major axis of the ellipse).4 As previously mentioned, when modulati on is performed according to the square root

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16 of attenuation, image noise is minimized; however, the anisotropic noise stru cture is still visible in the image. If tube current control is incr eased to being directly proportional to attenuation, image noise levels rise slightly, but become simila r for all projection angles and thus isotropic in the reconstructed image ( Figure 1-6 ). According to the study, a homogeneous, isotropic noise pattern is generally considered superior both aesthetically and diagnostically for CT images, pointing to the fact that absolute lower noise levels may not be optimal for image reconstruction.4 The mathematical phantom simulations also showed the dependency of image standard deviation on the inverse square root of photons registered by th e detector. For smaller numbers of registered photons, the functi on changes rapidly, but does not a ppreciably change for larger numbers of photons. Therefore reducing the number of photons in low atte nuation regions will have a minimal impact of image noise, thus allowing for lower patient doses.4 Overall the simulation studies confirmed the theory that tube current modulation has the potential to lower patient doses while maintaining or improving image noise characteristics in CT imaging.4 In the second part of the German study, act ual phantoms with the same dimensions as those utilized in mathematical simulations were used on a Siemens four-slice scanner.6 The scanner was equipped with an early prototype of the CAREdose angular modulation system, in which online tube current is modulated based on the previous 180 degr ee scan of patient attenuation. Tests were also performed with ion chambers in the phantoms to determine the extent of correlation between mAs reducti on and actual dose reducti on. Utilizing the hip phantom and mathematical simulations, a predic ted mAs, and thus dose reduction, of 39.4% was calculated.6 This value was similar to the actual m easured dose reduction (measured in mGy) in the center of the phantom of 45.1%, proving that actual CT dose reductions are possibly even

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17 larger than would be predicte d based solely on mAs reduction.6 Since most radiosensitive organs are located in a more or less centr al position in the body s cross-section, it was determined that the reduction of mAs in studies is a valid and conservative estimate of organ effective dose reduction.6 In general, the phantom measurements corr esponded to within 10% of the previous mathematical simulations regarding dose reduction and image noise ( Table 1-1 ). Dose reductions of up to 56% were found in highly as ymmetrical phantoms such as the shoulder phantom, while maintaining fairly constant image noise. Conversely, in tests where dose remained constant, scans utilizing tube curren t modulation showed a reduction in image noise compared to scans with constant tube current.6 Overall, the mathematical and phantom studi es concluded that on line, attenuation-based, tube current control systems showed a significant potential for clinical dose reduction without compromising image quality. Furthermore, the reduc tion in tube current w ould reduce x-ray tube load and thus result in lower operational costs of CT scanners.6 Clinical Studies in Tube Current Modulation Multiple clinical studies have been publis hed using Z-axis tube current modulation (AutomA on GE scanners). The majority of th ese studies come from the Radiology Department at Massachusetts General Hospital.7-9 As previously mentioned, the GE AutomA system requires an input of the noise index, as well as maximum and minimum tube current thresholds. These thresholds make sure that tube current re mains in a usable range during the entirety of the scan without unexpectedly high out puts through regions in order to maintain the constant noise index.7 One such clinical study involved utilizing t ube current modulation in abdominal and pelvic CT exams with a sixteen slice scanner.7 Sixty-two patients underwent follow-up

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18 abdominal CT scans using Z-axis modulation. Thes e images were then compared with previous images obtained using fixed tube current techniques from the same patients, but otherwise using identical imaging parameters. The mean interv al between the scans was 5 months (range 2-8 months). The two sets of images were graded by subspecialty radiologists on the basis of diagnostic acceptability and image noise. Images were graded on a five point scale with 1 being unacceptable, 3 being acceptable and 5 being excellent Images at five anatomic levels in the abdomen and pelvis were used, including the upper liver at the level of th e diaphragm, the porta hepatic, right kidney hilum, iliac crest, and upper margin of aceta bulum. The total mAs for each exam was also recorded for comparison.7 The results of the abdominal-pelvic study ( Table 1-2 ) showed that scores for image noise and diagnostic acceptability of im ages at the levels of the uppe r liver and the acetabulum were slightly lower with Z-axis modul ation then compared to fixed tube current scans, although the results were not statistically sign ificant (p = 0.34). At all other levels there were no significant differences in the scores for image nois e and diagnostic acceptability between the two techniques. Any lesions detectab le on the manual tube current scans were also detectable on the Z-axis modulation scans. An average overa ll mAs reduction of 31.9% (range 18.8% to 87.5%) was found for Z-axis modulated scans compar ed to fixed tube current techniques ( Table 1-3 ). The use of Z-axis modulation resulted in an mAs reduction in 87% of all exams. An increase in mAs was found in 13% of exams using Z-axis modulat ion and is attributable to the larger mean weight of patients in this cat egory as compared to those patients that had reduced mAs. Although mAs increased in these exams, a significan t (p = 0.1) improvement in image noise and diagnostic acceptability was noted for all cases. This fact shows that previous scans with overweight patients possibly utiliz ed inappropriately low tube currents resulting in below

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19 average image quality. Overall, the study showed that significant dose reduction is possible with Z-axis modulation and highlighted the need for proper technique settings of both the minimum and maximum current limits in order to en sure appropriate noise levels in images.7 Another clinical test performed by the sa me group evaluated dose reduction and scanner performance in the detection of urinar y tract stones using Z-axis modulation.8 This study used both phantom and patient studies. In the phant om study, sixteen calcium oxalate or calcium phosphate kidney stones were embedded in the collecting systems of two freshly harvested bovine kidneys. The size of the stones range d from 2.5 to 19.2mm. The ki dneys were placed in an elliptical Plexiglas container that was then filled with a ph ysiologic saline solution. The phantom was scanned on a Siemens 16 slice CT scanner six times. The phantom was scanned once with a fixed tube current t echnique, and then once with Z-axis tube current modulation at 5 different noise index settings; 14, 20, 25, 35, and 50. The remaining scanning and reconstruction parameters were kept the same for all scans. The phantom images were then viewed by two separate radiologists who were blinded to the s canning techniques and graded using the same 5 point scale described above to describe the detectability of the stones in the images. In addition to the phantom study, patient studi es were conducted in 22 patients using Zaxis modulation. As in the previously describe d study, all patients had be en previously scanned using standard fixed current techniques with a ll other scanning parameters kept the same. The same two radiologists evaluated both sets of images using the same five point scale for conspicuity and margins around the stones. In the phantom study, both radiologists identifi ed all 16 stones in the fixed tube current technique images, as well as in the Z-axis modul ated images at noise indexes of 14, 20, and 25. Three stones smaller than 5 mm were not identified by either radi ologist in images from Z-axis

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20 modulation with a noise index of 35 and 50. There was no signif icant difference (p > 0.5) in stone conspicuity, image noise, or diagnostic acceptance between fixed current and Z-axis modulated images at a noise index of 14, 20 or 25.8 Dose reduction, by means of reduced mAs, was found in all Z-axis modulated scans ranging from 51% in the noise index 14 case, to 92% in the noise index 50 case, although as previously men tioned, not all stones were localized with this technique. An mAs reduction of 76% was found in the noise i ndex 25 case, the highest noise index where all stones were detected.8 Kidney phantom images at each noise level are shown in Figure 1-7 Similar dose reductions were found in the pa tient studies, although onl y noise indexes of 14 and 20 were used. All stones lo cated with the fixed tube current scans were also located with Z-axis modulation, and no statistically signi ficant difference (p > 0.7) was found in the radiologists ratings for conspicuity, im age noise, or diagnostic acceptability.8 An overall reduction in mAs of 43% was found with a noise index of 14, and a 66% reduction was found at a noise index of 20. Overall the study demonstrated that doses in urinary tract stone de tection exams can be greatly reduced by utilizing Z-axis modulation techniques. These reductions can be higher than in the cases of chest or abdominal imaging due to the high contrast nature of urinary tract stones in soft tissue, allowing for slightly noisier imag es than required for low contrast detection cases.8 It was also found that for these exams, a 5% reduction in the noise index correlates to a dose (mAs) increase of approximately 10%. The same research group conducted a similar study of chest CT studies comparing fixed tube current methods to Z-axis modulation.9 Very similar methodologies were used as in both the previous studies. The study used 53 patients, wi th two radiologists read ing the results of CT

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21 exams and rating them based on image noise, diagnos tic acceptability and stre ak artifacts. Again the study found that Z-axis modulation provided acceptable imaging of the chest, including lung parenchyma and mediastinal structures, with no si gnificant differences in image noise, diagnostic acceptability, or streak artifacts, as comp ared to fixed tube current techniques.8 The use of Z-axis modulation showed average mAs reductions of 18 %, 26%, and 38%, with a noise index setting of 12, 12.5, and 15 respectively, when compared to fixed tube current protocols. Again tube current was found to increase slightly in heavie r patients, which is a result of the system attempting to keep image noise constant throughout the thicker area of the scan. It was noted that care should be taken by physicians to pay sp ecial attention to noise requirements from larger patients to avoid unnecessary increases in dose.8 Current State of the Art While all previous mentions of tube current modulation have been with respect to either angular or Z-axis techniques, there are CT scan ners currently available that make use of both systems simultaneously. This t echnology is currently only available from high-end, top of the line scanners, but is expected to trickle down and become more pr evalent in the industry as time progresses. A study has been published10 in which a Siemens Somatom Sensation 16 scanner was used to compare the combined modulation technique with angular modulation and fixed tube current modes of operation (CARE Dose 4D). In this scanner, a scout image performs the normal task of assigning tube currents for each slice along the Z-axis in order to maintain constant image noise. While the scan is in prog ress, the tube current is modulated as the tube rotates around the patient in order to adjust pho ton flux to match patient specific attenuation for each projection angle. This combined modulation is referred to as xyz modulation. In this particular study 152 patients were used who underw ent contrast enhanced CT examinations of the abdomen and pelvis. Seventy-nine patients were scanned using xyz-axes modulation with

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22 forty-two using the weak decrea se-strong increase operation mode and thirty-seven using the average decrease-average increase operation mode (explained below). Forty-two patients were then scanned using only the angul ar modulation technique, and thir ty-one patients were scanned using a fixed tube current technique. As with the GE system, a user input of an acceptable noise level is required, along with a Siemens specific modulation characteristic for sl im or obese patients. This modulation setting differs based on patient size, and decreases tube current for slim patients and increases current for obese patients. The extent to which the t ube current is increase d or decreased can be controlled via the setting of modulation strength, clas sified as either strong average, or weak. A strong setting for obese patients results in a larger increase in dose corresponding to lower image noise, while a weak setting for an obese patient spares patient dose at the expense of increased image noise. For slim patients, a strong modulat ion setting results in more image noise and lower patient dose, while a weak modulation setting results in higher dose and less noise.10 It is this authors opinion that the method for noise level selection employed by Siemens is infinitely more complex than that set by GE for Z-axis modulation, and could resu lt in improper technique settings by technologists. As in the previous clinical studies, images from each set of patie nts were graded by two radiologists on the basis of image noise, diagnostic acceptability, streak artifacts, and visibility of small structures. A quantitative measure of image noise was also recorded for each exam in the liver parenchyma at the leve l of the porta hepatis. The study found no significant statistical differen ce in the weights (p = 0.3) or ages (p = 0.5) of patients in each of the scanning technique groups.10 In evaluating valu es of image quality parameters (radiologist measured), there wa s no significant difference found for examinations

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23 performed with fixed tube cu rrent, angular modulation, and weak decrease-strong increase method combined modulation The scores from the average increase -average decrease method of combined modulation were found to be significantly lower than other techniques however (p = 0.0001). Of these examinations, several were labele d as below diagnostic quality with more than acceptable image noise. In comparing objective image noise, again there was no significant difference in values for exams performed with fi xed tube current, angul ar modulation, and weak decrease-strong increase combined modulation. Again, the noise values for average increaseaverage decrease mode of combined modulati on were significantly higher than any other technique (p > 0.1). In comparing doses measured for each technique, a significant reduction (p < 0.0001) was found in the angular and combined methods of tube current modulation. Compared to fixed current techniques, there was an average dos e reduction of 19% for angular modulation technique exams, and 42% for w eak decrease-strong increase comb ined modulation exams. The average decrease-averag e increase method of combined modu lation resulted in 44% average dose reduction, although at the expe nse of image quality as previ ously mentioned. It should be noted that dose changes in this st udy were reported as changes in CTDIvol and not in mAs as reported in previous studies, making a dire ct comparison between studies impossible. The combined modulation technique study showed the increased dose savings of combining angular and Z-axis tube current modul ation, but also highlight ed the importance of proper technique settings correlating to patient size. Conclusions As has been discussed, tube current modul ation techniques represent technological advances in CT scanners that allow for a re duction of patient dose while maintaining image quality. Three methods have been described fo r tube current modulati on; angular modulation,

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24 which modulates tube current in the x-y plane as the tube ro tates around the patient, Z-axis modulation in which tube current remains constant for a give rotation, but changes as the patient advances though the scanner, and xyz modula tion, which combines the two techniques. Mathematical simulations as well as phantom an d patient studies have shown the dose reduction benefits of current modulation, and dose reductio ns for both angular and Z-axis modulation have been shown to be comparable for similar exam inations. Combined tube current modulation makes use of the advantages of each method to further reduce patient doses, but at the present time is only available on high end scanners. While tube current modulation can provide numerous benefits to clinical CT examinations, it is not without its drawbacks. While the basic principles in use by different manufacturers remains similar, the method of im plementation can vary greatly between scanners, which can make clinical use in a clinic with multiple manufacturers difficult. The lack of uniformity between vendors, as evident with th e noise index value of GE compared to the decrease-increase method of Siemens, makes t echnologist training and understanding of the technology paramount to proper dia gnostic use of the equipment, as any dose savings in a single scan are nullified if scans must be repeated due to improper techniques. Purpose of Study The benefits and limitations of tube current modulation systems in CT scanners have been discussed, but as of now there is no simple method for medical physicists to ensure proper functionality of such systems from a quality assurance standp oint. An improperly functioning tube current modulation system would obvi ously pose numerous problems to a radiology department, including the possibility of un acceptable image quality as well as unknowingly higher patient doses. The purpose of this body of wo rk was to develop a simple, effective method to test the functionality of in -plane tube current modulation sy stems in order to ensure proper

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25 clinical operation. Variations in performance of the system could be tracked on a monthly basis, and proper maintenance or repairs could be re quested if system parameters vary to an unacceptable level. Overview A preliminary idea to test the functionality of an in-plane tube current modulation system was to create a series of ellipt ical tissue equivalent phantoms of the same minor axes, but with varying major axes. The overall concept was that such phantoms could all be scanned using the same image quality setting of the tube curr ent modulation system, and the respective image noises from each phantom scan should be similar, regardless of major axis length. Similar image noise values for varying sized phantoms would s how that the in-plane tube current modulation was indeed varying tube current based on attenuation values in order to maintain a constant image noise. If image noise values were found to vary greatly for differe nt sized phantoms, one could assume something was wrong with the modul ation system. Such a test would be easy to perform, take little time, and the results could be recorded for long term tracking of scanner performance. Approach We decided to use existing CT image quali ty and dose measurement phantoms for the study, and to develop tissue equivale nt elliptical attachments. This was chosen in the interest of using less material and for the flexibility in allowing for dose, as well as image quality measurements to be taken without having to manufacture two sets of phantoms. In order to fabricate such elliptical phantoms a tissue equivalent material was developed. This material was to have a density si milar to that of soft tissue (1.04 g/cm3), as well as similar x-ray attenuation properties in the diagnostic energy range (80-120 kVp) used in CT imaging. The developed material was also desired to be flexible and comp ressible in order to maintain

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26 close contact with the existing CatPhan (The Phantom Laboratory, Salem NY) and standard CTDI head phantom, as well as for applications in other phantom development projects dealing with mammography and anatomic tomographic phantoms. Once such a tissue equivalent material was deve loped, a series of five elliptical phantoms was created. All phantoms had a minor axis of 16cm, corresponding to the diameter of commercially available CTDI and CatPhan phantoms. The major axes of the created phantoms ranged from 26 to 37.5 cm. The created phantoms were then used w ith a Siemens Somaton Sensation 16 slice scanner equipped with CareDose4D tube current modulation system. This system uses angular as well as Z-axis modulation to adjust tube output on the fly within a sing le tube rotation as well as along the length of the scan based on a preexamination topogram. Both of these systems work together to ensure uniform image noise throughout a CT scan, while reducing patient doses as compared to fixed tube output protocols. The user input for image noise has changed slightly from that mentioned in the studies a bove. The user input for the CareDose4D software is now a single reference mAs value and the strong, normal, and weak modifications are no longer used. The reference mAs value corresponds to the mean effective mAs value that the system will use for a reference patient with the same protocol The reference patient is described by Siemens as a typical adult weighing 155-180 pounds for adu lt protocols, and a typical child weighing 45 pounds for childrens protocols. Based on the reference mAs value, the system adapts the tube current to the individual patient size. A higher reference mAs se tting indicates a higher tube current used in order to produce less image noise in the reconstructed image. A lower reference mAs setting can be used for lower patient dose in scans where image noise is not of crucial importance.

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27 Each of the created phantoms was imaged with the Siemens CT system at six different reference mAs values. These reference values were taken for a standard adult abdominal routine at values above and below the default setting. The pixel uniformity values for each phantom at each setting were then plotted in order to de termine proper functionality of the angular component of the CareDose 4D tube current modul ation system, as well as general trends in dose and image quality.

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28 Figure 1-1: Phantom demonstration of angul ar tube current modulation Tube current remains constant for the first 180 degrees rotation about the shoul der phantom before being modulated to match attenuation. Reprinted with permission from C. Suess, and X. Chen, Dose Optimization in Pediatric CT: current technology and future innovations, Pediatr. Radiol 32, 729-734 (2002). Figure 1-2: Real-time t ube current modulation Tube current is adjusted in real time to match patient attenuation. Larger currents are n eeded through the shoulder region with less required through the thorax due to lower attenuation. Reprinted with permission from C. Suess, and X. Chen, Dose Optimization in Pediatric CT: current technology and future innovations, Pediatr. Radiol 32, 729-734 (2002).

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29 Figure 1-3: Z-axis modulation. A scout radiographic imag e is used to determine the appropriate mAs per rotation based on patient attenuation characteristics. Reprinted with permission from C. Suess, and X. Chen, Dose Optimization in Pediatric CT: current technology and future innovations, Pediatr. Radiol 32, 729-734 (2002).

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30 Figure 1-4: Z-axis modulation. T ube current (A) varies per sl ice as determined by a scout radiograph (B). Reprinted with permission from J. Althen, Automatic Tube current Modulat io in CTA Comparison between Different Solutions, Radiation Protection Dosimetry 114, 308-312 (2005).

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31 Figure 1-5: Summary of phantoms and associated path-lengths. A) Phantom shapes used in simulation studies B) path-length as a function of angle of rotation for each phantom, demonstrating attenuation differe nces for noncircular shapes. Reprinted with permission from M. Geis, W. Kalender, H. Wolf, and C. Suess, Dose Reduction in CT by Anatomically Adapted Tube current Modulation I Simulation Studies, Med. Phys 26, 2235-2247 (1999).

