Patient doses and image quality in interventional neuroradiology

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Patient doses and image quality in interventional neuroradiology
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
        Page v
    List of Tables
        Page vi
        Page vii
    List of Figures
        Page viii
        Page ix
        Page x
    Abstract
        Page xi
        Page xii
    Chapter 1. Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
    Chapter 2. Literature review
        Page 8
        Page 9
        Page 10
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    Chapter 3. Surface doses
        Page 32
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    Chapter 4. Energy imparted and effective dose in neuroradiology
        Page 60
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    Chapter 5. Image quality
        Page 86
        Page 87
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        Page 90
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    Chapter 6. Conclusions
        Page 116
        Page 117
        Page 118
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        Page 120
        Page 121
    Bibliography
        Page 122
        Page 123
        Page 124
        Page 125
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    Biographical sketch
        Page 131
        Page 132
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        Page 134
Full Text











PATIENT DOSES AND IMAGE QUALITY
IN INTERVENTIONAL NEURORADIOLOGY













By


NIKOLAOS A. GKANATSIOS


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1998















ACKNOWLEGEMENTS


I would like to gratefully acknowledge the following who have helped me

throughout my graduate work:


Dr. Walter Huda, my advisor, for his invaluable guidance, time and patience
throughout the course of this research and the preparation of this dissertation.
I am very grateful for his continuous advice and suggestions throughout my
graduate work.

My Ph.D. committee members, Prof. James S. Tulenko, Dr. Wesley E. Bolch,
Dr. Janice C. Honeyman, Dr Keith R. Peters and Dr. Irvine F. Hawkins, for
reviewing my progress and guiding me through my Ph.D. research.

Ms. Lynn Rill, for her valuable time evaluating all the radiographic images
and for her review and comments on the manuscript.

Mr. Dennis Pinner from Toshiba America Medical Systems, for his valuable
insights into understanding the imaging equipment and for providing me with
the requested information and documentation on the imaging system.

The Department of Radiology, for giving me the graduate assistantship to
pursue my graduate studies, and for all the resources they made available for
me throughout my graduate research.

My beloved parents, Anastasia and Argyrios Gkanatsios, for their love,
encouragement, and support throughout all my endeavors. They are the ones
who made this possible for me.















TABLE OF CONTENTS



ACKN OW LEGEM ENTS................................................................................................... ii

LIST OF TABLES............................................................................................................. vi

LIST OF FIGURES ......................................................................................................... viii

ABSTRA CT....................................................................................................................... xi

CHAPTERS

1 INTRODUCTION ....................................................................................................... 1

Interventional Neuroradiology..................................................................................... 1
Patient Dosim etry........................................................................................................ 2
Dose M monitoring System s............................................................................................ 4
Im age Quality .............................................................................................................. 5
Purpose of This W ork.................................................................................................. 6

2 LITERATURE REVIEW ............................................................................................ 8

Introduction.................................................................................................................. 8
Interventional Neuroradiologic Procedures.......................................................... 8
Determ inistic Radiation Effects............................................................................ 9
Stochastic Radiation Effects............................................................................... 12
Dosim etry .................................................................................................................. 13
Surface Dose....................................................................................................... 13
Energy Im parted ................................................................................................. 15
Effective Dose .................................................................................................... 18
Im age Quality ............................................................................................................ 21
Im age Contrast.................................................................................................... 21
Im age Noise........................................................................................................ 23
Spatial Resolution............................................................................................... 26
Im aging Technique Factors ....................................................................................... 27
Tube Potential..................................................................................................... 28
Input Exposure to the Im age Receptor............................................................... 29
M agnification ..................................................................................................... 30

3 SURFACE DOSES.................................................................................................... 32









N euroradiologic Im aging........................................................................................... 32
Clinical Practice.................................................................................................. 32
Im aging Equipm ent ............................................................................................ 33
Operation............................................................................................................ 35
Im aging Techniques ........................................................................................... 36
The Patient D osim etry System .................................................................................. 38
System Description............................................................................................. 38
Calibration.......................................................................................................... 40
Evaluation........................................................................................................... 42
Data Acquisition................................................................................................. 45
Fluoroscopy ............................................................................................................... 47
X-Ray Beam Localization.................................................................................. 47
Surface Doses ..................................................................................................... 48
Surface Dose Rates............................................................................................. 49
Fluoroscopic Tim es and Intervals....................................................................... 51
Radiography............................................................................................................... 53
X-Ray Beam Localization.................................................................................. 53
Surface Doses ..................................................................................................... 54
Surface Dose Rates............................................................................................. 54
Radiographic Fram es.......................................................................................... 56
Conclusions................................................................................................................ 57