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32 Figure 1-6: Image noise as a function of m odulation parameter. A) With no modulation anisotropic noise is visible in the dir ection of highest attenuation. B) Modulation according to the square root of attenuation: minimizes noise but anisotropic effects are still visible. C) Modulation according to attenuation, slightly higher noise than b, but isotropic. Reprinted with permission from M. Geis, W. Kalender, H. Wolf, and C. Suess, Dose Reduction in CT by Anatomically Adapted Tube Current Modulation I Simulation Studies, Med. Phys 26, 2235-2247 (1999).

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33 Table 1-1: Results of phantom dose reduction studies Reprinted with permission from W. Kalender, H. Wolf, and C. Suess, Dos e Reduction in CT by Anatomically Adapted Tube Current Modulation II Phantom Measurements, Med. Phys 26, 2248-2253 (1999).

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34 Table 1-2: Comparison of image noise and diagnostic acceptability Z-Axis Modulation Fixed Tube current Anatomic Level Image Noise Diagnostic Acceptability Image Noise Diagnostic Acceptability Upper Liver 2.8 +/0.7 3.0 +/ 0.6 2.9 +/0.7 3.1 +/0.8 Porta hepatis 2.9 +/0.6 3.2 +/ 0.6 2.8 +/0.7 3.2 +/0.7 Renal hilum 2.8 +/0.7 3.2 +/ 0.7 2.7 +/0.7 3.2 +/0.7 Illiac crest 3.0 +/0.8 3.3 +/ 0.6 2.9 +/0.7 3.4 +/0.7 Acetabulum 2.9 +/0.7 3.3 +/0.6 3.0 +/0.7 3.4 +/0.5 NoteDate are means and standard deviations. No statistically significant differences were found between CT images aquired with Z-axis automatic m odulation and those aquired with fixed tube current (P = .34-.84) Reprinted with permission from W. Kalender, H. Wolf, and C. Suess, Dos e Reduction in CT by Anatomically Adapted Tube Current Modulation II Phantom Measurements, Med. Phys 26, 2248-2253 (1999). Table 1-3: Reductions in tube current time product with Z-axis modulation Tube Current-Time Product Tube Current-Time Product Tube Current-Time Product Anatomic Level with Z-Axis Modulation with Fixed Tube Current Reduction with Z-Axis Modulation* Upper Liver 103.6 +/50.3 187.9 +/21.4 84.3 +/45.7 (44.9) Porta hepatis 121.0 +/51.6 187.9 +/21.4 66.9 +/47.7 (35.6) Renal hilum 119.9 +/54.2 187.9 +/21.4 68.0 +/49.4 (36.2) Illiac crest 115.4 +/56.4 187.9 +/ 21.4 72.5 +/51.7 (38.6) Acetabulum 123.8 +/50.0 187.9 +/21.4 64.1 +/47.1 (34.1) NoteData are mean milliampere-seconds +/standard deviations Data in parentheses indicate per cent change. For examinations in which mean tube current time product decreased with Z-axis m odulation compared with fixed tube current, the difference was statistically significant (P<0.001) Reprinted with permission from W. Kalender, H. Wolf, and C. Suess, Dos e Reduction in CT by Anatomically Adapted Tube Current Modulation II Phantom Measurements, Med. Phys 26, 2248-2253 (1999).

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35 Figure 1-7: Kidney phantom Kidney stones visualized in bovine kidneys scanned with a) fixed tube current technique, b) Z-axis modulat ion noise index =50, c) noise index=35, d) noise index=25, e) noise i ndex=20, f) noise index=14. Scan s at higher noise indexes show a substantial increase in image noise and lowered diagnostic acceptability. Reprinted with permission from M. Kalra, M. Maher, R. DSouza, S. Rizzo, E. Halpern, M. Blake, and S. Saini, Detection of Urinary Tract Stones at Low-Ra diation-Dose CT with Z-Ax is Automatic Tube Current Modulation: Phantom and Clinical Studies, Radiology 235, 523-529 (2005).

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36 CHAPTER 2 TISSUE EQUIVALENT MATERIAL DEVELOPEMT The first portion of this work centered on th e development of a flexible, compressible, tissue-equivalent material for us e in radiological phantom constr uction. The material was to be equivalent to human soft tissue in density a nd x-ray attenuation properties in the diagnostic energy range (80-120 kV). Phantoms previously created by the lab have been epoxy based, giving them a solid consistency similar to acry lic. While the epoxy based material was tissue equivalent, it was difficult to work with and limited in its scope of use. It was also hoped that the new compressible material coul d be used for applications in more realistic mammography phantoms to better model breast tissue properties under clin ical conditions. Acrylic was used as a benchmark for comp arison of the attenuation properties of the created material. Acrylic has previously been us ed for commercially available phantoms due to its similar x-ray attenuation propert ies in the diagnostic energy range. The density of acrylic is 1.17 g/cm3 though, and as such the density of acrylic was not used as a benchmark for the phantom material development. A target density of 1.04 g/cm3, the density of human soft tissue, was used as a target density value. Silicone-Based Rubber Material Testing EcoflexTM, a silicone-based rubber compound produced by Smooth-On Inc. (Easton PA), was first tested for its utility in creating a compre ssible tissue equivalent ph antom material. EcoflexTM consists of two parts, A and B, which are mixed in a 1:1 ratio by weight and allowed to cure for 12 hours to produce a stretchy, compressible material that returns to its original form after deformation. Preliminary tests showed the compressibility of EcoflexTM was as desired and it had a published density (1.07 g/cm3) close to that of soft tissue (1.04 g/cm3). Based on these preliminary tests, molds were produced in order to better quantify the proper ties of the material.

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37 Sample Preparation Epoxy molds were constructed in order to make slabs of EcoflexTM of a reproducible thickness. Solid pieces of epoxy were poured and allowed to harden overnight. Cutouts were then machined into the epoxy slabs with a Vision Pro Engraver (Dade City, FL) to create four 10x10 cm square molds with a depth of approx 1 cm. Using these molds, slabs of EcoflexTM were poured with various additives in order to test their radiographi c attenuation and density properties. Pictures of the epoxy molds and the EcoflexTM slabs are shown in Figure 2-1 Additives used included phenolic microspheres (System Three, Auburn WA), powdered calcium carbonate, and powdered polyethylene (Fisher, Fa irlawn NJ). The by-weight percentages of additives used in the samples are given in Table 2-1 The EcoflexTM material easily peeled out of the molds and did not require the mold release previously needed with epoxy based tissue equivalent materials, making sample preparatio n clean and simple as compared to epoxy based materials. Sample Testing After the previously mentioned samples were mixed, poured, and cured, they were tested for density and radiographic attenuation propert ies. As mentioned, a density of 1.04 g/cm3, the density of soft tissue, was used for a target de nsity. A 1.25 cm thick Acrylic slab was used as a benchmark for attenuation properties as acry lic is commonly used in commercial phantom production. A portable x-ray gene rator (Source-Ray Inc. mode l SR-115, Boheimia NY) and CCD detector were utilized for testing the atte nuation properties of the samples. The samples were placed directly on the CCD de tector, and two slabs of EcoflexTM based material, the acrylic slab, and a piece of BR-12 breast tissue equivalent tissue were imaged at a time. The average pixel values from a 156x156 pixel square in the ce nter of each sample for each of three images was taken and then averaged in order to pr ovide a relative compar ison of the attenuation

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38 properties of each material. If an EcoflexTM based slab had an average pixel value similar to that of acrylic, its radiological attenua tion properties were in the range required. All images were acquired with an SID of 43 cm, and at 80kVp and 4mA. A sample of the collected x-ray images is shown in Figure 2-2 Upon inspection, it was found that all prepared EcoflexTM samples had a much lower average pixel value than acrylic indicating a higher attenuati on of the incoming x-ray beam (it should be noted that the CCD detector utili zed assigns pixel values oppositely to that of a conventional digital radiographi c unit, in that darker areas represent areas of higher attenuation and lighter areas indicate regions of lower attenuation). The results of the attenuation testing can be found in Figure 2-3 Even the EcoflexTM samples with up to 10% by weight phenolic microspheres, a low atomic number material added to reduce the density and attenuation of the sample, had substantially higher attenuation values. Density tests were not performed on the EcoflexTM samples since it was apparent that the material could not successfully be used as a base in tissue equi valent materials due to the high attenuation attributed to its silicone base. Urethane-Based Rubber Material Testing After it was realized that EcoflexTM was unsuitable for use as a base in tissue equivalent materials, other solutions were explored. A ra nge of urethane based compounds with varying material properties were orde red from Smooth-On for further testing. The product numbers and properties of the tested materials can be found in Table 2-2 These materials were selected from multiple other products based on the listed properties. A larger value for elongation at break, as we ll as a lower hardness va lue indicates a more compressible material. All rubbers come in two parts which must be mixed in proportion. A mix ratio of 1:1 by weight was desired for precision and ease of use in prepar ing samples, but other

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39 options were tested in case their other propert ies ended up justifying a slightly more complex mixing process. Some materials were offered in both a wet and dry option, with the wet having a built in release agent to aid in the de-molding of plasters or concrete, the intended use of the products. When available, the dry opti on was chosen but the PMC 121/50 material was available in only a wet option. All materials had a similar de-mold time, which required overnight curing before samples could be removed from their molds. The same epoxy molds used for making EcoflexTM slabs of similar thicknesses were utilized to produce slabs of the urethane based materials. Each material was mixed as per its instructions and with no additives in order to compare their base x-ray attenuation va lues to that of acrylic. Based on these tests, the best material were selected and further tested with various additives in order to reach the desired tissue equi valent properties. Slabs of each of the urethane based materials are shown in Figure 2-4 Material Testing Unlike the previously described EcoflexTM sample testing, where CCD images were taken and pixel values compared, a new appr oach was taken for ease of calculations and comparison in testing the urethane based mate rials. A Keithley 35050A ion chamber (Cleveland, Ohio) was used with a portable x-ray unit (Source-Ray Inc. model SR-115) to measure attenuation through each sample. The ion chamber was suspended on a stand to prevent backscatter effects from altering data, and the sl abs of material were placed approximately 18 inches above the chamber on the stand. The beam wa s collimated so as to not interact with the metal arms of the stand holding the samples, and to fall across the active region of the detector. A tube current of 80 kVp was used with 400 mAs and an SID of 38 for all measurements. The experimental setup can be seen in Figures 2-5 and 2-6

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40 For testing, an initial exposure measurement was taken of open air with no slab in place. Three exposure measurements were then taken each with an acrylic slab, a BR-12 breast tissue equivalent material, and each of the SmoothOn urethane compounds in place above the ion chamber. Based on the measured exposures and sl ab thicknesses for each material, a relative attenuation coefficient was calculated using Equation 2-1. R=R0*e-u*t (Eq. 2-1) With: R = the average measured exposure (R) R0 = the initial exposure with no slab in place (R) u = relative attenuation coefficient (cm-1) t = the slab thickness (cm) These relative attenuation coefficient values allow for easy comparison of x-ray attenuation properties between materials and allow slabs of sli ghtly differing thicknesses to be quantitatively compared. Density measurements of each sample were th en taken utilizing Arch imedess principle. A dry sample of each material was weighed on a scale with 0.001 gram precision. The samples were then weighed submerged in a beaker of de-ionized water. Using both these measurements, as well as the known density of the de-ionized wa ter, the density of each sample was calculated using Equation 2-2. Dry weight / [(dry weight -wet weight)/density of H20] (Eq.2-2) Results of Attenuation and Density Testing The results of the initial round of material testin g are shown in Figure 2-7 and 2-8 Based on these tests, the 121/30 material was selected for use as a base in the tissue equivalent material. It was chosen based on its density and attenuation values being within a r easonable range of that of the desired values of both acrylic and BR-12, a breast tissue equivalent material. It was hoped

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41 that the 121/30 material could be successfully used to develop bot h a soft tissue and breast tissue equivalent phantom material. The individual atte nuation and density properties can be altered by the addition of additives such as phenolic microspheres, calcium carbonate, and polyvinyl chloride in order to reach the de sired target values. The 121/30 ma terial was also chosen because its elasticity and compressibility were qualitatively determined to be the best of the group of materials. Other materials such as PMC 744 were found to be extremely firm and not compressible enough. The PMC 121/50 wet material had an oily residue upon firming which was not suitable for long term phantom use if only b ecause of the messy residue it left on anything it came into contact with. Materials Testing with PMC 121/30 Once the 121/30 material had been selected, fu rther testing was done utilizing a range of previously described additives in order to get it s attenuation and density properties to the desired values. The same procedures for measuring atte nuation and density described in the previous section were used for this portion of testing. Multi ple slabs of material we re poured with varying amounts of microspheres, calcium chloride, and/ or polyethylene additives. Once the slabs had cured, their relative x-ray attenuation values we re measured using the Keithley ion chamber and the previously described method. For the first round of testing, the 121/30 material was mixed with 5% by weight each of calcium carbonate, polyethylene, and microspheres to gauge thes e additives effects on the properties of the base urethane material. It was decided to try to match attenuation values first, and once that was accomplished to then move to adjust the materials density through further additives. The results of the first round of testing are shown in Figures 2-9 and 2-10. Figure 2-9 shows that the addition of 5% polyethylene to the 121/30 compound did little to alter the

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42 attenuation, while the addition of 5% pheno lic microspheres significantly lowered the attenuation. Both of these materials had a lower a ttenuation coefficient than that of acrylic. The addition of 5% CaC03 increased the atte nuation of the 121/30 to a value higher than that of acrylic. This indicated that the addition of a lower byweight percentage of CaC03 would bring the attenuation of the material clos e to the target value of acrylic. The measured densities presented in Figure 2-10 show that the addition of CaC03 increased the density of PMC 121/30 to the desired value of 1.04 g/cm3. The addition of microspheres and polyethylene lowered the dens ity to less than that of water (~1 g/cm3). The specific densities of these materials were not able to be calculated with the previously described method because, being less dense than water, a wet sample weight could not be calculated since the samples floated. Isopropyl alcohol, which has a density lower than that of water, could have been used to calculate these materials densit ies, but this was deemed unnecessary since they were below the target value. Further Testing Based on the results of the in itial 121/30 testing, several new samples were poured with varying amounts of CaC03 and microspheres in order to get the material properties closer to tissue equivalent. CaC03 was added in order to raise the atte nuation coefficient of the material, while microspheres acted to lower both the atte nuation and the density. It was hoped that some combination of the two materials would produce a tissue equivalent rubber. A list of the newly created material mixtures is provided in Table 2-3 The samples were tested for attenuation and de nsity in the same ma nner as the previous samples. The results of this round of testing are shown in Figure 2-11 The densities of all materials with microspheres added were once again less than 1 g/cm3. The densities of both the 2.5% and 3.2% CaCO3 samples were 1.03 g/cm3, and the density of the 5% CaCO3 sample

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43 remained at 1.04 g/cm3. These density values indicate that increases on the order of 1% in CaCO3 to the 121/30 mixture produce relatively sma ll changes in density. Based on these test results, the addition of only CaCO3 seemed to be the most promising, as these samples had a density and attenuation close to the target valu es of soft tissue. While samples with added microspheres also had attenuation values both ab ove and below that of acrylic, indicating the ability to zero in on the exact attenuation, their low densities made then unsuitable for a tissue equivalent material. Given the results of the previous tests, th e relative attenuation values of the 2.5% and 3.2% CaCO3 sample were plotted as a func tion of their percentage of CaCO3, since the samples lay on either side of the target attenuation of acrylic. A linear-fit trend line was then fit to the data points, and used to calcu late the percentage of CaCO3 that would produce the same relative attenuation as acrylic, 0.302 cm-1 in this case. This calculati on yielded a value of 2.8% CaCO3 to be added to the PMC 121/30 urethane in order to provide a soft tissue equivalent attenuation coefficient. A sample of 2.8% CaCO3 was then poured in order to verify this calculation. The attenuation of this 2.8% sample was found to be equal to that of acrylic. Density measurements of the 2.8% CaCO3 sample showed that it has a density of 1.037 g/cm3 which is extremely close to the target value for soft tissue and essentiall y tissue equivalent within bounds of experimental error. It was determined that the combination of x-ray attenuation char acteristics and density very close to the ICRP standard for soft ti ssue was suitable to make the 2.8% by weight CaCO3 added to PMC 121/30 urethane mixture a suitab le tissue equivalent material for phantom construction.

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44 Table 2-1: EcoflexTM samples prepared with various additives (percentages by weight) EcoflexTM no additives 5% polyethylene 10% polyethylene 15% polyethylene 20% polyethylene 10% CaCO3 20% CaCO3 5% poly 5% CaCO3 10% poly/10% CaCO3 10% microspheres Figure 2-1: Three tissue equivalent slabs curing in epoxy molds Figure 2-2: Relative attenuati on of samples: Two EcoflexTM based samples on top below a breast equivalent (lower left) and acrylic (lower right) slab.

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45 Attenuation of Ecoflex Based Materials70.00 75.00 80.00 85.00 90.00 95.00 100.00 105.00 110.00 115.00Acryllicecoflex10% microspheres 10% CaCO320% CaCO35% polyethelene 10% polyethelene 15% polyethelene 20% polyethelene 25% polyethelene 5% poly 5% CaCO3 10% poly 10% CaCO3Average Pixel Value Figure 2-3: Relative attenuation of EcoflexTM based tissue equivalent materials Table 2-2: Material properties of various Smooth-On rubbers Rubber Compound A:B Mix Ratio HardnessElongation at Break PMC 780 dry 2:1 by weight 80 700% PMC 744 2:1 by weight 45 400% PMC 121/30 dry 1:1 by weight 30 1000% PMC 121/50 wet 1:1by volume 50 500%

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46 Figure 2-4: Slabs of PMC 780, 744, 121/30 and 121/50 ur ethane-based rubbers offered by Smooth-On. Figure 2-5: Experimental setup for attenuation measurem ent. Slabs of material were placed 18 above an ion chamber.

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47 Figure 2-6: Full experimental setup with portable x-ra y generator 38above the ion chamber. Relative Attenuation of Urethane Based Compounds0.264 0.272 0.251 0.295 0.248 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 121/50780744121/30acryllic Material Relative Attenuation (cm^-1 Figure 2-7: Comparison of a ttenuation coefficients of urethane based rubbers.

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48 Densitites of Urethane Based Compounds1.03 1.36 1.01 1.06 1.17 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 121/50780744121/30acryllic MaterialDensity (g/cm^ 3 Figure 2-8: Comparison of densitie s of urethane based rubbers. Relative Attenuation Values for PMC 121/30 Based Compounds0.251 0.255 0.295 0.215 0.326 0 0.05 0.1 0.15 0.2 0.25 0.3 0.3512 1 /3 0 5% polyethylene 5 % C aC O3 5% mi cr os ph eres a cr y ll i cMaterial Relative Attenuation (cm^-1 Figure 2-9: Attenuation coefficients of PMC 121/30 based materials.