4 ENERGY IMPARTED AND EFFECTIVE DOSE IN NEURORADIOLOGY....... 60

Introduction................................................................................................................ 60
M ethod....................................................................................................................... 62
Energy Im parted................................................................................................. 62
Adult Effective Doses......................................................................................... 66
Pediatric Effective Dose..................................................................................... 69
Adult Patient Doses ................................................................................................... 71
Energy Im parted................................................................................................. 71
Effective Doses................................................................................................... 74
Pediatric Patient Doses.............................................................................................. 75
Energy Im parted................................................................................................. 76
Effective Doses................................................................................................... 79
Discussion.................................................................................................................. 80
Conclusions................................................................................................................ 83

5 IM AGE QUALITY .................................................................................................... 86

Im age Acquisition...................................................................................................... 86
Phantom Description.......................................................................................... 86
Acquisition of Digitally Subtracted Im ages....................................................... 88
Dosim etry and Im age Quality.................................................................................... 94
Dosim etry ........................................................................................................... 94
Im age Quality Evaluation................................................................................... 95
Precision of M easurem ents................................................................................. 96









Results........................................................................................................................ 97
Tube Voltage ...................................................................................................... 97
Image Intensifier Input Exposure..................................................................... 100
Geometric Object M agnification...................................................................... 102
Discussion................................................................................................................ 107
Patient Surface Dose......................................................................................... 107
Energy Imparted............................................................................................... 110
Image Quality................................................................................................... 113
Conclusions.............................................................................................................. 114

6 CONCLUSIONS ..................................................................................................... 116

Patient Dosim etry .................................................................................................... 116
Surface Doses ................................................................................. ................. 116
Effective Doses................................................................................................. 117
Im age Quality .......................................................................................................... 119
Future W ork............................................................................................................. 120

BIBLIOGRAPHY ........................................................................................................... 122

BIOGRAPHICAL SCHETCH........................................................................................ 131





























V















LIST OF TABLES


Table age

2-1. Deterministic Effects of the Skin after Single-Fraction Irradiation....................... 11

3-1. List of the Input Signals Interfaced to the PEMNET Dosimetry System from
the Toshiba Neurobiplane Imaging Unit................................................................ 40

3-2. Experimental Arrangements for Evaluating the Patient Dosimetry System..........44

3-3. Summary of the Ratios of the Measured to Calculated Surface Doses, Xh/Xc,
Obtained During Testing of the Accuracy of the Patient Exposure System..........45

4-1. Computed a and fi Coefficients and Half-Value Layers for X-Ray Beams as
a Function of Tube Voltage ................................................................................... 64

4-2. Backscatter Fractions of Radiation Exposure at Different Tube Voltages............ 65

4-3. Patient Thickness and Area of Exposure Corresponding to the Head Region
of Different Age Groups........................................................................................ 66

4-4. Patient Thickness and Area of Exposure Corresponding to the Trunk Region
of Different Age Groups........................................................................................ 67
4-5. Values of Effective Dose per Unit Energy Imparted, E/l in mJ/Sv, for
Different Body Projections as a Function of Tube Voltage................................... 70

4-6. Standard Patient Mass for Different Age Groups .................................................. 70

5-1. Iodine Contrast Concentration in Each Vessel of the Vessel Insert ...................... 89

5-2. Imaging Techniques During Tube Voltage Experiments ...................................... 92

5-3. Imaging Techniques During Geometric Object Magnification Experiments ........93

5-4. Score Describing the Visibility of Each Iodine Contrast Concentration ...............95

5-5. Tube Voltage Dependency at 120 p.R/frame ......................................................... 98

5-6. Tube Voltage Dependency at 440 .R/frame ......................................................... 98

vi









5-7. Image Intensifier Input Exposure Dependency.................................................... 102