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49 Densitites of PMC 121/30 Based Compounds1.01 1.04 0.6 0.7 0.8 0.9 1 1.1 121/305% polyethylene5% CaCO35% microspheres MaterialDensity (g/cm^ 3 Figure 2-10: Densities of PMC 121/30 based materials Table 2-3: List of slabs poured with 121/30 as a base (Ca= CaCO3 & mico =microspheres, percents are by weight) 2.5% CaCO3 3.2% CaCO3 5% CaCO3 2.25% Ca/1% micro 2.25% Ca/2.25% micro 4.5% Ca/ 4.5%micro

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50 Attenuation of 121/30 Based Materials 0.305 0.299 0.334 0.231 0.358 0.316 0.276 0.272 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400acr yl l ic 2.5 % CaC O3 3. 2% CaC O3 5 % CaCO3 2.25% C a/ 1 % m i cro 2.25% C a/ 2.25% micro 4.5% Ca/ 4.5%mic r o 5% micr oMaterialrelative atten (cm^-1 ) Figure 2-11: Attenuation values of 121/30 with various additives

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51 CHAPTER 3 ELLIPTICAL PHANTOM CONSTRUCTION Once a tissue equivalent urethane based ma terial was developed, it was used in the construction of several elliptical phantoms. These phantoms were to be used in testing proper functionality of tube current m odulation systems in clinical CT scanners. A total of five such phantoms were created, all with a height of 15cm, a minor axis of 16cm, and major axes varying between 26 and 37.25cm. A basic diagram of the proposed phantoms is shown in Figure 3-1 The phantoms were designed to fit an existing CTDI head phantom with a height of 15cm and a diameter of 16cm. Materials and Methods The original idea for building the phantoms wa s to utilize several la rge pieces of four inch thick packing foam already in the lab as a frame for pouring the phantoms. Elliptical cutouts would be cut into each of three slabs, which w ould be stacked and then filled with the tissue equivalent rubber. Before constructing the phant oms, small scale testi ng was done in order to verify the design concept. Two small rectangular cutouts were made in a 4 thick piece of packing foam, and a piece of wax paper was epoxie d to the bottom to act as a base. The walls of one cutout were lined with wax pa per, while the other was left as bare foam. Both cutouts were filled with the tissue equivalent PMC 121/30 materi al and left to cure overnight. It was found that the cured rubber easily pulle d out of both cutouts. The cutout lined with wax paper produced a sample with cleaner edges, while the sample from the foam cutout was rough and uneven as the liquid rubber material filled in the small voids in the foam. Based on the small scale test, the first ellipti cal phantom was constructed. An ellipse with a 26cm major axis and 16cm minor axis was traced onto and cut out of three large slabs of the 4 thick foam. Wax paper was epoxied to the bottom of one of the pieces of foam to act as the

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52 bottom. The three slabs were stacked on top of each other with small blocks acting as spacers in between levels in order to get the mold to the re quired height of 15 cm. The inside edge of the ellipse was lined with wax paper and a CTDI head phantom was placed in the center of the elliptical cutout. Immediately upon pouring the urethane mi xture into the phantom mold it became apparent that the wax paper lining the inside was not strong enough to hol d in the large volume of material. The wax paper bulged out at all ga ps in the foam and much of the liquid rubber material spilled out. After the rubber had cu red, the resulting ellipt ical phantom was not acceptable and the entire process was dubbed a lear ning experience. It was obvious that the foam and wax paper combination was not sufficient to contain the fairly visc ous tissue equivalent material. It was also difficult to cut perfectly geometrical ellips es out of the thick foam by hand, and as such alternative methods were sought. Improved Construction Methods After the failure of the first phantom, a ne w method of constructi on was devised. Instead of the foam used previously, plywood was purch ased for use in the mold. An ellipse was cut out of three pieces of wood with the Vision Pro engraving system in the lab. The outline of the same sized ellipse was then engraved halfway through a fourth sheet of plywood that would act as the base. The Vision Pro engraving system ensu red that all elliptical cutouts were uniform and of the same size. The three pieces were then stacked to a height of 15 on top of the base using wooden blocks as spacers. The interior was line d with a thick rubber sh eet originally designed for lining garden ponds. Plastic molding was placed along the groove machined into the base to provide more support for the walls of the phantom mold. The CTDI head phantom was placed in the center of the mold, which was then filled with the liquid 121/30 mixtur e. The new setup did a

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53 much better job of containing the liquid urethane and the phantom was left to cure overnight. Pictures of the mold and phantom building process can be found in Figures 3-2 through 3-4 After the urethane rubber had cured, the pha ntom was removed from the mold, although with some difficulty as the uretha ne tissue-equivalent material st uck to the wooden base layer of the mold. Other than a hard time removing the ph antom, the first effort was a success and four more phantoms were constructed in a similar fashion, but with increasing major axis size while maintaining the same minor axis. The major axis le ngths used in each of the five phantoms were 26cm, 28.5cm, 31.25cm, 32.6cm, and 37.25cm (+/.25c m variation from top to bottom). The five completed phantoms are shown in Figure 3-5 Each of the five phantoms fit around a 16cm CTDI head phantom for use in CT imaging and domes measurement. The tissue equivalent ma terial adheres to the acrylic CTDI phantom fairly well on its own, but medical tape was utili zed to ensure a tight consistent fit for testing. After construction was complete, the phantoms we re used to test the functionality of the angular component of the CareDose4D tube current modulation system on a Siemens Sensation 16 scanner in the Shands Orthopedi c and Sports Medicine Institute at the University of Florida.

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54 Figure 3-1: Diagram of proposed phantom design. Phanto ms were designed to fit an existing CTDI phantom shown in light gray. Figure 3-2: Elliptical phantom mold. Cutouts fr om other sized phantoms are visible in the plywood.

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55 Figure 3-3: CTDI head phantom centered in phantom mo ld. Rubber sheeting is taped in place to contain the urethane rubber. Figure 3-4: Phantom mold filled with urethane liquid rubber.

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56 Figure 3-5 : Five elliptical tissue equivalent phantoms of increasing major axis.

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57 CHAPTER 4 PHANTOM TESTING The five created elliptical phantoms were used to test the clinical functionality of the angular portion of the CareDose4D tube current modulation sy stem on a Siemens Somatom Sensation 16 CT scanner, as well as to compare doses in CT scans with fixed versus modulated tube current techniques. As pr eviously discussed, the CareDose4D system acts to modulate current along the Z-axis according to a scout scan, as well as within each tube rotation based on attenuation information from the previous 180 degrees of tube rotation. The purpose of the current modulation is to maintain a constant phot on flux at the detector elements and as a result, to possibly reduce patient dose as compared to fixed current techniques. If the in-plane modulation system is working properly, image noi se should remain constant for all scans at a given reference mAs setting regardle ss of phantom size due to the fact that image noise is related to the number of detected photons. If a larger sized phantom is being scanned, the system will compensate for the higher attenuation by increasi ng the tube current in order to maintain a constant number of photons at the detector, thus keeping imag e noise relatively constant. It should be noted that the elliptical phantoms do not test the functi onality of the Z-axis modulation of the system since they are of uniform thickness along their length. Materials and Methods The general experimental method for th e study was to measure both dose and image quality for each of the five phantoms at several reference mAs settings, an d to observe trends in dose as a function of CareDose4D setting and phantom size. Phantom Dose Measurement For all phantom measurements a standard adult abdominal rou tine with the same reconstruction kernel was used. The entire leng th of the phantom was scanned with the tube

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58 voltage set to 120 kVp, a 5.6 second scan time, 0.5 second rotation time, 16 x 1.5mm collimation, and 18mm table feed per rotation. These parameters were held constant for all scans. The entire length of the phantom was scanned in order to allow the system to modulate the tube current. Since the angular modul ation portion of the CareDose4D system modulates current on the fly based on the previous 180 degrees of the s can, a single axial-slice scan, as is done for CTDI measurements, would not allow the system to properly adjust to the changes in phantom attenuation. By scanning the entire length of th e phantom, which takes multiple rotations of the x-ray tube, the in-plane tube current m odulation system is allowed to operate. In order to collect dose measurements, each elliptical phantom was attached to a standard CTDI head phantom (15cm tall, 16cm di ameter). The phantom was placed on the CT table and a scout image (topogram) was performed. Th e entirety of the phantom was selected to be scanned using six differe nt reference mAs values (115, 130, 145, 160, 175, and 190 mAs). Each of the five elliptical phantoms was scanne d at each of the previously mentioned reference mAs settings. The default setting for the adult abdominal routine is 160 mAs, and the other settings were chosen to provide a range of data points both above and belo w the default setting in order to check the functi onality of the CareDose4D system as well as to as certain the effects of the setting on both dose and image quality in the various sized phantoms. A Capintec (Ramsey, NJ) PC-4P pencil ion cham ber was used in the center hole of the CTDI phantom to measure exposur e during the scans. Exposure meas urements were recorded for each scan over the entire 15cm length of th e phantom. These exposure measurements were converted into integral dose measurements by multiplying each by the F-factor for soft tissue (0.94 Rad/R). This calculation is possible since the total scan le ngth (15 cm) and table feed per rotation (1.8cm) were held constant for each phantom measurement. The integral dose, as

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59 described by Dixon12, is the line integral of dose measured along the Z-axis of a phantom. In the case of these measurements, the length of the sc an (15 cm) was longer than the active region of the pencil ion chamber (10 cm), but this discrepancy in length wa s not an issue due to the width of the x-ray cone beam (approx 2.5 cm). The expe rimental setup for dose measurements can be seen in Figure 4-1 Phantom Image Quality Measurement In order to measure image quality as a f unction of reference mAs setting and phantom size, the uniformity portion of a Catphan 440 (The Phantom Laboratory, Salem NY) image quality phantom was used. Scans were performed at each of the previously mentioned reference mAs settings with each of the five elliptica l phantoms. Minimum a nd maximum CT number values were measured in five regions of intere steach 4 cm in diameterin the reconstructed image, as shown in Figure 4-2 The values from each of these five regions were averaged together to provide the aver age minimum and maximum CT nu mbers for the uniformity region of the Catphan. Based on these average maximu m and minimum CT numb er values, the image uniformity for each h scan was calculated using Equation 4-1. Im age Uniformity = [1-((max-min)/(max+min))] (Eq. 4-1) A uniformity value of one indicates that the region is completely uniform, while lower uniformity values indicate a greater degree of vari ation in CT number values as a result of image noise. The uniformity portion of the Catphan 440 wa s used for image quality measurements instead of the CTDI head phantom used for dos e measurements because it provided an entirely uniform volume for measurement. The CTDI phantom has five drilled out segments (one in the

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60 center, and four around the periphery) for pencil ion chamber placement. While these drilled out portions are filled with acrylic rods when not in use for dose measurements, there was concern that the small air gaps around each rod could alter the minimum and maximum CT number values in the reconstructed image and provide a source for variation and error in the results. Comparison of Modulated and Fixed Tube Current Techniques Tests were also performed in order to determ ine the differences in dose and image quality between tube current modulated scans and those performed with fixed-current techniques. For purposed of clarity and simplicit y, only three elliptical phantom s (with major axes of 26, 31.25, and 37.25 cm) were used in these experiments. Each phantom was scanned with the CareDose system at reference mAs settings of 100, 150, and 200 mAs. The phantoms were then scanned with a fixed tube current technique with e ffective mAs settings of 100, 150, and 200 mAs. Dose for each scan was measured in the same manner prev iously described. In the interest of time, CT number standard deviation was used in place of uniformity as a measure of image quality. A lower CT number deviation indicates a more uni form reconstructed image. All other scan parameters were the same as previously descri bed under the dose and image quality sections. Results and Discussion The image uniformity and dose measurements for each phantom at each of the six reference mAs values were plotted in or der to verify proper in-plane CareDose4D function, as well as to observe trends in dose and image qua lity as a function of reference mAs setting and phantom size. Image Uniformity Measurements The effects of phantom major axis length and reference mAs setting on image uniformity are illustrated in Figure 4-3 As expected, image uniformity re mains constant (within bounds of experimental error) for each reference mAs setting, regardless of phantom axis length. Error bars

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61 were not included in Figure 4-3 for purposes of clarity, but image uniformity for each reference mAs setting with associated error bars can be found in Appendix A. Image uniformity is also seen to increase as the reference mAs setting was increased. As expected, each increase in the reference mAs setting yields an increase in the image uniformity, indicating a decrease in image noise. This is a result of the tube current modulation system increasing the photon output of the x-ray tube in order to attempt to match the total ef fective mAs of the scan to that of the reference setting. An interesting trend is observable in Figure 4-3 in which image uniformity for phantoms 2 through 4 is slightly lower than for phantoms 1 and 5. This trend is s een across all reference mAs settings, although the uniformity values for pha ntom 4 migrate towards that of phantom 5 starting at a reference mAs setting of 160. While a ll uniformity values were found to be within bounds of experimental error, the repeating trend for all measurem ents is worth notice, and can be attributed to one of two sources. The first and most probable source for this trend is the reconstruction algorithm itself. While no changes we re made to the reconstruction kernel used for each scan, the system itself makes adjustme nts in its reconstruction method in order to produce the reconstructed image. It is possibl e that such changes t ook place for the three midsized phantoms in this testing. A second possible source of the trend could be attributable to systematic human error during the testing. Alth ough every step was made to use the same procedure and techniques when performing each sca n, it is not outside the realm of possibility that some such changes, such as exact phantom placement on the CT table, were made. A repeat of all tests would be required to determine if the trend is repr oducible, which would suggest its roots were in the reconstruction algorithm.

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62 The percentage increase in uniformity for each increase in reference mAs (referenced to the 115 reference mAs setting) can be seen in Figure 4-4 Again, error bars are not included for purposes of clarity. Although it appe ars that a trend towards larger increases in uniformity for larger phantom sizes exists, when the data is plotted with e rror bars (shown in Figure 4-5 for only the 190 reference mAs setting) it is apparent that all points are well within bounds of experimental error and no such trend can be inferred. Dose Measurements The effects of phantom major axis and reference mAs setting on integral dose are shown in Figure 4-6 As expected, dose increased as the phant om major axis increased for each given reference mAs setting. This is at tributable to the CareDose4D system increasing the tube current in response to the added attenuating material in order to maintain a constant photon flux at the detector elements. The larger phantoms attenuate more of the incoming beam and thus require larger tube currents in order to maintain the same im age quality. Thes e larger tube currents in turn cause a higher dose in the phantom. Integral dose measurements were also found to increase as the reference mAs setting was increased. The increase as a result of changing refe rence mAs setting was fairly uniform for each phantom size as is seen by the even spacing between each reference mAs setting line in Figure 46 across all phantom sizes. This in crease in integral dose averaged an increase of 13% (+/0.75) per increase of 15 in the reference mAs setting for all sized phantoms. A noticeable trend found in Figure 4-6 is the larger integral dose values for the largest phantom (phantom 5) as compared to those of the other phantoms within each reference mAs grouping. This is most likely attributable to non uniform spacing of the phantoms major axes. The phantoms major axes measure 26 cm, 28.5 cm, 31.25cm, 31.6 cm and 37 cm respectively, with approximately 2 cm gaps between each of th e first four phantoms, and a relatively larger

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63 (~5 cm) gap between phantoms 4 and 5. This larg er gap equates to a phantom with a larger amount of attenuating material which could expl ain the jump in dose to phantom 5 as the modulation system attempts to maintain a constant photon flux at the detector. Overall Trends The average increase in integral dose from one reference mAs setting to the next was found to be fairly constant at 13% (+/0.75%). The average in crease in image uniformity for each increase of 15 in the reference mAs setting was found to be 2.5% (+/0.79%). Both of these measurements were found to increase linearly acr oss the range of reference mAs values utilized. It is hypothesized that if the reference mAs setting we re increased further, dose measurements would continue to rise, but gains in image uniformity would fall off as the limits of the scanners detectors were reached. Comparison of Modulated and Fixed Tube Current Techniques The results of the comparison study between mo dulated and fixed tube current techniques are shown in Figures 4-7 and 4-8 In comparing the image quality between the two modes of acquisition, several trends are visibl e. As expected, pixel deviation in the reconstructed images is seen to decrease as the tube current is increase d for both fixed and modulated techniques. This is expected as the higher number of photons reaching the detector at higher tube currents reduces the effects of random image noise. The pixel deviat ion for the tube current modulated techniques is seen to remain fairly constant regardless of phantom size. As previously mentioned, this indicates that the in-plane tube current modulation system was properly working to maintain a constant photon flux at the detector In the fixed tube current scan s, pixel deviation is seen to increase with phantom size. This is also as e xpected since the larger phantoms attenuated more of the fixed photon output, a situation that resu lts in fewer photons reaching the detector for larger phantom sizes. Fewer photons reaching the de tector cause an increase in image noise, and

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64 as such, an increase in the standard deviation of pixel values in a unifo rm area. These results demonstrate the need for features such as tube current modulation, since image noise in a patient scan can vary from slice to slice depending on anatomy with fixed curr ent techniques. Systems such as CareDose act to maintain a consta nt image noise throughout an entire scan. The effects of dose in the comparison study at seen in Figure 4-8 As a general trend, doses were seen to increase as tube current setting was increa sed for both modulated and fixed technique scans. As seen previ ously, the dose from the tube curre nt modulated scans was seen to increase slightly as phantom size increased. This was a result of a larger x-ray tube output in order to compensate for more attenuation in the phantom in order to maintain a constant flux at the detector. Conversely, doses were seen to decrease as phantom size increased in the fixed current scans. This again is explained by the higher photon attenuation in the larger phantoms. Unlike in the modulated techniqu e scans, the fixed current tec hnique does not adjust for the increase in attenuation in larger phantoms and as such, the dose in the center of the phantom is lower, but at the expense of noisier images. In comparing the doses between the two techniques, the tube current modulated techniques all produced lower doses than fixe d tube techniques. The magnitude of the dose savings increased with phantom size, and ranged from 49% for the smallest phantom to 62% for the largest phantom size. These percentage dos e savings were constant for each mAs setting. That is, regardless of mAs setti ng, the dose reduction by switching to the tube current modulated scan versus the fixed current technique was the same for each phantom size. These decreases in dose came at the expense of slightly noisier images in the tube current modulated scans. As seen in Figure 4-7 the pixel deviation was found to be higher in the tube current modulated scans as compared to the fixe d technique scans at similar settings. These

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65 differences again were found to be based on pha ntom size, and ranged from 64% to 90% lower pixel deviations in the fixed tube current scan s for the small to large phantoms respectively. Conclusions A compressible tissue equivalent material wa s developed and successfully used to build five elliptical phantoms of varying major axis length. These phantoms were then used to show the proper functionality of the angular por tion of the Siemens CareDose4D tube current modulation system in a Somatom Sensation 16 CT scanner. The image uniformity values in reconstructed images from each phantom we re found to be the same, within bounds of experimental error, at each of six tested refe rence mAs values for an adult abdominal routine, indicating that the modulation system was properly adjusting the tube curren t in response to the amount of attenuating material in the beam. This method could easily be worked into a regular monthly or quarterly quality a ssurance plan for the CT scanner in order to ensure proper operation of the tube current modulation syst em. Individual scanner performance could be tracked over its useful life and act ion limits could be set if unifo rmity values begin to drift. In addition to quality assurance, general trends in image quality and dose were observed as a function of variation in reference mAs setting and phantom major axis. An increase in integral dose of 13% was found for each increase of 15 in the reference mAs setting, while image uniformity increased by approximately 2.5% for each such increase. Knowledge of such trends could help aid radiologists and technologists in selected a proper CareDose4D reference mAs setting for a given exam in order to minimi ze patient doses while maintaining clinically acceptable image quality. In comparing the modulated to fixed tube current techniques, it was shown that dose reductions of over 50% were possible by using an in-plane modulation system. These reductions did come at the cost of increased image noise, and further tests would be required in order to

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66 determine an acceptable level of image noise in di agnostic images. It is possible that the increase in image noise as a result of these dose reducti ons would not affect th e diagnostic acceptability of the images. Figure 4-1: Experimental setup for dose measurements A pencil ion chamber is placed in the center of a CTDI head phantom with a tissu e equivalent elliptical phantom during a CT scan.

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67 Figure 4-2: Screen capture of a CT scan of an ellip tical phantom surrounding a uniform region of an image quality phantom. The five regions of interest used for measuring CT # uniformity values are shown as blac k circles in the center phantom.

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68 Effect of Phatom Axis on CT # Uniformity 0.60 0.65 0.70 0.75 0.80 25272931333537 Phantom Major Axis Length (cm)CT# uniformit y 115 130 145 160 175 190 Figure 4-3: CT number uniformity as a function of phantom major axis and reference mAs setting Percent Increase in Ima g e Uniformit y for Various reference mAs Settings0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 25272931333537 Phaontom Major Axis Length (cm)% Increase in Uniformit y 115 130 145 160 175 190 Figure 4-4: Percent increase in CT number uniformity for increasing reference mAs settings

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69 Percent Increase in Ima g e Uniformit y for Various Reference mAs Settings0.000 0.050 0.100 0.150 0.200 0.250 25272931333537 Phaontom Major Axis Length (cm) 190 Figure 4-5: Percent increase in CT uniformity with error bars for 190 reference mAs setting Effect of Phatom Axis on Dose4.000 5.000 6.000 7.000 8.000 9.000 10.000 11.000 25272931333537 Phantom Major Axis Length (cm)Integral Dose (mGy ) 115 130 145 160 175 190 Figure 4-6 : Increases in dose as a result of changes in phantom major axis length and reference mAs setting.

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70 Comparison of Pixel Deviation for Modulated vs Fixed Techniques4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 2527293133353739 Phantom Major Axis (cm)Standard Deviation of Pixel Valu e 100 M 150 M 200 M 100 F 150 F 200 F Figure 4-7: Comparison of image quality betwee n modulated and fixed tu be current techniques. In the legend, M indicates tube current m odulation, and F indicates fixed tube current techniques.

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71 Comparison of Dose for Modulated vs Fixed Settings0.0 500.0 1000.0 1500.0 2000.0 2500.0 2527293133353739 Phantom major axis (cm)Dose (mR ) 100 M 150 M 200 M 100 F 150 F 200 F Figure 4-8: Comparison of dose between modulated and fixed tube current techniques. In the legend, M indicates tube current modulati on, and F indicates fi xed tube current techniques.

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72 CHAPTER 5 FUTURE WORK Future work is planned in further characteriz ing the tissue equivalent material developed. The characterization of attenuation properties will be expanded to ensure that the material is equivalent to human soft tissues at a wider range of beam energi es. The radiological properties of the tissue equivalent material will also be documented in more detail, paralleling the work of Kyle Jones, a recent doctoral graduate of th e program who developed the epoxy based tissue equivalent materials previ ously in use in the lab. There are also several outlets for future work utilizing the phantoms and methods described in this thesis. The tissue equivalent mate rial is already being util ized in another project to create a tomographic adult CT phantom. Su ch a phantom would serve to provide specific organ dose measurements from CT scans which c ould be correlated to Monte Carlo simulations using the data the phantom was created from. The Smooth-On PMC 121/30 urethane rubber util ized to make a soft tissue equivalent material is also being used as a base for the development of a breast tissue equivalent material. Recipes for materials equivalent to breast tissues of varying com position and ratios of glandular to fatty tissue are in development. It is hoped that such materials can be used in the construction of more realistic mammographic phantoms than those currently in widespread use. Such anatomical phantoms could be used to better characterize equipmen t and patient doses associated with mammography. The five created phantoms could also be us ed to further characterize dose and image quality in next generation 32 and 64 slice CT scanners. Characterization of dose and image quality as a function of tube current modulation setting and patient size is of paramount importance as wider beams and more slices can drastically increase patient doses. The same

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73 benefits of this current work would thus be ex tended to these newer scanners. Plans are also in place to develop phantoms to test Z-axis tube current modulation specifically. Such phantoms would need to change dimensions along the Z-axis of a CT scan, something the current phantoms do not accomplish. The phantoms would also be extremely useful in comparing the dose savings and gains in image quality across CT scanners from different manufacturers. Currently some form of tube current modulation is in place in most commerci ally available CT scanners, but the methodology utilized by companies can vary greatly. Often the specifics of the algorithms used by the companies are not fully disclosed due to their pr oprietary nature. The created phantoms could be used to qualitatively compare th e functionality of these varied. The created phantoms could also be used to characterize location dependant image quality in phantoms. It was qualitatively observed during the course of this research that image noise was higher towards the cen ter of the phantoms as compar ed to around the periphery. Further studies could clarify and quantify th ese differences in image quality, and their dependence on tube current modulation setting and phantom size. This information would obviously be helpful to radiologists in knowi ng the degree to which image noise changes throughout an image. Lastly, plans for position dependant dose measurements within the phantoms are being developed along with a fiber optic dosimetry system.

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74 APPENDIX IMAGE UNIFORMITY GRAPHS The following are the graphs of image uniformity as a function of phantom major axis for each of the individual reference mAs settings. Error bars are in cluded to show that uniformity remains constant within bounds of experime ntal error regardless of phantom size. Effect of Phantom Major Axis on Uniformity at 115 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y A Effect of Phantom Major Axis on Uniformity at 130 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y B Figure A-1: CT number uniformity as a function of phantom major axis for a reference mAs setting of A) 115, B) 130, C)145, D) 160, E) 175, F) 190.

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75 Effect of Phantom Major Axis on Uniformity at 145 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y C Effect of Phantom Major Axis on Uniformity at 160 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y D

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76 Effect of Phantom Major Axis on Uniformity at 175 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y E Effect of Phantom Major Axis on Uniformity at 190 ref mAs0.60 0.65 0.70 0.75 0.80 2527293133353739 Phantom Major Axis (cm)Image Uniformit y F

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77 LIST OF REFERENCES 1 M. Kalra, M. Maher, T. Toth, L. Hamberg, M. Bl ake, J. Shepard, and S. Saini, Strategies for CT Radiation Dose Optimization, Radiology 230 619-628 (2004). 2 H. Grees, J. Lutze, H. Wolf, T. Hothorn, and W. Bautz, Dose Reduction in Subsecond Multislice Spiral CT Examination of Children by Online Tube Current Modulation, Eur. Radiol. 14, 995-999 (2004). 3 M. Kalra, M. aher, T. Toth, B. Schmidt, B. Westerman, H. Morgan, and S. Saini, Techniques and Applications of Automatic Tube Current Modulation for CT, Radiology 233, 649-657 (2004). 4 M. Geis, W. Kalender, H. Wolf, and C. Su ess, Dose Reduction in CT by Anatomically Adapted Tube Current Modulation I Simulation Studies, Med. Phys 26, 2235-2247 (1999). 5 J. Althen, Automatic Tube Current Modulat ion in CTA Comparison between Different Solutions, Radiation Protection Dosimetry 114, 308-312 (2005). 6W. Kalender, H. Wolf, and C. Suess, Dos e Reduction in CT by Anatomically Adapted Tube Current Modulation II Phantom Measurements, Med. Phys 26, 2248-2253 (1999). 7M. Kalra, M. Maher, T. Toth, R. Kamath, E. Halpern, and S. Saini, Comparison of Z-Axis Automatic Tube Current Modulat ion Technique with Fixed T ube Current CT Scanning of Abdomen and Pelvis, Radiology 232, 347-353 (2004). 8M. Kalra, M. Maher, R. DSouza, S. Rizzo, E. Halpern, M. Blake, and S. Saini, Detection of Urinary Tract Stones at Low-Ra diation-Dose CT with Z-Ax is Automatic Tube Current Modulation: Phantom and Clinical Studies, Radiology 235, 523-529 (2005). 9M. Kalra, S. Rizzo, M. Maher, E. Halpern, T. Toth, J. Shepard, and S. Aquino, Chest CT Performed with Z-Axis Modulation: Scanning Protocol and Radiation Dose, Radiology 237, 303-308 (2005). 10S. Rizzo, M. Kalra, B. Schmidt, T. Dalal, C. Suess, T. Flohr, M. Blake, and S. Saini, Comparison of Angular and Combined Automatic Tube Current Modulation Techniques with Constant Tube Current CT of the A bdomen and Pelvis, Am J Roentgenol 186, 673-679 (2006). 11C. Suess, and X. Chen, Dose Optimization in Pediatric CT: current technology and future innovations, Pediatr. Radiol 32, 729-734 (2002). 12 R. Dixon Restructuring CT dosimetryA realistic strategy for the future Requiem for the pencil chamber, MedPhys 33 (10), 3973-3976 (2006).

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78 BIOGRAPHICAL SKETCH Ryan Fisher graduated from Brookwood Hi gh School in 2000. Brookwood is located in Snellville Georgia, a suburban town 25 miles northeast of Atlanta. Ryan then attended The Georgia Institute of Technology (Georgia Tech) and graduated with a Bachelor of Science degree in biomedical engineering in 2004. He then enrolled at the University of Florida to pursue masters and doctorate degrees in medical physics.


Permanent Link: http://ufdc.ufl.edu/UFE0017940/00001

Material Information

Title: Tissue equivalent phantoms for evaluating in-plane tube current modulated CT dose and image quality
Physical Description: Mixed Material
Language: English
Creator: Fisher, Ryan F. ( Dissertant )
Hintenlang, David E. ( Thesis advisor )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2006
Copyright Date: 2006

Subjects

Subjects / Keywords: Nuclear Engineering Sciences thesis, M.S
Dissertations, Academic -- UF -- Nuclear and Radiological Engineering

Notes

Subject: A compressible, flexible, urethane-based tissue equivalent material was developed and utilized in the production of five ellipsoid-shaped phantoms for evaluating the in-plane tube current modulation performance of multi-slice CT scanners. The created phantoms were designed to be integrated with a computed tomography dose index (CTDI) dose assessment head phantom. Each phantom has a minor axis of 16 cm (corresponding to the diameter of the CTDI head phantom), with major axes ranging from 26 to 36 cm. A Siemens Somatom Sensation 16 CT scanner (Malvern, PA) was used to take central axis ion chamber dose measurements of each phantom using six different reference mAs settings with Siemens CARE Dose4D super script TM tube current modulation system. Image uniformity from each scan was measured as a method of tracking changes in image quality as a result of changes in the reference mAs setting. Tests were also performed comparing dose and image quality for scans using modulated and fixed tube current techniques. Image uniformity was found to be relatively constant for each reference mAs setting regardless of phantom major axis length, proving the proper functionality of the in-plane component of the CARE Dose4Dsuperscript TM system. Image uniformity was found to increase as the reference mAs setting was increased, at the expense of higher doses. In comparing modulated versus fixed tube current techniques, dose savings of up to 63% were observed, but at the expense of slightly noisier images. Elliptical phantoms of varying major axis length can be easily and effectively used to test the performance of in-plane tube current modulation systems in multi-slice CT scanners. Such phantoms can also prove useful in comparing image quality and dose measurements between differing commercial CT scanners.
Subject: computed, current, equivalent, material, modulation, tissue, tomography, tube
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 78 pages.
General Note: Includes vita.
Thesis: Thesis (M.S.)--University of Florida, 2006.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017940:00001

Permanent Link: http://ufdc.ufl.edu/UFE0017940/00001

Material Information

Title: Tissue equivalent phantoms for evaluating in-plane tube current modulated CT dose and image quality
Physical Description: Mixed Material
Language: English
Creator: Fisher, Ryan F. ( Dissertant )
Hintenlang, David E. ( Thesis advisor )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2006
Copyright Date: 2006

Subjects

Subjects / Keywords: Nuclear Engineering Sciences thesis, M.S
Dissertations, Academic -- UF -- Nuclear and Radiological Engineering

Notes

Subject: A compressible, flexible, urethane-based tissue equivalent material was developed and utilized in the production of five ellipsoid-shaped phantoms for evaluating the in-plane tube current modulation performance of multi-slice CT scanners. The created phantoms were designed to be integrated with a computed tomography dose index (CTDI) dose assessment head phantom. Each phantom has a minor axis of 16 cm (corresponding to the diameter of the CTDI head phantom), with major axes ranging from 26 to 36 cm. A Siemens Somatom Sensation 16 CT scanner (Malvern, PA) was used to take central axis ion chamber dose measurements of each phantom using six different reference mAs settings with Siemens CARE Dose4D super script TM tube current modulation system. Image uniformity from each scan was measured as a method of tracking changes in image quality as a result of changes in the reference mAs setting. Tests were also performed comparing dose and image quality for scans using modulated and fixed tube current techniques. Image uniformity was found to be relatively constant for each reference mAs setting regardless of phantom major axis length, proving the proper functionality of the in-plane component of the CARE Dose4Dsuperscript TM system. Image uniformity was found to increase as the reference mAs setting was increased, at the expense of higher doses. In comparing modulated versus fixed tube current techniques, dose savings of up to 63% were observed, but at the expense of slightly noisier images. Elliptical phantoms of varying major axis length can be easily and effectively used to test the performance of in-plane tube current modulation systems in multi-slice CT scanners. Such phantoms can also prove useful in comparing image quality and dose measurements between differing commercial CT scanners.
Subject: computed, current, equivalent, material, modulation, tissue, tomography, tube
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 78 pages.
General Note: Includes vita.
Thesis: Thesis (M.S.)--University of Florida, 2006.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017940:00001


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TIS SUE EQUIVALENT PHANTOMS FOR EVALUATING IN-PLANE TUBE CURRENT
MODULATED CT DOSE AND IMAGE QUALITY





















By

RYAN F. FISHER


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2006
































Copyright 2006

by

Ryan F. Fisher









ACKNOWLEDGMENTS

I thank my supervisory committee chair (Dr. David E. Hintenlang) for his guidance, help,

and patience throughout the course of this research. I also thank Dr. Manuel Arreola for his

expertise and direction in the clinical aspects of the research.












TABLE OF CONTENTS


page

ACKNOWLEDGMENTS .............. ...............3.....


LIST OF TABLES ................ ...............6............ ....


LIST OF FIGURES .............. ...............7.....


AB S TRAC T ......_ ................. ............_........9


CHAPTER


1 INTRODUCTION AND BACKGROUND ................. ...............11................


Introducti on .................... .. .. ... .... .. ....... .... .............1
Basic Principles of Tube Current Modulation ................. ...............12...............
Angular M odulation .............. ............... 13....
Z-Axis Modulation .................. .... ........ ...... ............... ..... ..........1
Simulation and Phantom Studies with Tube Current Modulation ................. ...............15
Clinical Studies in Tube Current Modulation .............. ...............17....
Current State of the Art ................. ...............21.......... ....
Conclusions .............. ...............23....

Purpose of Study ................. ...............24...............
Overview .............. ...............25....

Approach .............. ...............25....

2 TIS SUE EQUIVALENT MATERIAL DEVELOPMENT ................ ........................3 6


Silicone-B ased Rubber Material Testing .......................... ...............36.....
Sample Preparation............... ..............3
Sam ple Testing .............. ......... .............3
Urethane-Based Rubber Material Testing .............. ...............38....
M material Testing................ ... ... ................3
Results of Attenuation and Density Testing ................. ...............40......_.._...
Materials Testing with PMC 121/30............... ...............41.
Further Testing .............. ...............42....

3 ELLIPTICAL PHANTOM CONSTRUCTION ................. ...............51................


Materials and Methods .............. ...............51....

Improved Construction Methods ................. ...............52................













4 PHANTOM TESTING .............. ...............57....


Materials and Methods ................ ...............57...
Phantom Dose Measurement ................. ...............57........... ....

Phantom Image Quality Measurement ................. ............. ........ ............5

Comparison of Modulated and Fixed Tube Current Techniques ................. ................60
Results and Discussion .............. ...............60...

Image Uniformity Measurements ................. ...............60........... ....
Dose Measurements............... ..............6
Overall Trends ................... .. ... ....... ........ ..... .. ......... ........6

Comparison of Modulated and Fixed Tube Current Techniques ................. ................63
Conclusions............... ..............6


5 FUTURE WORK............... ...............72..


APPENDIX .............. ...............74....


LIST OF REFERENCES ................. ...............77................


BIOGRAPHICAL SKETCH .............. ...............78....










LIST OF TABLES


Table page

1-1 Results of phantom dose reduction studies ................. ...............33......__. ..

1-2 Comparison of image noise and diagnostic acceptablity ................. ........................34

1-3 Reductions in tube current time product with Z-axis modulation .............. ...................34

2-1 Ecoffex samples prepared with various additives ................. ....___ .............. .....44

2-2 Material properties of various Smooth-On rubbers ................. ......___ .........__ ....45

2-3 List of slabs poured with 121/30 as a base .............. ...............49....











LIST OF FIGURES


Figure page

1-1 Phantom demonstration of angular tube current modulation ................. ............... .....28

1-2 Real-time tube current modulation. ............. ...............28.....

1-3 Z-axis modulation. ............. ...............29.....

1-4 Z-axis modulation. ............. ...............30.....

1-5 Summary of phantoms and associated path-lengths ................ ................ ......... .3 1

1-6 Image noise as a function of modulation parameter. ................. ................. ..........32

1-7 Kidney phantom ................. ...............35........... ....

2-1 Three tissue equivalent slabs curing in epoxy molds............... ...............44.

2-2 Relative attenuation of samples. ............. ...............44.....

2-3 Relative attenuation of EcoffexThl based tissue equivalent materials .............. .................45

2-4 Slabs of PMC 780, 744, 121/30 and 121/50 urethane-based rubbers ............... .... ........._..46

2-5 Experimental setup for attenuation measurement. ....._.. .............._ .........._..__....46

2-6 Full experimental setup ..........._._ ......_.. ...............47....

2-7 Comparison of attenuation coefficients of urethane based rubbers. .............. ..............47

2-8 Comparison of densities of urethane based rubbers. ........._._ ......_. ........_.......48

2-9 Attenuation coefficients of PMC 121/30 based materials. ............. ......................4

2-10 Densities of PMC 121/30 based materials............... ...............4

2-11 Attenuation values of 121/30 with various additives............... ...............5

3-1 Diagram of proposed phantom design. ................ .............. ........ ......... .....54

3-2 Elliptical phantom mold. ...._.._................. ...............54. ....

3-3 CTDI head phantom centered in phantom mold ................. ...............55......_.._..

3-4 Phantom mold filled with urethane liquid rubber. .............. ...............55....

3-5 Five elliptical tissue equivalent phantoms of increasing maj or axis ................. ...............56










4-1 Experimental setup for dose measurements ................. ...............66......_.._...

4-2 Screen capture of a CT scan of an elliptical phantom surrounding a uniform region of
an image quality phantom. .............. ...............67....

4-3 CT number uniformity as a function of phantom maj or axis and reference mAs
setting ................. ...............68.................

4-4 Percent increase in CT number uniformity for increasing reference mAs settings.......... .68

4-5 Percent increase in CT uniformity with error bars for 190 reference mAs setting............ 69

4-6 Increases in dose as a result of changes in phantom maj or axis length and reference
m As setting. ............. ...............69.....