5-8. Geometric Object Magnification Dependency at 120 pR/frame ......................... 104

5-9. Geometric Object Magnification Dependency at 440 p.R/frame ......................... 104

5-10. Comparison of the Effects of Tube Voltage, Input Exposure and Geometric
Magnification on the Surface Dose for a Range of Changes in Threshold
Iodine Concentration at 120 ptR/frame ................................................................ 108

5-11. Comparison of the Effects of Tube Voltage, Input Exposure and Geometric
Magnification on the Surface Dose for a Range of Changes in Threshold
Iodine Concentration at 440 gR/frame ................................................................ 108

5-12. Comparison of the Effects of Tube Voltage, Input Exposure and Geometric
Magnification on the Energy Imparted for a Range of Changes in Threshold
Iodine Concentration at Low Input Exposures .................................................... 111

5-13. Comparison of the Effects of Tube Voltage, Input Exposure and Geometric
Magnification on the Energy Imparted for a Range of Changes in Threshold
Iodine Concentration at High Input Exposures.................................................... 111















LIST OF FIGURES


Figure page

1-1. Unsubtracted image (left) where anatomical details are mixed with diagnostic
information. Digitally subtracted angiogram (right) where anatomical
information has been subtracted to allow easier visualization ofvasculature.........2

2-1. Key parameters that affect patient dose and image quality in x-ray imaging........28

3-1. Histogram of surface dose contribution at different x-ray tube voltages from
frontal plane (black bars) and lateral plane (gray bars) fluoroscopy for an
average interventional neuroradiologic procedure................................................. 37

3-2. Histogram of surface dose contribution at different x-ray tube voltages from
frontal plane (black bars) and lateral plane (gray bars) radiography for an
average interventional neuroradiologic procedure................................................. 38

3-3. Calibration setup of the frontal plane (left) and lateral plane (right) using an
RSD RS-235 anthropomorphic head phantom ...................................................... 42

3-4. Sample page from the PEMNET database showing all recorded information
for the frontal im aging plane.................................................................................. 46

3-5. Histogram distribution of surface doses for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) fluoroscopy........................................... 49

3-6. Histogram distribution of surface dose rates for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) fluoroscopy........................................... 50

3-7. Histogram distribution of fluoroscopic times to 175 patients from frontal plane
(black bars) and later plane (gray bars) fluoroscopy.............................................. 52

3-8. Histogram distribution of fluoroscopic intervals for 175 patients from frontal
plane (black bars) and lateral plane (gray bars) fluoroscopy ................................. 53

3-9. Histogram distribution of surface doses for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) radiography........................................... 55
3-10. Histogram distribution of surface dose rates for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) radiography........................................... 56

viii









3-11. Histogram distribution of the number of radiographic frames for 175 patients
from frontal plane (black bars) and lateral plane (gray bars) radiography ............. 57

3-12. Histogram distribution of the total surface doses to 175 patients from frontal
plane (black bars) and later plane (gray bars) fluoroscopy and radiography
com bined................................................................................................................ 59

4-1. Values of wo as a function of water phantom thickness for tube voltages of 60
kVp, 80 kVp and 100 kVp ..................................................................................... 63

4-2. Effective dose as a function of patient mass for one joule of uniform whole
body irradiation...................................................................................................... 71

4-3. Histogram distribution of energy imparted to patients from use of fluoroscopy
during interventional neuroradiologic procedures ................................................. 72

4-4. Histogram distribution of energy imparted to patients from radiographic
acquisitions during interventional neuroradiologic procedures ............................. 73

4-5. Histogram distribution of the total energy imparted to patients undergoing
diagnostic angiographic and therapeutic embolization neuroradiologic
procedures .............................................................................................................. 75

4-6. Histogram distribution of the total effective dose to patients from biplane
neuroradiologic exam nations ................................................................................ 76

4-7. Energy imparted as a function of patient mass from fluoroscopy during
interventional neuroradiologic procedures on pediatric patients. Line shows
the linear fit between energy imparted and patient mass ....................................... 77

4-8. Energy imparted as a function of patient mass from radiographic acquisitions
during interventional neuroradiologic procedures on pediatric patients. Line
shows the linear fit between energy imparted and patient mass ............................ 78