4-7 Comparison of image quality between modulated and fixed tube current techniques. .....70

4-8 Comparison of dose between modulated and fixed tube current techniques ................... ..71

A-1 CT number uniformity as a function of phantom maj or axis ................. ......._.._.. ......74









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

TIS SUE EQUIVALENT PHANTOMS FOR EVALUATING IN-PLANE TUBE CURRENT
MODULATED CT DOSE AND IMAGE QUALITY

By

Ryan F. Fisher

December 2006

Chair: David Hintenlang
Major: Nuclear Engineering Sciences

A compressible, flexible, urethane-based tissue equivalent material was developed and

utilized in the production of five ellipsoid-shaped phantoms for evaluating the in-plane tube

current modulation performance of multi-slice CT scanners. The created phantoms were

designed to be integrated with a computed tomography dose index (CTDI) dose assessment head

phantom. Each phantom has a minor axis of 16 cm (corresponding to the diameter of the CTDI

head phantom), with major axes ranging from 26 to 36 cm. A Siemens Somatom Sensation 16

CT scanner (Malvern, PA) was used to take central axis ion chamber dose measurements of each

phantom using six different reference mAs settings with Siemens CARE Dose4DTM tube current

modulation system. Image uniformity from each scan was measured as a method of tracking

changes in image quality as a result of changes in the reference mAs setting. Tests were also

performed comparing dose and image quality for scans using modulated and fixed tube current

techniques.

Image uniformity was found to be relatively constant for each reference mAs setting

regardless of phantom maj or axis length, proving the proper functionality of the in-plane

component of the CARE Dose4DTM System. Image uniformity was found to increase as the

reference mAs setting was increased, at the expense of higher doses. In comparing modulated









versus fixed tube current techniques, dose savings of up to 63% were observed, but at the

expense of slightly noisier images.

Elliptical phantoms of varying maj or axis length can be easily and effectively used to test

the performance of in-plane tube current modulation systems in multi-slice CT scanners. Such

phantoms can also prove useful in comparing image quality and dose measurements between

differing commercial CT scanners.









CHAPTER 1
INTRODUCTION AND BACKGROUND

Introduction

X-ray Computed Tomography exams have become increasingly popular because of

recent technological developments (such as multi detector spiral CT and greatly reduced scan

times that allow a large amount of diagnostic information to be collected in a short period of

time). As volumetric CT becomes more commonplace, concerns have arisen over increases in

patient dose as a result of wider beams and more frequent exams. Studies have shown that

although CT accounts for only 1 1% of x-ray based examinations in the United States, it delivers

over 65% of the total radiation dose associated with medical imaging.' Effective doses for

standard protocols of neck, chest abdomen and lumbar spine examinations can range from 3-15

mSv in adults, and currently there are no limits in place on the amount of radiation delivered per

scan in the US.2 Radiation exposure from CT is of particular concern for pediatric studies due

to children's relative increased lifetime cancer risk and higher radiosensitivity compared to

adults.2 It should be noted that despite these statistics and facts, CT remains a low dose imaging

modality. However, in the interest of keeping radiation exposure as low as reasonably

achievable, methods to reduce CT dose and improve image quality have been explored.

Numerous solutions have been suggested in response to dose concerns, including a

general lowering of tube current techniques for all exams.2 Although this suggestion would

reduce patient doses, such a reduction would come at the expense of noisier images, given that

image noise is related to the number of photons incident on the detector.3 A decrease in tube

current could thus compromise image resolution and low contrast delectability in medical

images, possibly leading to misdiagnosis.3 As an alternate approach, CT scanner manufacturers

have developed technologies for lowering patient doses without sacrificing image quality. These









technologies are referred to as tube current modulation, and act to adjust the x-ray tube output

during the CT scan in either the x-y plane (angular modulation) or in the Z direction (Z-axis

modulation) in response to changes in patient anatomy and attenuation in order to reduce dose

and maintain a constant image quality throughout the scan.3

Basic Principles of Tube Current Modulation

In Computed Tomography exams, selectable techniques such as tube current and tube

potential determine the photon fluence output of the x-ray htbe.l This fluence, along with the

attenuation characteristics of the patient, determines patient dose as well as the number of

photons reaching the detectors, which in turn determines reconstructed image noise

characteristics.1-3 If all other variables are held constant, a reduction in tube current leads to a

reduction in patient dose, but an increase in quantum noise or mottle in the reconstructed image.

Images with too much quantum noise may obscure low contrast lesions or tumors that would

normally be visible in less noisy images.1-3

In conventional CT, a technologist selects the tube current and tube potential based on

patient characteristics such as size and weight, as well as based on the particular exam being

performed. These techniques are held constant for each slice throughout the exam. Since

patients are not homogeneous in composition, nor circular in exterior body shape, these fixed

techniques lead to variable attenuation though the body and as such, a variable number of

photons reaching the detectors on the opposite side of the patient for different proj section angles.4

Certain anatomical areas, such as the shoulders, are problematic in that lateral views have much

higher attenuation, up to three orders of magnitude higher,4 than anteroposterior views of the

same area. In these instances, tube current must be increased to allow more photons to reach the

detector in order to minimize reconstruction artifacts due to high image noise in those planes.









The result of this increase in tube current is an increased patient dose throughout the entire scan

area of the exam if the techniques are held constant for the entire exam area.4

In response to these problems, CT manufacturers have developed automatic tube current

modulation techniques that allow the tube current to be automatically adjusted during a CT

examination in order to provide lower patient doses and constant image noise characteristics.

Tube current in low attenuation proj sections can be greatly reduced without loss of image quality,

thus reducing patient dose for the exam. There are currently two maj or strategies employed by

manufacturers to accomplish this task; angular and Z-axis modulation.l

Angular Modulation

Angular modulation was first developed by GE Medical Systems and works to modulate

tube current in the x-y plane within a single rotation of the tube.3 Photon fluence is increased in

areas of higher attenuation, such as lateral views though the shoulders, and decreased in areas

with lower attenuation, such as AP views through the chest. The first angular modulation system

from GE appeared in 1994 (SmartScan), and used two localizer radiographic images, AP and

lateral, to determine attenuation values of the patient.3 The tube current was then modulated in a

preprogrammed sinusoidal pattern that matched the attenuation characteristics. These systems

attained a dose reduction of up to 20% while maintaining a relatively constant level of image

noise.3 More recent offerings of the technology by Siemens employ an online real-time

anatomy-adapted system (CARE Dose) that automatically adjusts the tube current for a given

proj section based on the attenuation calculated from the previous rotation.' Thus, tube current is

modulated 'on the fly' without the need for localizer radiographic images, and is adjusted by

attenuation information provided from the previous 180 degree proj section as seen in Figures 1-1

and 1-2. The CARE Dose system has been shown to produce dose reductions of up to 90% for

the anteroposterior projection in regions such as the shoulders with marked asymmetry.3












Philips Medical Systems also uses an angular tube current modulation system (Dose-

Right Dose Modulation) in their CT scanners. The Philips technology modulates current within

a single tube rotation according to the square root of the attenuation measured during the

previous rotation. This modulation technique is based on the fact that image noise is inversely

related to the square root of the number of photons captured.3

Z-Axis Modulation

The second major type of tube current modulation technique is Z-axis modulation. This

technology also acts to adjust the photon output of the x-ray tube according to patient specific

attenuation characteristics, but unlike angular modulation, it does not alter tube current within a

single rotation of the x-ray source. Instead, a scout radiographic image is taken of the patient,

and the system calculates the photon flux required in each slice in order to maintain a user

designated noise level in the reconstructed image.3 The tube current remains constant for each

rotation around the patient, but is altered along the length of the patient as the table translates

through the beam. Lower current values are thus used in lower attenuation regions, such as the

chest, lowering patient dose in comparison to higher attenuation regions such as the pelvis

(Figures 1-3 and 1-4).

The user can choose between several noise index values depending on the quality of

images required by the exam. The noise index value is approximately equal to the standard

deviation of pixel values in the central region of an image of a uniform phantom.3 A higher noise

index corresponds to a greater standard deviation of pixel values for similar tissues in an image,

but also to a lower overall tube current during the exam and thus lower patient doses. It should

be noted that the noise index chosen will not always exactly match the noise in a reconstructed

patient image since reconstruction parameters also influence image noise.3 Z-axis modulation is









currently offered by GE medical systems (AutomA) as well as Toshiba and Siemens medical

systems, and acts to keep image noise the same in each slice image in an exam.3

Simulation and Phantom Studies with Tube Current Modulation

A two-part study was conducted in 1999 by Michael Gies et al. at the University of

Erlangen-Nuremberg in Germany4 that tested the theory of tube current modulation with both

mathematical simulations as well as phantom studies. In the first part of the study,

computational simulations of CT imaging were run on a series of four geometric phantoms

including: an elliptical, water-fi11ed, 'shoulder phantom,' an oval shaped, acrylic, 'hip phantom,'

an oval, water-filled, 'abdominal phantom,' and a standard water-fi11ed circular phantom (Figure

1-5).

Mathematical simulations were run on all phantoms for a range of modulation factors

ranging from 0, corresponding to no modulation (fixed tube current cases), to 1, corresponding to

modulation proportional to attenuation. For these varying parameters, image noise in the central

pixel of each phantom was computed mathematically to quantify the possible noise reduction

and efficiency of tube current modulation.4 The effect of tube current modulation was also

evaluated on noise in reconstructed images. For all simulations, both a sinusoidal and an

attenuation based modulation function were used.

The results of the mathematical simulations showed that a sinusoidal modulation function

provides much less noise reduction than attenuation-based methods for all noncircular shaped

obj ects.4 It was also shown that image noise is minimized when an attenuation based modulation

factor of 0.5 is used, corresponding to current control proportional to the square root of obj ect

attenuation. In reconstructed images without tube current modulation, anisotropic noise patterns

were visible in the direction of highest attenuation in the obj ect (along the maj or axis of the

ellipse).4 As previously mentioned, when modulation is performed according to the square root









of attenuation, image noise is minimized; however, the anisotropic noise structure is still visible

in the image. If tube current control is increased to being directly proportional to attenuation,

image noise levels rise slightly, but become similar for all proj section angles and thus isotropic in

the reconstructed image (Figure 1-6). According to the study, a homogeneous, isotropic noise

pattern is generally considered superior both aesthetically and diagnostically for CT images,

pointing to the fact that absolute lower noise levels may not be optimal for image

reconstruction.4

The mathematical phantom simulations also showed the dependency of image standard

deviation on the inverse square root of photons registered by the detector. For smaller numbers

of registered photons, the function changes rapidly, but does not appreciably change for larger

numbers of photons. Therefore reducing the number of photons in low attenuation regions will

have a minimal impact of image noise, thus allowing for lower patient doses.4 Overall the

simulation studies confirmed the theory that tube current modulation has the potential to lower

patient doses while maintaining or improving image noise characteristics in CT imaging.4

In the second part of the German study, actual phantoms with the same dimensions as

those utilized in mathematical simulations were used on a Siemens four-slice scanner.6 The

scanner was equipped with an early prototype of the CAREdose angular modulation system, in

which online tube current is modulated based on the previous 180 degree scan of patient

attenuation. Tests were also performed with ion chambers in the phantoms to determine the

extent of correlation between mAs reduction and actual dose reduction. Utilizing the hip

phantom and mathematical simulations, a predicted mAs, and thus dose reduction, of 39.4% was

calculated.6 This value was similar to the actual measured dose reduction (measured in mGy) in

the center of the phantom of 45.1%, proving that actual CT dose reductions are possibly even










larger than would be predicted based solely on mAs reduction.6 Since most radiosensitive

organs are located in a more or less central position in the body's cross-section, it was

determined that the reduction of mAs in studies is a valid and conservative estimate of organ

effective dose reduction.6

In general, the phantom measurements corresponded to within 10% of the previous

mathematical simulations regarding dose reduction and image noise (Table 1-1). Dose

reductions of up to 56% were found in highly asymmetrical phantoms such as the shoulder

phantom, while maintaining fairly constant image noise. Conversely, in tests where dose

remained constant, scans utilizing tube current modulation showed a reduction in image noise

compared to scans with constant tube current.6

Overall, the mathematical and phantom studies concluded that online, attenuation-based,

tube current control systems showed a significant potential for clinical dose reduction without

compromising image quality. Furthermore, the reduction in tube current would reduce x-ray tube

load and thus result in lower operational costs of CT scanners.6

Clinical Studies in Tube Current Modulation

Multiple clinical studies have been published using Z-axis tube current modulation

(AutomA on GE scanners). The maj ority of these studies come from the Radiology Department

at Massachusetts General Hospital.7- As previously mentioned, the GE AutomA system

requires an input of the noise index, as well as maximum and minimum tube current thresholds.

These thresholds make sure that tube current remains in a usable range during the entirety of the

scan without unexpectedly high outputs through regions in order to maintain the constant noise

mndex.'

One such clinical study involved utilizing tube current modulation in abdominal and

pelvic CT exams with a sixteen slice scanner.' Sixty-two patients underwent follow-up









abdominal CT scans using Z-axis modulation. These images were then compared with previous

images obtained using fixed tube current techniques from the same patients, but otherwise using

identical imaging parameters. The mean interval between the scans was 5 months (range 2-8

months). The two sets of images were graded by subspecialty radiologists on the basis of

diagnostic acceptability and image noise. Images were graded on a five point scale with 1 being

unacceptable, 3 being acceptable and 5 being excellent. Images at five anatomic levels in the

abdomen and pelvis were used, including the upper liver at the level of the diaphragm, the porta

hepatic, right kidney hilum, iliac crest, and upper margin of acetabulum. The total mAs for each

exam was also recorded for comparison.7

The results of the abdominal-pelvic study (Table 1-2) showed that scores for image noise

and diagnostic acceptability of images at the levels of the upper liver and the acetabulum were

slightly lower with Z-axis modulation then compared to fixed tube current scans, although the

results were not statistically significant (p = 0.34). At all other levels there were no significant

differences in the scores for image noise and diagnostic acceptability between the two

techniques. Any lesions detectable on the manual tube current scans were also detectable on the

Z-axis modulation scans. An average overall mAs reduction of 31.9% (range 18.8% to 87.5%)

was found for Z-axis modulated scans compared to fixed tube current techniques (Table 1-3).

The use of Z-axis modulation resulted in an mAs reduction in 87% of all exams. An increase in

mAs was found in 13% of exams using Z-axis modulation and is attributable to the larger mean

weight of patients in this category as compared to those patients that had reduced mAs.

Although mAs increased in these exams, a significant (p = 0.1) improvement in image noise and

diagnostic acceptability was noted for all cases. This fact shows that previous scans with

overweight patients possibly utilized inappropriately low tube currents resulting in below









average image quality. Overall, the study showed that significant dose reduction is possible with

Z-axis modulation and highlighted the need for proper technique settings of both the minimum

and maximum current limits in order to ensure appropriate noise levels in images.'

Another clinical test performed by the same group evaluated dose reduction and scanner

performance in the detection of urinary tract stones using Z-axis modulation.8 This study used

both phantom and patient studies. In the phantom study, sixteen calcium oxalate or calcium

phosphate kidney stones were embedded in the collecting systems of two freshly harvested

bovine kidneys. The size of the stones ranged from 2.5 to 19.2mm. The kidneys were placed in

an elliptical Plexiglas container that was then filled with a physiologic saline solution. The

phantom was scanned on a Siemens 16 slice CT scanner six times. The phantom was scanned

once with a fixed tube current technique, and then once with Z-axis tube current modulation at 5

different noise index settings; 14, 20, 25, 35, and 50. The remaining scanning and reconstruction

parameters were kept the same for all scans. The phantom images were then viewed by two

separate radiologists who were blinded to the scanning techniques and graded using the same 5

point scale described above to describe the detectability of the stones in the images.

In addition to the phantom study, patient studies were conducted in 22 patients using Z-

axis modulation. As in the previously described study, all patients had been previously scanned

using standard fixed current techniques with all other scanning parameters kept the same. The

same two radiologists evaluated both sets of images using the same five point scale for

conspicuity and margins around the stones.

In the phantom study, both radiologists identified all 16 stones in the fixed tube current

technique images, as well as in the Z-axis modulated images at noise indexes of 14, 20, and 25.

Three stones smaller than 5 mm were not identified by either radiologist in images from Z-axis









modulation with a noise index of 35 and 50. There was no significant difference (p > 0.5) in

stone conspicuity, image noise, or diagnostic acceptance between Eixed current and Z-axis

modulated images at a noise index of 14, 20 or 25.8 Dose reduction, by means of reduced mAs,

was found in all Z-axis modulated scans ranging from 51% in the noise index 14 case, to 92% in

the noise index 50 case, although as previously mentioned, not all stones were localized with this

technique. An mAs reduction of 76% was found in the noise index 25 case, the highest noise

index where all stones were detected.8 Kidney phantom images at each noise level are shown in

Figure 1-7.

Similar dose reductions were found in the patient studies, although only noise indexes of

14 and 20 were used. All stones located with the Eixed tube current scans were also located with

Z-axis modulation, and no statistically significant difference (p > 0.7) was found in the

radiologists ratings for conspicuity, image noise, or diagnostic acceptability.8 An overall

reduction in mAs of 43% was found with a noise index of 14, and a 66% reduction was found at

a noise index of 20.

Overall the study demonstrated that doses in urinary tract stone detection exams can be

greatly reduced by utilizing Z-axis modulation techniques. These reductions can be higher than

in the cases of chest or abdominal imaging due to the high contrast nature of urinary tract stones

in soft tissue, allowing for slightly noisier images than required for low contrast detection cases.8

It was also found that for these exams, a 5% reduction in the noise index correlates to a dose

(mAs) increase of approximately 10%.

The same research group conducted a similar study of chest CT studies comparing Eixed

tube current methods to Z-axis modulation.9 Very similar methodologies were used as in both the

previous studies. The study used 53 patients, with two radiologists reading the results of CT









exams and rating them based on image noise, diagnostic acceptability and streak artifacts. Again

the study found that Z-axis modulation provided acceptable imaging of the chest, including lung

parenchyma and mediastinal structures, with no significant differences in image noise, diagnostic

acceptability, or streak artifacts, as compared to fixed tube current techniques.8 The use of Z-axis

modulation showed average mAs reductions of 18%, 26%, and 38%, with a noise index setting

of 12, 12.5, and 15 respectively, when compared to fixed tube current protocols. Again tube

current was found to increase slightly in heavier patients, which is a result of the system

attempting to keep image noise constant throughout the thicker area of the scan. It was noted

that care should be taken by physicians to pay special attention to noise requirements from larger

patients to avoid unnecessary increases in dose.8

Current State of the Art

While all previous mentions of tube current modulation have been with respect to either

angular or Z-axis techniques, there are CT scanners currently available that make use of both

systems simultaneously. This technology is currently only available from high-end, top of the

line scanners, but is expected to trickle down and become more prevalent in the industry as time

progresses. A study has been published in which a Siemens Somatom Sensation 16 scanner

was used to compare the combined modulation technique with angular modulation and fixed

tube current modes of operation (CARE Dose 4D). In this scanner, a scout image performs the

normal task of assigning tube currents for each slice along the Z-axis in order to maintain

constant image noise. While the scan is in progress, the tube current is modulated as the tube

rotates around the patient in order to adjust photon flux to match patient specific attenuation for

each proj section angle. This combined modulation is referred to as xyz modulation. In this

particular study 152 patients were used who underwent contrast enhanced CT examinations of

the abdomen and pelvis. Seventy-nine patients were scanned using xyz-axes modulation with










forty-two using the weak decrease-strong increase operation mode, and thirty-seven using the

average decrease-average increase operation mode (explained below). Forty-two patients were

then scanned using only the angular modulation technique, and thirty-one patients were scanned

using a fixed tube current technique.

As with the GE system, a user input of an acceptable noise level is required, along with a

Siemens specific modulation characteristic for slim or obese patients. This modulation setting

differs based on patient size, and decreases tube current for slim patients and increases current

for obese patients. The extent to which the tube current is increased or decreased can be

controlled via the setting of modulation strength, classified as either strong, average, or weak. A

strong setting for obese patients results in a larger increase in dose corresponding to lower image

noise, while a weak setting for an obese patient spares patient dose at the expense of increased

image noise. For slim patients, a strong modulation setting results in more image noise and

lower patient dose, while a weak modulation setting results in higher dose and less noise.10 It is

this author's opinion that the method for noise level selection employed by Siemens is infinitely

more complex than that set by GE for Z-axis modulation, and could result in improper technique

settings by technologists.

As in the previous clinical studies, images from each set of patients were graded by two

radiologists on the basis of image noise, diagnostic acceptability, streak artifacts, and visibility of

small structures. A quantitative measure of image noise was also recorded for each exam in the

liver parenchyma at the level of the porta hepatis.

The study found no significant statistical difference in the weights (p = 0.3) or ages (p =

0.5) of patients in each of the scanning technique groups.'0 In evaluating values of image quality

parameters (radiologist measured), there was no significant difference found for examinations










performed with Eixed tube current, angular modulation, and weak decrease-strong increase

method combined modulation The scores from the average increase -average decrease method

of combined modulation were found to be significantly lower than other techniques however (p =

0.0001). Of these examinations, several were labeled as below diagnostic quality with more than

acceptable image noise. In comparing obj ective image noise, again there was no significant

difference in values for exams performed with Eixed tube current, angular modulation, and weak

decrease-strong increase combined modulation. Again, the noise values for average increase-

average decrease mode of combined modulation were significantly higher than any other

technique (p > 0.1).

In comparing doses measured for each technique, a significant reduction (p < 0.0001)

was found in the angular and combined methods of tube current modulation. Compared to Eixed

current techniques, there was an average dose reduction of 19% for angular modulation

technique exams, and 42% for weak decrease-strong increase combined modulation exams. The

average decrease-average increase method of combined modulation resulted in 44% average

dose reduction, although at the expense of image quality as previously mentioned. It should be

noted that dose changes in this study were reported as changes in CTDIvol and not in mAs as

reported in previous studies, making a direct comparison between studies impossible.

The combined modulation technique study showed the increased dose savings of

combining angular and Z-axis tube current modulation, but also highlighted the importance of

proper technique settings correlating to patient size.

Conclusions

As has been discussed, tube current modulation techniques represent technological

advances in CT scanners that allow for a reduction of patient dose while maintaining image

quality. Three methods have been described for tube current modulation; angular modulation,









which modulates tube current in the x-y plane as the tube rotates around the patient, Z-axis

modulation in which tube current remains constant for a give rotation, but changes as the patient

advances though the scanner, and xyz modulation, which combines the two techniques.

Mathematical simulations as well as phantom and patient studies have shown the dose reduction

benefits of current modulation, and dose reductions for both angular and Z-axis modulation have

been shown to be comparable for similar examinations. Combined tube current modulation

makes use of the advantages of each method to further reduce patient doses, but at the present

time is only available on high end scanners.

While tube current modulation can provide numerous benefits to clinical CT

examinations, it is not without its drawbacks. While the basic principles in use by different

manufacturers remains similar, the method of implementation can vary greatly between scanners,

which can make clinical use in a clinic with multiple manufacturers difficult. The lack of

uniformity between vendors, as evident with the noise index value of GE compared to the

decrease-increase method of Siemens, makes technologist training and understanding of the

technology paramount to proper diagnostic use of the equipment, as any dose savings in a single

scan are nullified if scans must be repeated due to improper techniques.

Purpose of Study

The benefits and limitations of tube current modulation systems in CT scanners have

been discussed, but as of now there is no simple method for medical physicists to ensure proper

functionality of such systems from a quality assurance standpoint. An improperly functioning

tube current modulation system would obviously pose numerous problems to a radiology

department, including the possibility of unacceptable image quality as well as unknowingly

higher patient doses. The purpose of this body of work was to develop a simple, effective method

to test the functionality of in-plane tube current modulation systems in order to ensure proper










clinical operation. Variations in performance of the system could be tracked on a monthly basis,

and proper maintenance or repairs could be requested if system parameters vary to an

unacceptable level.

Overview

A preliminary idea to test the functionality of an in-plane tube current modulation system

was to create a series of elliptical tissue equivalent phantoms of the same minor axes, but with

varying maj or axes. The overall concept was that such phantoms could all be scanned using the

same image quality setting of the tube current modulation system, and the respective image

noises from each phantom scan should be similar, regardless of maj or axis length. Similar image

noise values for varying sized phantoms would show that the in-plane tube current modulation

was indeed varying tube current based on attenuation values in order to maintain a constant

image noise. If image noise values were found to vary greatly for different sized phantoms, one

could assume something was wrong with the modulation system. Such a test would be easy to

perform, take little time, and the results could be recorded for long term tracking of scanner

performance .

Approach

We decided to use existing CT image quality and dose measurement phantoms for the

study, and to develop tissue equivalent elliptical attachments. This was chosen in the interest of

using less material and for the flexibility in allowing for dose, as well as image quality

measurements to be taken without having to manufacture two sets of phantoms.

In order to fabricate such elliptical phantoms, a tissue equivalent material was developed.

This material was to have a density similar to that of soft tissue (1.04 g/cm3), as well as similar

x-ray attenuation properties in the diagnostic energy range (80-120 kVp) used in CT imaging.

The developed material was also desired to be flexible and compressible in order to maintain









close contact with the existing CatPhan (The Phantom Laboratory, Salem NY) and standard

CTDI head phantom, as well as for applications in other phantom development proj ects dealing

with mammography and anatomic tomographic phantoms.

Once such a tissue equivalent material was developed, a series of five elliptical phantoms

was created. All phantoms had a minor axis of 16cm, corresponding to the diameter of

commercially available CTDI and CatPhan phantoms. The maj or axes of the created phantoms

ranged from 26 to 37.5 cm.

The created phantoms were then used with a Siemens Somaton Sensation 16 slice

scanner equipped with CareDose4D" tube current modulation system. This system uses angular

as well as Z-axis modulation to adjust tube output on the fly within a single tube rotation as well

as along the length of the scan based on a pre-examination topogram. Both of these systems

work together to ensure uniform image noise throughout a CT scan, while reducing patient doses

as compared to fixed tube output protocols. The user input for image noise has changed slightly

from that mentioned in the studies above. The user input for the CareDose4D" software is now a

single reference mAs value and the strong, normal, and weak modifications are no longer used.

The reference mAs value corresponds to the mean effective mAs value that the system will use

for a "reference patient" with the same protocol. The reference patient is described by Siemens

as a typical adult weighing 155-180 pounds for adult protocols, and a typical child weighing 45

pounds for children's protocols. Based on the reference mAs value, the system adapts the tube

current to the individual patient size. A higher reference mAs setting indicates a higher tube

current used in order to produce less image noise in the reconstructed image. A lower reference

mAs setting can be used for lower patient dose in scans where image noise is not of crucial

importance.










Each of the created phantoms was imaged with the Siemens CT system at six different

reference mAs values. These reference values were taken for a standard adult abdominal routine

at values above and below the default setting. The pixel uniformity values for each phantom at

each setting were then plotted in order to determine proper functionality of the angular

component of the CareDose 4D tube current modulation system, as well as general trends in dose

and image quality.













~---
I rts~li


II5 .II ]5 2.0 ?.5
Neanimein r
* ** th.*l liennu m -- Mralncksknluhe turrn


Figure 1-1: Phantom demonstration of angular tube current modulation. Tube current
remains constant for the first 180 degrees rotation about the shoulder phantom before
being modulated to match attenuation.
Reprinted with permission from
C. Suess, and X. Chen, "Dose Optimization in Pediatric CT: current technology and future
innovations," Pediatr. Radiol 32, 729-734 (2002).



















Reprinted with1 permisson fro
C. Sess andX. hen "Doe Otimiatin i Pedatrc CT curen tecnolgy ad ftur
innoatios," ediar. Rdiol32, 29-74 (202)






























Figure 1-3: Z-axis modulation. A scout radiographic image is used to determine the appropriate
mAs per rotation based on patient attenuation characteristics.
Reprinted with permission from
C. Suess, and X. Chen, "Dose Optimization in Pediatric CT: current technology and future
innovations," Pediatr. Radiol 32, 729-734 (2002).
















0 0 0 0 0 0
e I m


% 1MB





Figur 1-:Zai ouain uecret()vaisprsiea eemndb cu












J. Alten "-.ZAuts omuatic n Tube current Mouati ain s CT- A lc Cmarso betwenDfer ene b ot


Solutions," Radiation Protection Dosimetry 114, 308-312 (2005).

























II


'Shoulder Phantopm'
14cm x 40am ellipse
water


" -
,


'Abdomen Phansorm'
20cm x 30c m oval
waer


'QuaHI)~ Phanrtomc'
10cm diamerer
ratrer


O so is2 5150 aco acse soo sso
Anrgle of Rotatbn in degrees
Figure 1-5: Summary of phantoms and associated path-lengths. A) Phantom shapes used in
simulation studies B) path-length as a function of angle of rotation for each phantom,
demonstrating attenuation differences for noncircular shapes.
Reprinted with permission from
M. Geis, W. Kalender, H. Wolf, and C. Suess, "Dose Reduction in CT by Anatomically Adapted
Tube current Modulation I Simulation Studies," Med. Phys 26, 223 5-2247 (1999).


'Hi Phntm'
15cm x 3cm oval
PMMlrA

















L ~I B








Figure 1-6: Image noise as a function of modulation parameter. A) With no modulation
anisotropic noise is visible in the direction of highest attenuation. B) Modulation
according to the square root of attenuation: minimizes noise but anisotropic effects
are still visible. C) Modulation according to attenuation, slightly higher noise than b,
but isotropic.
Reprinted with permission from
M. Geis, W. Kalender, H. Wolf, and C. Suess, "Dose Reduction in CT by Anatomically Adapted
Tube Current Modulation I Simulation Studies," Med. Phys 26, 223 5-2247 (1999).



















Sigma Measurd Norma~lized Docse repduction
Phantoma~ Miodtulatioln ty~e in HU m~~s ~ mL~s in %

(a)
Noane (connt. mAl 2.9 494 494 0.0%(
Cylindrical wyate phantom .
~Sinusoldal 2.9 491 491 0 66.>
20 em dzamt
AnttenuatioEln-b~a-se 2.9 487 487 14
Oval waater phantom None (connt. mA) 5.7 493 493 0.0%(
20 emX 30 cmr Sinusoidal 6.1 393 45i0 9.7%I
AnttenuatioEln-b~a-se 6.1 367 420 I 6.
Shourldezr phantom1 Nocne (connt. mAl 19.0 213 213 0.0%i~
w~ithouZ inserts. [Fig. 4(a)]
14 enX 40 cmu Sinusoidal 14.6 213 126 34 I r
[Fig. 4(b)]
Attenneation-based 13.2 220 103 44.6%C
[Fig. 4(c)]
(b)
Cval w~ater phantomr None (conn. mnAl 0.0
20 cm -30 cm Sinuscidal 9.4
Attennation-based 9.5
Shoulder phantomu Noe (conn. mnA) 0.00,
wkithozut insestts Sinusoidatl :29.70,
14 cmX40 cm Attenltuation-based 39.30,


Reprinted with permission from
W. Kalender, H. Wolf, and C. Suess, "Dose Reduction in CT by Anatomically Adapted
Tube Current Modulation II Phantom Measurements," Med. Phys 26, 2248-2253 (1999).


Table 1-1: Results of phantom dose reduction studies
TABLE 1. (a) Comparison of dose reduction using a cylindrical, oval and elliptical w~ater phantom, and two
mocdulation types. (Scan parameters: 120 kY, 5 mmt, 1 s, 70% modulationa ampituderrri I (bl Dose rieductiona values


(Ref. 1). (Scan parameters: ]120 kV:, 5 imn, 1 4, 70%;


for the shoulder phantom obtained by 4insiunatos
rmoduflaion amplitude.)










Table 1-2: Comparison of image noise and diagnostic acceptability
Anatomic Z-Axis Modulation Fixed Tube current
Level Image Noise Diagnostic Acceptability Image Noise Diagnostic Acceptability
Upper Liver 2.8 +/- 0.7 3.0 +/- 0.6 2.9 +/- 0.7 3.1 +/- 0.8
Porta hepatis 2.9 +/- 0.6 3.2 +/- 0.6 2.8 +/- 0.7 3.2 +/- 0.7
Renal hilum 2.8 +/- 0.7 3.2 +/- 0.7 2.7 +/- 0.7 3.2 +/- 0.7
Illiac crest 3.0 +/- 0.8 3.3 +/- 0.6 2.9 +/- 0.7 3.4 +/- 0.7
Acetabulum 2.9 +/- 0.7 3.3 +/- 0.6 3.0 +/- 0.7 3.4 +/- 0.5
Note- Date are means and standard deviations. No statistically significant differences were found
between CT images acquired with Z-axis automatic modulation and those acquired with fixed tube current
(P = .34-.84)
Reprinted with permission from
W. Kalender, H. Wolf, and C. Suess, "Dose Reduction in CT by Anatomically Adapted
Tube Current Modulation II Phantom Measurements," Med. Phys 26, 2248-2253 (1999).


Table 1-3: Reductions in tube current time product with Z-axis modulation
Tube Current-Time Tube Current-Time Tube Current-Time
An ato mic Prod uct Product Product
Level .Reduction with Z-Axis
with Z-Axis Modulation with Fixed Tube Current
Modulation"
Upper Liver 103.6 +/- 50.3 187.9 +/- 21.4 84.3 +/- 45.7 (44.9)
Porta hepatis 121.0 +/- 51.6 187.9 +/- 21.4 66.9 +/- 47.7 (35.6)
Renal hilum 119.9 +/- 54.2 187.9 +/- 21.4 68.0 +/- 49.4 (36.2)
Illiac crest 115.4 +/- 56.4 187.9 +/- 21.4 72.5 +/- 51.7 (38.6)
Acetabulum 123.8 +/- 50.0 187.9 +/- 21.4 64.1 +/- 47.1 (34.1)
Note- Data are mean milliampere-seconds +/- standard deviations
Data in parentheses indicate percent change. For examinations in which mean tube
current time product decreased with Z-axis modulation compared with fixed tube current, the
difference was statistically significant (P<0.001)
Reprinted with permission from
W. Kalender, H. Wolf, and C. Suess, "Dose Reduction in CT by Anatomically Adapted
Tube Current Modulation II Phantom Measurements," Med. Phys 26, 2248-2253 (1999).


























I~II














Figure 1-7: Kidney phantom. Kidney stones visualized in bovine kidneys scanned with a) fixed
tube current technique, b) Z-axis modulation noise index =50, c) noise index=3 5, d)
noise index=25, e) noise index=20, f) noise index=14. Scans at higher noise indexes
show a substantial increase in image noise and lowered diagnostic acceptability.
Reprinted with permission from
M. Kalra, M. Maher, R. D'Souza, S. Rizzo, E. Halpern, M. Blake, and S. Saini, "Detection of
Urinary Tract Stones at Low-Radiation-Dose CT with Z-Axis Automatic Tube Current
Modulation: Phantom and Clinical Studies," Radiology 235, 523-529 (2005).









CHAPTER 2
TIS SUE EQUIVALENT MATERIAL DEVELOPEMT

The first portion of this work centered on the development of a flexible, compressible,

tissue-equivalent material for use in radiological phantom construction. The material was to be

equivalent to human soft tissue in density and x-ray attenuation properties in the diagnostic

energy range (80-120 kV). Phantoms previously created by the lab have been epoxy based,

giving them a solid consistency similar to acrylic. While the epoxy based material was tissue

equivalent, it was difficult to work with and limited in its scope of use. It was also hoped that the

new compressible material could be used for applications in more realistic mammography

phantoms to better model breast tissue properties under clinical conditions.

Acrylic was used as a benchmark for comparison of the attenuation properties of the

created material. Acrylic has previously been used for commercially available phantoms due to

its similar x-ray attenuation properties in the diagnostic energy range. The density of acrylic is

1.17 g/cm3 though, and as such the density of acrylic was not used as a benchmark for the

phantom material development. A target density of 1.04 g/cm3, the density of human soft tissue,

was used as a target density value.

Silicone-Based Rubber Material Testing

EcoffexTM, a silicone-based rubber compound produced by Smooth-On Inc. (Easton PA), was

first tested for its utility in creating a compressible tissue equivalent phantom material. EcoffexTM

consists of two parts, A and B, which are mixed in a 1:1 ratio by weight and allowed to cure for

12 hours to produce a stretchy, compressible material that returns to its original form after

deformation. Preliminary tests showed the compressibility of EcoffexTM was as desired and it had

a published density (1.07 g/cm3) ClOse to that of soft tissue (1.04 g/cm3). Based on these

preliminary tests, molds were produced in order to better quantify the properties of the material.









Sample Preparation

Epoxy molds were constructed in order to make slabs of EcoffexTM Of a reproducible

thickness. Solid pieces of epoxy were poured and allowed to harden overnight. Cutouts were

then machined into the epoxy slabs with a Vision Pro Engraver (Dade City, FL) to create four

10x10 cm square molds with a depth of approx 1 cm. Using these molds, slabs of EcoffexTM

were poured with various additives in order to test their radiographic attenuation and density

properties. Pictures of the epoxy molds and the EcoffexTM Slabs are shown in Figure 2-1.

Additives used included phenolic microspheres (System Three, Auburn WA), powdered calcium

carbonate, and powdered polyethylene (Fisher, Fairlawn NJ). The by-weight percentages of

additives used in the samples are given in Table 2-1. The EcoffexTM material easily peeled out of

the molds and did not require the mold release previously needed with epoxy based tissue

equivalent materials, making sample preparation clean and simple as compared to epoxy based

materials.

Sample Testing

After the previously mentioned samples were mixed, poured, and cured, they were tested for

density and radiographic attenuation properties. As mentioned, a density of 1.04 g/cm3, the

density of soft tissue, was used for a target density. A 1.25 cm thick Acrylic slab was used as a

benchmark for attenuation properties as acrylic is commonly used in commercial phantom

production. A portable x-ray generator (Source-Ray Inc. model SR-115, Boheimia NY) and

CCD detector were utilized for testing the attenuation properties of the samples. The samples

were placed directly on the CCD detector, and two slabs of EcoffexTM based material, the acrylic

slab, and a piece of BR-12 breast tissue equivalent tissue were imaged at a time. The average

pixel values from a 156x156 pixel square in the center of each sample for each of three images

was taken and then averaged in order to provide a relative comparison of the attenuation









properties of each material. If an EcoffexTM based slab had an average pixel value similar to that

of acrylic, its radiological attenuation properties were in the range required. All images were

acquired with an SID of 43 cm, and at 80kVp and 4mA. A sample of the collected x-ray images

is shown in Figure 2-2.

Upon inspection, it was found that all prepared EcoffexTM Samples had a much lower

average pixel value than acrylic, indicating a higher attenuation of the incoming x-ray beam

(it should be noted that the CCD detector utilized assigns pixel values oppositely to that of a

conventional digital radiographic unit, in that darker areas represent areas of higher

attenuation and lighter areas indicate regions of lower attenuation). The results of the

attenuation testing can be found in Figure 2-3. Even the EcoffexTM Samples with up to 10%

by weight phenolic microspheres, a low atomic number material added to reduce the density

and attenuation of the sample, had substantially higher attenuation values. Density tests were

not performed on the EcoffexTM Samples since it was apparent that the material could not

successfully be used as a base in tissue equivalent materials due to the high attenuation

attributed to its silicone base.

Urethane-Based Rubber Material Testing

After it was realized that Ecoff exTM was unsuitable for use as a base in tissue equivalent

materials, other solutions were explored. A range of urethane based compounds with varying

material properties were ordered from Smooth-On for further testing. The product numbers and

properties of the tested materials can be found in Table 2-2.

These materials were selected from multiple other products based on the listed properties.

A larger value for elongation at break, as well as a lower hardness value indicates a more

compressible material. All rubbers come in two parts which must be mixed in proportion. A mix

ratio of 1:1 by weight was desired for precision and ease of use in preparing samples, but other










options were tested in case their other properties ended up justifying a slightly more complex

mixing process. Some materials were offered in both a "wet" and "dry" option, with the "wet"

having a built in release agent to aid in the de-molding of plasters or concrete, the intended use

of the products. When available, the dry option was chosen but the PMC 121/50 material was

available in only a wet option. All materials had a similar de-mold time, which required

overnight curing before samples could be removed from their molds.

The same epoxy molds used for making EcoffexTM Slabs of similar thicknesses were

utilized to produce slabs of the urethane based materials. Each material was mixed as per its

instructions and with no additives in order to compare their base x-ray attenuation values to that

of acrylic. Based on these tests, the best material were selected and further tested with various

additives in order to reach the desired tissue equivalent properties. Slabs of each of the urethane

based materials are shown in Figure 2-4.

Material Testing

Unlike the previously described EcoffexTM Sample testing, where CCD images were

taken and pixel values compared, a new approach was taken for ease of calculations and

comparison in testing the urethane based materials. A Keithley 35050A ion chamber (Cleveland,

Ohio) was used with a portable x-ray unit (Source-Ray Inc. model SR-115) to measure

attenuation through each sample. The ion chamber was suspended on a stand to prevent

backscatter effects from altering data, and the slabs of material were placed approximately 18

inches above the chamber on the stand. The beam was collimated so as to not interact with the

metal arms of the stand holding the samples, and to fall across the active region of the detector.

A tube current of 80 kVp was used with 400 mAs and an SID of 38" for all measurements. The

experimental setup can be seen in Figures 2-5 and 2-6.









For testing, an initial exposure measurement was taken of open air with no slab in place.

Three exposure measurements were then taken each with an acrylic slab, a BR-12 breast tissue

equivalent material, and each of the Smooth-On urethane compounds in place above the ion

chamber. Based on the measured exposures and slab thicknesses for each material, a relative

attenuation coefficient was calculated using Equation 2-1.

R=Ro*e-Ult (Eq. 2-1)
With :
R = the average measured exposure (R)
Ro = the initial exposure with no slab in place (R)
u = relative attenuation coefficient (cm l)
t = the slab thickness (cm)


These relative attenuation coefficient values allow for easy comparison of x-ray attenuation

properties between materials and allow slabs of slightly differing thicknesses to be quantitatively

compared.

Density measurements of each sample were then taken utilizing Archimedes's principle.

A dry sample of each material was weighed on a scale with 0.001 gram precision. The samples

were then weighed submerged in a beaker of de-ionized water. Using both these measurements,

as well as the known density of the de-ionized water, the density of each sample was calculated

using Equation 2-2.

Dry weight / [(dry weight-wet weight)/density of H20] (Eq.2-2)



Results of Attenuation and Density Testing

The results of the initial round of material testing are shown in Figure 2-7 and 2-8. Based

on these tests, the 121/30 material was selected for use as a base in the tissue equivalent material.

It was chosen based on its density and attenuation values being within a reasonable range of that

of the desired values of both acrylic and BR-12, a breast tissue equivalent material. It was hoped









that the 121/30 material could be successfully used to develop both a soft tissue and breast tissue

equivalent phantom material. The individual attenuation and density properties can be altered by

the addition of additives such as phenolic microspheres, calcium carbonate, and polyvinyl

chloride in order to reach the desired target values. The 121/30 material was also chosen because

its elasticity and compressibility were qualitatively determined to be the best of the group of

materials. Other materials such as PMC 744 were found to be extremely firm and not

compressible enough. The PMC 121/50 wet material had an oily residue upon firming which was

not suitable for long term phantom use if only because of the messy residue it left on anything it

came into contact with.

Materials Testing with PMC 121/30

Once the 121/30 material had been selected, further testing was done utilizing a range of

previously described additives in order to get its attenuation and density properties to the desired

values. The same procedures for measuring attenuation and density described in the previous

section were used for this portion of testing. Multiple slabs of material were poured with varying

amounts of microspheres, calcium chloride, and/or polyethylene additives. Once the slabs had

cured, their relative x-ray attenuation values were measured using the Keithley ion chamber and

the previously described method.

For the first round of testing, the 121/30 material was mixed with 5% by weight each of

calcium carbonate, polyethylene, and microspheres to gauge these additives' effects on the

properties of the base urethane material. It was decided to try to match attenuation values first,

and once that was accomplished to then move to adjust the material's density through further

additives.

The results of the first round of testing are shown in Figures 2-9 and 2-10. Figure 2-9

shows that the addition of 5% polyethylene to the 121/30 compound did little to alter the









attenuation, while the addition of 5% phenolic microspheres significantly lowered the

attenuation. Both of these materials had a lower attenuation coefficient than that of acrylic. The

addition of 5% CaCO3 inCreaSed the attenuation of the 121/30 to a value higher than that of

acrylic. This indicated that the addition of a lower by-weight percentage of CaCO3 WOuld bring

the attenuation of the material close to the target value of acrylic.

The measured densities presented in Figure 2-10 show that the addition of CaCO3

increased the density ofPMC 121/30 to the desired value of 1.04 g/cm3. The addition of

microspheres and polyethylene lowered the density to less than that of water (~1 g/cm3). The

specific densities of these materials were not able to be calculated with the previously described

method because, being less dense than water, a wet sample weight could not be calculated since

the samples floated. Isopropyl alcohol, which has a density lower than that of water, could have

been used to calculate these material's densities, but this was deemed unnecessary since they

were below the target value.

Further Testing

Based on the results of the initial 121/30 testing, several new samples were poured with

varying amounts of CaCO3 and microspheres in order to get the material properties closer to

tissue equivalent. CaCO3 WAS added in order to raise the attenuation coefficient of the material,

while microspheres acted to lower both the attenuation and the density. It was hoped that some

combination of the two materials would produce a tissue equivalent rubber. A list of the newly

created material mixtures is provided in Table 2-3.

The samples were tested for attenuation and density in the same manner as the previous

samples. The results of this round of testing are shown in Figure 2-11. The densities of all

materials with microspheres added were once again less than 1 g/cm3. The densities of both the

2.5% and 3.2% CaCO3 Samples were 1.03 g/cm3, and the density of the 5% CaCO3 Sample









remained at 1.04 g/cm3. These density values indicate that increases on the order of 1% in

CaCO3 to the 121/30 mixture produce relatively small changes in density. Based on these test

results, the addition of only CaCO3 Seemed to be the most promising, as these samples had a

density and attenuation close to the target values of soft tissue. While samples with added

microspheres also had attenuation values both above and below that of acrylic, indicating the

ability to zero in on the exact attenuation, their low densities made then unsuitable for a tissue

equivalent material.

Given the results of the previous tests, the relative attenuation values of the 2.5% and

3.2% CaCO3 Sample were plotted as a function of their percentage of CaCO3, Since the samples

lay on either side of the target attenuation of acrylic. A linear-fit trend line was then fit to the

data points, and used to calculate the percentage of CaCO3 that would produce the same relative

attenuation as acrylic, 0.302 cml in this case. This calculation yielded a value of 2.8% CaCO3 to

be added to the PMC 121/30 urethane in order to provide a soft tissue equivalent attenuation

coefficient. A sample of 2.8% CaCO3 WAS then poured in order to verify this calculation. The

attenuation of this 2.8% sample was found to be equal to that of acrylic. Density measurements

of the 2.8% CaCO3 Sample showed that it has a density of 1.037 g/cm3 which is extremely close

to the target value for soft tissue and essentially tissue equivalent within bounds of experimental

error. It was determined that the combination of x-ray attenuation characteristics and density

very close to the ICRP standard for soft tissue was suitable to make the 2.8% by weight CaCO3

added to PMC 121/30 urethane mixture a suitable tissue equivalent material for phantom

construction.










Table 2-1: EcoflexTM Samples prepared with various additives (percentages by weight)
EcoflexT no additives
5% polyethylene
10% poytyen
15% polyethylene
20% polyethylene
10% CaCO3
20% CaCO3
5% poly 5% CaCO3
10% poly/10% CaCO3
10% microspheres


Figure 2-1: Three tissue equivalent slabs curing in epoxy molds


Figure 2-2: Relative attenuation of samples: Two Ecoflex'"' based samples on top below a breast
equivalent (lower left) and acrylic (lower right) slab.




















110000

905 00




90 00

8500

80 00

7500


Table 2-2: Material properties of various Smooth-On rubbers
A:B Mix
Rubber Compround Ratio Hardness Elongation at Break
PMC 780 dry 2:1 by weight 80 700%
PMC 744 2:1 by weight 45 400%
PMC 121/30 dry 1:1 by weight 30 1000%
PMC 121/50 wet 1:1by volume 50 500%


Attenuation of Ecoflex Based Materials


Acryllic ecodex


10%b 10% CaCO3 20% CaCO3 5%b 10%b 15%b 20%b 25%b 5% poly 5% 10% poly -
microspheres polyethelene polyethelene polyethelene polyethelene polyethelene CaCO3 10%b CaCO3


Figure 2-3: Relative attenuation of EcoffexTM based tissue equivalent materials



























Figure 2-4: Slabs of PMC 780, 744, 121/30 and 121/50 urethane-based rubbers offered by
Smooth-On.


Figure 2-5: Experimental setup for attenuation measurement. Slabs of material were placed 18"
above an ion chamber.







































Figure 2-6: Full experimental setup with portable x-ray generator 38"above the ion chamber.





Relative Attenuation of Urethane Based

Compounds


0.3

0.29

0.28

0.27

0.26

0.25

0.24

0.23

0.22


n ~ne


0.272


0.G


0.251


0.248


121/50


744

Mate rialI


121/30


acryllic


Figure 2-7: Comparison of attenuation coefficients of urethane based rubbers.















1.06


1.36


1.4
1.3
<1.2



E 0.9
e8 0.8
0.7
0.6


117


~MM





I


121/50


121/30


acryllic


Figure 2-8: Comparison of densities of urethane based rubbers.


0.326


0.35
0.3
0.25
0.2
0.15
0.1
0.05
0


0.295


0.251


U.25b


n 31F


c~


~c~~'
3s~;;

~\Q


Densitites of Urethane Based Compounds


744
Mate rialI


Relative Attenuation Values for PMC 121/30 Based
Compounds


Mate rialI

Figure 2-9: Attenuation coefficients of PMC 121/30 based materials.














IV+


0.6 L
121/30 5% polyethylene 5% CaCO3 5%
microspheres
Material


Figure 2-10: Densities of PMC 121/30 based materials


Table 2-3: List of slabs poured with 121/30 as a base (Ca= CaCO3 & miCO =microspheres, percent
are by weight)
2.5% CaCO3
3.2% CaCO3
5% CaCO3
2.25% Call% micro
2.25% Ca/2.25% micro
4.5% Cal 4.5%micro


Densitites of PMC 121/30 Based Compounds





















-C


0.400
0.350
0.300
0.250
0.200
0.150
0.100
0.050
0.000


0.334


00 .;


U"Of 0.276 0.272


0.231


B
o
C5'


~c""
G~b


Figure 2-11: Attenuation values of 121/30 with various additives


Attenuation of 121/30 Based Materials


Mate rial











CHAPTER 3
ELLIPTICAL PHANTOM CONSTRUCTION

Once a tissue equivalent urethane based material was developed, it was used in the

construction of several elliptical phantoms. These phantoms were to be used in testing proper

functionality of tube current modulation systems in clinical CT scanners. A total of five such

phantoms were created, all with a height of 15cm, a minor axis of 16cm, and maj or axes varying

between 26 and 37.25cm. A basic diagram of the proposed phantoms is shown in Figure 3-1. The

phantoms were designed to fit an existing CTDI head phantom with a height of 15cm and a

diameter of 16cm.

Materials and Methods

The original idea for building the phantoms was to utilize several large pieces of four

inch thick packing foam already in the lab as a frame for pouring the phantoms. Elliptical cutouts

would be cut into each of three slabs, which would be stacked and then filled with the tissue

equivalent rubber. Before constructing the phantoms, small scale testing was done in order to

verify the design concept. Two small rectangular cutouts were made in a 4" thick piece of

packing foam, and a piece of wax paper was epoxied to the bottom to act as a base. The walls of

one cutout were lined with wax paper, while the other was left as bare foam. Both cutouts were

filled with the tissue equivalent PMC 121/30 material and left to cure overnight. It was found

that the cured rubber easily pulled out of both cutouts. The cutout lined with wax paper produced

a sample with cleaner edges, while the sample from the foam cutout was rough and uneven as the

liquid rubber material filled in the small voids in the foam.

Based on the small scale test, the first elliptical phantom was constructed. An ellipse with

a 26cm maj or axis and 16cm minor axis was traced onto and cut out of three large slabs of the 4"

thick foam. Wax paper was epoxied to the bottom of one of the pieces of foam to act as the









bottom. The three slabs were stacked on top of each other with small blocks acting as spacers in

between levels in order to get the mold to the required height of 15 cm. The inside edge of the

ellipse was lined with wax paper and a CTDI head phantom was placed in the center of the

elliptical cutout.

Immediately upon pouring the urethane mixture into the phantom mold it became

apparent that the wax paper lining the inside was not strong enough to hold in the large volume

of material. The wax paper bulged out at all gaps in the foam and much of the liquid rubber

material spilled out. After the rubber had cured, the resulting elliptical phantom was not

acceptable and the entire process was dubbed a learning experience. It was obvious that the foam

and wax paper combination was not sufficient to contain the fairly viscous tissue equivalent

material. It was also difficult to cut perfectly geometrical ellipses out of the thick foam by hand,

and as such alternative methods were sought.

Improved Construction Methods

After the failure of the first phantom, a new method of construction was devised. Instead

of the foam used previously, %/" plywood was purchased for use in the mold. An ellipse was cut

out of three pieces of wood with the Vision Pro engraving system in the lab. The outline of the

same sized ellipse was then engraved halfway through a fourth sheet of plywood that would act

as the base. The Vision Pro engraving system ensured that all elliptical cutouts were uniform and

of the same size. The three pieces were then stacked to a height of 15" on top of the base using

wooden blocks as spacers. The interior was lined with a thick rubber sheet originally designed

for lining garden ponds. Plastic molding was placed along the groove machined into the base to

provide more support for the walls of the phantom mold. The CTDI head phantom was placed in

the center of the mold, which was then filled with the liquid 121/30O mixture. The new setup did a









much better j ob of containing the liquid urethane and the phantom was left to cure overnight.

Pictures of the mold and phantom building process can be found in Figures 3-2 through 3-4.

After the urethane rubber had cured, the phantom was removed from the mold, although

with some difficulty as the urethane tissue-equivalent material stuck to the wooden base layer of

the mold. Other than a hard time removing the phantom, the first effort was a success and four

more phantoms were constructed in a similar fashion, but with increasing maj or axis size while

maintaining the same minor axis. The maj or axis lengths used in each of the five phantoms were

26cm, 28.5cm, 31.25cm, 32.6cm, and 37.25cm (+/- .25cm variation from top to bottom). The

five completed phantoms are shown in Figure 3-5.

Each of the five phantoms fit around a 16cm CTDI head phantom for use in CT imaging

and domes measurement. The tissue equivalent material adheres to the acrylic CTDI phantom

fairly well on its own, but medical tape was utilized to ensure a tight consistent fit for testing.

After construction was complete, the phantoms were used to test the functionality of the

angular component of the CareDose4D" tube current modulation system on a Siemens Sensation

16 scanner in the Shands Orthopedic and Sports Medicine Institute at the University of Florida.



























I |
Variable Major Axis 28 cm-37,25 em

Figure 3-1: Diagram of proposed phantom design. Phantoms were designed to fit an existing
CTDI phantom shown in light gray.


Figure 3-2: Elliptical phantom mold. Cutouts from other sized phantoms are visible in the
plywood.






























Figure 3-3: CTDI head phantom centered in phantom mold. Rubber sheeting is taped in place to
contain the urethane rubber.


Figure 3-4: Phantom mold filled with urethane liquid rubber.





















.~n!


;;


J


Figure 3-5: Five elliptical tissue equivalent phantoms of increasing major axis.









CHAPTER 4
PHANTOM TESTING

The Hyve created elliptical phantoms were used to test the clinical functionality of the

angular portion of the CareDose4D" tube current modulation system on a Siemens Somatom

Sensation 16 CT scanner, as well as to compare doses in CT scans with Eixed versus modulated

tube current techniques. As previously discussed, the CareDose4D" system acts to modulate

current along the Z-axis according to a scout scan, as well as within each tube rotation based on

attenuation information from the previous 180 degrees of tube rotation. The purpose of the

current modulation is to maintain a constant photon flux at the detector elements and as a result,

to possibly reduce patient dose as compared to Eixed current techniques. If the in-plane

modulation system is working properly, image noise should remain constant for all scans at a

given reference mAs setting regardless of phantom size due to the fact that image noise is related

to the number of detected photons. If a larger sized phantom is being scanned, the system will

compensate for the higher attenuation by increasing the tube current in order to maintain a

constant number of photons at the detector, thus keeping image noise relatively constant. It

should be noted that the elliptical phantoms do not test the functionality of the Z-axis modulation

of the system since they are of uniform thickness along their length.

Materials and Methods

The general experimental method for the study was to measure both dose and image

quality for each of the Hyve phantoms at several reference mAs settings, and to observe trends in

dose as a function of CareDose4D" setting and phantom size.

Phantom Dose Measurement

For all phantom measurements a standard adult abdominal routine with the same

reconstruction kernel was used. The entire length of the phantom was scanned with the tube









voltage set to 120 kVp, a 5.6 second scan time, 0.5 second rotation time, 16 x 1.5mm

collimation, and 18mm table feed per rotation. These parameters were held constant for all scans.

The entire length of the phantom was scanned in order to allow the system to modulate the tube

current. Since the angular modulation portion of the CareDose4D" system modulates current on

the fly based on the previous 180 degrees of the scan, a single axial-slice scan, as is done for

CTDI measurements, would not allow the system to properly adjust to the changes in phantom

attenuation. By scanning the entire length of the phantom, which takes multiple rotations of the

x-ray tube, the in-plane tube current modulation system is allowed to operate.

In order to collect dose measurements, each elliptical phantom was attached to a

standard CTDI head phantom (15cm tall, 16cm diameter). The phantom was placed on the CT

table and a scout image (topogram) was performed. The entirety of the phantom was selected to

be scanned using six different reference mAs values (115, 130, 145, 160, 175, and 190 mAs).

Each of the five elliptical phantoms was scanned at each of the previously mentioned reference

mAs settings. The default setting for the adult abdominal routine is 160 mAs, and the other

settings were chosen to provide a range of data points both above and below the default setting in

order to check the functionality of the CareDose4D" system as well as to ascertain the effects of

the setting on both dose and image quality in the various sized phantoms.

A Capintec (Ramsey, NJ) PC-4P pencil ion chamber was used in the center hole of the

CTDI phantom to measure exposure during the scans. Exposure measurements were recorded for

each scan over the entire 15cm length of the phantom. These exposure measurements were

converted into integral dose measurements by multiplying each by the F-factor for soft tissue

(0.94 Rad/R). This calculation is possible since the total scan length (15 cm) and table feed per

rotation (1.8cm) were held constant for each phantom measurement. The integral dose, as










described by Dixonl2, iS the line integral of dose measured along the Z-axis of a phantom. In the

case of these measurements, the length of the scan (15 cm) was longer than the active region of

the pencil ion chamber (10 cm), but this discrepancy in length was not an issue due to the width

of the x-ray cone beam (approx 2.5 cm). The experimental setup for dose measurements can be

seen in Figure 4-1.

Phantom Image Quality Measurement

In order to measure image quality as a function of reference mAs setting and phantom

size, the uniformity portion of a Catphan 440 (The Phantom Laboratory, Salem NY) image

quality phantom was used. Scans were performed at each of the previously mentioned reference

mAs settings with each of the five elliptical phantoms. Minimum and maximum CT number

values were measured in five regions of interest- each 4 cm in diameter- in the reconstructed

image, as shown in Figure 4-2. The values from each of these five regions were averaged

together to provide the average minimum and maximum CT numbers for the uniformity region

of the Catphan. Based on these average maximum and minimum CT number values, the image

uniformity for each h scan was calculated using Equation 4-1.



Image thrifornzity = [1-((nzax-nzin) (nzax nain))] (Eq. 4-1)



A uniformity value of one indicates that the region is completely uniform, while lower

uniformity values indicate a greater degree of variation in CT number values as a result of image

noise.

The uniformity portion of the Catphan 440 was used for image quality measurements

instead of the CTDI head phantom used for dose measurements because it provided an entirely

uniform volume for measurement. The CTDI phantom has five drilled out segments (one in the










center, and four around the periphery) for pencil ion chamber placement. While these drilled out

portions are filled with acrylic rods when not in use for dose measurements, there was concern

that the small air gaps around each rod could alter the minimum and maximum CT number

values in the reconstructed image and provide a source for variation and error in the results.

Comparison of Modulated and Fixed Tube Current Techniques

Tests were also performed in order to determine the differences in dose and image quality

between tube current modulated scans and those performed with fixed-current techniques. For

purposed of clarity and simplicity, only three elliptical phantoms (with maj or axes of 26, 3 1.25,

and 37.25 cm) were used in these experiments. Each phantom was scanned with the CareDose

system at reference mAs settings of 100, 150, and 200 mAs. The phantoms were then scanned

with a fixed tube current technique with effective mAs settings of 100, 150, and 200 mAs. Dose

for each scan was measured in the same manner previously described. In the interest of time, CT

number standard deviation was used in place of uniformity as a measure of image quality. A

lower CT number deviation indicates a more uniform reconstructed image. All other scan

parameters were the same as previously described under the dose and image quality sections.

Results and Discussion

The image uniformity and dose measurements for each phantom at each of the six

reference mAs values were plotted in order to verify proper in-plane CareDose4D" function, as

well as to observe trends in dose and image quality as a function of reference mAs setting and

phantom size.

Image Uniformity Measurements

The effects of phantom maj or axis length and reference mAs setting on image uniformity

are illustrated in Figure 4-3. As expected, image uniformity remains constant (within bounds of

experimental error) for each reference mAs setting, regardless of phantom axis length. Error bars









were not included in Figure 4-3 for purposes of clarity, but image uniformity for each reference

mAs setting with associated error bars can be found in Appendix A. Image uniformity is also

seen to increase as the reference mAs setting was increased. As expected, each increase in the

reference mAs setting yields an increase in the image uniformity, indicating a decrease in image

noise. This is a result of the tube current modulation system increasing the photon output of the

x-ray tube in order to attempt to match the total effective mAs of the scan to that of the reference

setting.

An interesting trend is observable in Figure 4-3 in which image uniformity for phantoms

2 through 4 is slightly lower than for phantoms 1 and 5. This trend is seen across all reference

mAs settings, although the uniformity values for phantom 4 migrate towards that of phantom 5

starting at a reference mAs setting of 160. While all uniformity values were found to be within

bounds of experimental error, the repeating trend for all measurements is worth notice, and can

be attributed to one of two sources. The first and most probable source for this trend is the

reconstruction algorithm itself. While no changes were made to the reconstruction kernel used

for each scan, the system itself makes adjustments in its reconstruction method in order to

produce the reconstructed image. It is possible that such changes took place for the three

midsized phantoms in this testing. A second possible source of the trend could be attributable to

systematic human error during the testing. Although every step was made to use the same

procedure and techniques when performing each scan, it is not outside the realm of possibility

that some such changes, such as exact phantom placement on the CT table, were made. A repeat

of all tests would be required to determine if the trend is reproducible, which would suggest its

roots were in the reconstruction algorithm.










The percentage increase in uniformity for each increase in reference mAs (referenced to

the 115 reference mAs setting) can be seen in Figure 4-4. Again, error bars are not included for

purposes of clarity. Although it appears that a trend towards larger increases in uniformity for

larger phantom sizes exists, when the data is plotted with error bars (shown in Figure 4-5 for

only the 190 reference mAs setting) it is apparent that all points are well within bounds of

experimental error and no such trend can be inferred.

Dose Measurements

The effects of phantom maj or axis and reference mAs setting on integral dose are shown

in Figure 4-6. As expected, dose increased as the phantom maj or axis increased for each given

reference mAs setting. This is attributable to the CareDose4D" system increasing the tube

current in response to the added attenuating material in order to maintain a constant photon flux

at the detector elements. The larger phantoms attenuate more of the incoming beam and thus

require larger tube currents in order to maintain the same image quality. These larger tube

currents in turn cause a higher dose in the phantom.

Integral dose measurements were also found to increase as the reference mAs setting was

increased. The increase as a result of changing reference mAs setting was fairly uniform for each

phantom size as is seen by the even spacing between each reference mAs setting line in Figure 4-

6 across all phantom sizes. This increase in integral dose averaged an increase of 13% (+/- 0.75)

per increase of 15 in the reference mAs setting for all sized phantoms.

A noticeable trend found in Figure 4-6 is the larger integral dose values for the largest

phantom (phantom 5) as compared to those of the other phantoms within each reference mAs

grouping. This is most likely attributable to non uniform spacing of the phantom's major axes.

The phantoms major axes measure 26 cm, 28.5 cm, 31.25cm, 31.6 cm and 37 cm respectively,

with approximately 2 cm gaps between each of the first four phantoms, and a relatively larger










(~5 cm) gap between phantoms 4 and 5. This larger gap equates to a phantom with a larger

amount of attenuating material which could explain the jump in dose to phantom 5 as the

modulation system attempts to maintain a constant photon flux at the detector.

Overall Trends

The average increase in integral dose from one reference mAs setting to the next was

found to be fairly constant at 13% (+/- 0.75%). The average increase in image uniformity for

each increase of 15 in the reference mAs setting was found to be 2.5% (+/- 0.79%). Both of

these measurements were found to increase linearly across the range of reference mAs values

utilized. It is hypothesized that if the reference mAs setting were increased further, dose

measurements would continue to rise, but gains in image uniformity would fall off as the limits

of the scanner' s detectors were reached.

Comparison of Modulated and Fixed Tube Current Techniques

The results of the comparison study between modulated and fixed tube current techniques

are shown in Figures 4-7 and 4-8. In comparing the image quality between the two modes of

acquisition, several trends are visible. As expected, pixel deviation in the reconstructed images is

seen to decrease as the tube current is increased for both fixed and modulated techniques. This is

expected as the higher number of photons reaching the detector at higher tube currents reduces

the effects of random image noise. The pixel deviation for the tube current modulated techniques

is seen to remain fairly constant regardless of phantom size. As previously mentioned, this

indicates that the in-plane tube current modulation system was properly working to maintain a

constant photon flux at the detector. In the fixed tube current scans, pixel deviation is seen to

increase with phantom size. This is also as expected since the larger phantoms attenuated more

of the fixed photon output, a situation that results in fewer photons reaching the detector for

larger phantom sizes. Fewer photons reaching the detector cause an increase in image noise, and










as such, an increase in the standard deviation of pixel values in a uniform area. These results

demonstrate the need for features such as tube current modulation, since image noise in a patient

scan can vary from slice to slice depending on anatomy with Eixed current techniques. Systems

such as CareDose act to maintain a constant image noise throughout an entire scan.

The effects of dose in the comparison study at seen in Figure 4-8. As a general trend,

doses were seen to increase as tube current setting was increased for both modulated and fixed

technique scans. As seen previously, the dose from the tube current modulated scans was seen to

increase slightly as phantom size increased. This was a result of a larger x-ray tube output in

order to compensate for more attenuation in the phantom in order to maintain a constant flux at

the detector. Conversely, doses were seen to decrease as phantom size increased in the Eixed

current scans. This again is explained by the higher photon attenuation in the larger phantoms.

Unlike in the modulated technique scans, the Eixed current technique does not adjust for the

increase in attenuation in larger phantoms and as such, the dose in the center of the phantom is

lower, but at the expense of noisier images.

In comparing the doses between the two techniques, the tube current modulated

techniques all produced lower doses than Eixed tube techniques. The magnitude of the dose

savings increased with phantom size, and ranged from 49% for the smallest phantom to 62% for

the largest phantom size. These percentage dose savings were constant for each mAs setting.

That is, regardless of mAs setting, the dose reduction by switching to the tube current modulated

scan versus the Eixed current technique was the same for each phantom size.

These decreases in dose came at the expense of slightly noisier images in the tube current

modulated scans. As seen in Figure 4-7 the pixel deviation was found to be higher in the tube

current modulated scans as compared to the Eixed technique scans at similar settings. These









differences again were found to be based on phantom size, and ranged from 64% to 90% lower

pixel deviations in the fixed tube current scans for the small to large phantoms respectively.

Conclusions

A compressible tissue equivalent material was developed and successfully used to build

five elliptical phantoms of varying maj or axis length. These phantoms were then used to show

the proper functionality of the angular portion of the Siemens CareDose4D~ tube current

modulation system in a Somatom Sensation 16 CT scanner. The image uniformity values in

reconstructed images from each phantom were found to be the same, within bounds of

experimental error, at each of six tested reference mAs values for an adult abdominal routine,

indicating that the modulation system was properly adjusting the tube current in response to the

amount of attenuating material in the beam. This method could easily be worked into a regular

monthly or quarterly quality assurance plan for the CT scanner in order to ensure proper

operation of the tube current modulation system. Individual scanner performance could be

tracked over its useful life and action limits could be set if uniformity values begin to drift.

In addition to quality assurance, general trends in image quality and dose were observed

as a function of variation in reference mAs setting and phantom maj or axis. An increase in

integral dose of 13% was found for each increase of 15 in the reference mAs setting, while image

uniformity increased by approximately 2.5% for each such increase. Knowledge of such trends

could help aid radiologists and technologists in selected a proper CareDose4D" reference mAs

setting for a given exam in order to minimize patient doses while maintaining clinically

acceptable image quality.

In comparing the modulated to fixed tube current techniques, it was shown that dose

reductions of over 50% were possible by using an in-plane modulation system. These reductions

did come at the cost of increased image noise, and further tests would be required in order to










determine an acceptable level of image noise in diagnostic images. It is possible that the increase

in image noise as a result of these dose reductions would not affect the diagnostic acceptability

of the images.


Figure 4-1: Experimental setup for dose measurements. A pencil ion chamber is placed in the
center of a CTDI head phantom with a tissue equivalent elliptical phantom during a
CT scan.




































Figure 4-2: Screen capture of a CT scan of an elliptical phantom surrounding a uniform region of
an image quality phantom. The five regions of interest used for measuring CT #
uniformity values are shown as black circles in the center phantom.





35 37


e *


Efect of Phatom Axis on CT # Ulkiformity


~115
-c 130
145
160
~t 175
~ 190


0.6506


25 27 29 31 33

Phantom Mlajor Axis Length (cm)


Figure 4-3: CT number uniformity as a function of phantom maj or axis and reference mAs
setting


Percent Increase in Image Uniformity for Various reference mAs
Settings


20.00

18.00
16.00

14.00
12.00



800

6.00

2.00
0.00


-*-115
-m- 130




-a- 175
-*-190


25 27 29 31 33

Phaontom Mlajor Axis Length (cm)


35 37


Figure 4-4: Percent increase in CT number uniformity for increasing reference mAs settings





27 29 31 33 35 37

Phantom Major Axis Length (cm)


0.250


0.200


0.150


0.100


0.050


0.000


-* 190|


25 27 29 31 33 35

Phaontom Mlajor Axis Length (cm)


Figure 4-5: Percent increase in CT uniformity with error bars for 190 reference mAs setting





Effect of Phatom Axis on Dose


11.000

10.000

9.000

8.000

7.000




4.000


-o-115


15
160
~175
-* 190


Figure 4-6: Increases in dose as a result of changes in phantom maj or axis length and reference
mAs setting.


Percent Increase in Image Uniformity for Various Reference
mAs Setti ngs










Comparison of Pixel Deviation for Modulated vs Fixed
Techniques





S10.00 -*- 100 M

'59.00--15M
200 M

.00 1 00 F




4.00
25 27 29 31 33 35 37 39
Phantom Major Axis (om)

Figure 4-7: Comparison of image quality between modulated and fixed tube current techniques.
In the legend, M indicates tube current modulation, and F indicates fixed tube current
techniques.











Comparison of Dose for Modulated vs Fixed Settings

2500.0


2000.0
-* 100 M

B 1500.0 -=- 150 M
E 200 M
100 F
1000.0
-t 150 F
m -*- 200 F
500.0*


0.0
25 27 29 31 33 35 37 39
Phantom major axis (cm)

Figure 4-8: Comparison of dose between modulated and fixed tube current techniques. In the
legend, M indicates tube current modulation, and F indicates fixed tube current
techniques.









CHAPTER 5
FUTURE WORK

Future work is planned in further characterizing the tissue equivalent material developed.

The characterization of attenuation properties will be expanded to ensure that the material is

equivalent to human soft tissues at a wider range of beam energies. The radiological properties

of the tissue equivalent material will also be documented in more detail, paralleling the work of

Kyle Jones, a recent doctoral graduate of the program who developed the epoxy based tissue

equivalent materials previously in use in the lab.

There are also several outlets for future work utilizing the phantoms and methods

described in this thesis. The tissue equivalent material is already being utilized in another proj ect

to create a tomographic adult CT phantom. Such a phantom would serve to provide specific

organ dose measurements from CT scans which could be correlated to Monte Carlo simulations

using the data the phantom was created from.

The Smooth-On PMC 121/30 urethane rubber utilized to make a soft tissue equivalent

material is also being used as a base for the development of a breast tissue equivalent material.

Recipes for materials equivalent to breast tissues of varying composition and ratios of glandular

to fatty tissue are in development. It is hoped that such materials can be used in the construction

of more realistic mammographic phantoms than those currently in widespread use. Such

anatomical phantoms could be used to better characterize equipment and patient doses associated

with mammography.

The five created phantoms could also be used to further characterize dose and image

quality in next generation 32 and 64 slice CT scanners. Characterization of dose and image

quality as a function of tube current modulation setting and patient size is of paramount

importance as wider beams and more slices can drastically increase patient doses. The same









benefits of this current work would thus be extended to these newer scanners. Plans are also in

place to develop phantoms to test Z-axis tube current modulation specifically. Such phantoms

would need to change dimensions along the Z-axis of a CT scan, something the current phantoms

do not accomplish.

The phantoms would also be extremely useful in comparing the dose savings and gains in

image quality across CT scanners from different manufacturers. Currently some form of tube

current modulation is in place in most commercially available CT scanners, but the methodology

utilized by companies can vary greatly. Often the specifies of the algorithms used by the

companies are not fully disclosed due to their proprietary nature. The created phantoms could be

used to qualitatively compare the functionality of these varied.

The created phantoms could also be used to characterize location dependant image

quality in phantoms. It was qualitatively observed during the course of this research that image

noise was higher towards the center of the phantoms as compared to around the periphery.

Further studies could clarify and quantify these differences in image quality, and their

dependence on tube current modulation setting and phantom size. This information would

obviously be helpful to radiologists in knowing the degree to which image noise changes

throughout an image.

Lastly, plans for position dependant dose measurements within the phantoms are being

developed along with a fiber optic dosimetry system.




















































t f'


APPENDIX
IMAGE UNIFORMITY GRAPHS

The following are the graphs of image uniformity as a function of phantom maj or axis for

each of the individual reference mAs settings. Error bars are included to show that uniformity

remains constant within bounds of experimental error regardless of phantom size.


Effect of Phantom Major Axis on Uniformity at 115 ref mAs


0.80

0.75

0.70-

0.65

0.60
25 27 29 31 33
Phantom Mlajor Axis (cm)


35 37 39


Effect of Phantom Major Axis on Uniformity at 130 ref mAs


0.80

0.75

S0.70

0.65

0.60


25 27 29 31 33
Phantom Mlajor Axis (cm)


35 37 39


B
Figure A-1: CT number uniformity as a function of phantom maj or axis for a reference mAs
setting of A) 115, B) 130, C)145, D) 160, E) 175, F) 190.















Efect of Phantom Major Axis on Uniformity at 145 ref mAs


0.80






0.65




0.60


25 27 29 31 33

Phantom Mlajor Axis (cm)


0.80


0.75


0.70


0.65


0.60


35 37 39


25 27 29 31 33

Phantom Mlajor Axis (cm)


35 37 39


Efect of Phantom Major Axis on Uniformity at 160 ref mAs













Effet of Phantom Major Axis on Uniformity at 175 ref mAs


0.80




0.75

a,0.70


0.65


0.60
25 27 29 31 33 35 37 39

Phantom Mlajor Axis (cm)


Efect of Phantom Major Axis on Uniformity at 190 ref mAs


0.80


0.75


S0.70


0.65


0.60
25 27 29 31 33

Phantom Mlajor Axis (cm)


35 37 39









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BIOGRAPHICAL SKETCH

Ryan Fisher graduated from Brookwood High School in 2000. Brookwood is located in

Snellville Georgia, a suburban town 25 miles northeast of Atlanta. Ryan then attended The

Georgia Institute of Technology (Georgia Tech) and graduated with a Bachelor of Science

degree in biomedical engineering in 2004. He then enrolled at the University of Florida to pursue

master' s and doctorate degrees in medical physics.