4-9. Energy imparted as a function of patient mass from interventional
neuroradiologic procedures on pediatric patients. Line shows the linear fit
between energy imparted and patient mass............................................................ 79

4-10. Effective dose as a function of patient mass from interventional
neuroradiologic procedures on pediatric patients. Line shows the linear fit
between effective dose and patient mass ............................................................... 80

4-11. Comparison of E/ls values vs. patient age as determined by Equation (4.4) and
by using the dosimetry data from Hart et al. (1996a) ............................................ 84

5-1. Schematic diagram of the acrylic phantom with the vessel and blank inserts
used to simulate small vessels for the purpose of evaluating image quality in
neuroradiology ....................................................................................................... 87

ix









5-2. Experimental setup for DSA acquisitions.............................................................. 90

5-3. Position of the two ionization chambers relative to the vessel insert (left).
Subtracted im age (right) ........................................................................................ 90

5-4. Surface dose and energy imparted as a function of tube voltage........................... 99

5-5. Threshold iodine concentration as a function of tube voltage. The circles
correspond to the 120 .tR/frame and have been fitted to kVp207. The squares
correspond to the 440 tR/frame and have been fitted to kVp56.......................... 100

5-6. Surface dose and energy imparted as a function of image intensifier input
exposure at 70 kV p .............................................................................................. 101

5-7. Threshold iodine concentration as a function of image intensifier input
exposure for a constant video level at 70 kVp ..................................................... 103

5-8. Surface dose and energy imparted as a function of image intensifier input
exposure at 70 kV p .............................................................................................. 105

5-9. Threshold iodine concentration as a function of geometric object
m agnification at 70 kVp....................................................................................... 106

5-10. Change in surface dose versus change in threshold iodine concentration
with tube voltage input exposure and magnification ........................................... 109

5-11. Change in energy imparted versus change in threshold iodine concentration
with tube voltage input exposure and magnification ........................................... 112













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


PATIENT DOSES AND IMAGE QUALITY
IN INTERVENTIONAL NEURORADIOLOGY

By

Nikolaos A. Gkanatsios

December, 1998

Chairman: James S. Tulenko
Cochairman: Walter Huda
Major Department: Nuclear and Radiological Engineering

Diagnostic and therapeutic interventional neuroradiologic procedures involve

imaging of catheter manipulation and vascular anomalies of the brain and generally

require extensive use of x-ray radiation. Knowledge of the surface dose allows one to

estimate the probability of inducing deterministic effects, whereas the corresponding

value of effective dose is related to the patient stochastic risk. Modification of key

imaging parameters (i.e., tube voltage, input exposure to the image receptor and

geometric magnification) impact on patient doses and image quality, with the latter being

defined as the lowest concentration of iodine in a vessel that may be visually detected in

the radiographic image. A dosimetry system was installed on a biplane neuroradiologic

imaging system to determine the doses to patients undergoing interventional

neuroradiologic procedures. The dosimetry system computed surface doses on the basis

of selected technique factors and information about patient location relative to the x-ray









tube. The energy imparted to the patient, e, was determined using the surface dose, x-ray

beam quality (i.e., kVp and HVL), exposure area and thickness of the patient and was

converted into the corresponding value of effective dose, E. Values of surface dose and E

were obtained for 175 patients, consisting of 149 adults and 26 pediatrics. Median values

of surface doses to the head region were 1.2 Gy in the frontal plane and 0.62 Gy in the

lateral plane. Median values of the effective doses were 36 mSv for adult patients and 44

mSv for pediatric patients. An acrylic phantom with 1-mm diameter vessels filled with

iodine contrast was used to evaluate the effects of varying imaging parameters on signal

detection and patient doses during digital subtraction angiography. Reducing the x-ray

tube voltage offered the largest improvement in image quality for a given increase in

patient dose. Increasing the image intensifier input exposure beyond 250 pR/frame

provided very little improvement in image quality, and this II exposure level should not

be exceeded in interventional neuroradiologic imaging. A linear relationship was

observed between magnification and threshold concentration, which offers significant

patient benefits when surface doses are not expected to exceed the threshold doses for the

induction of deterministic effects.














CHAPTER 1
INTRODUCTION



Interventional Neuroradiology

Neuroradiology is a multi-imaging science, which utilizes all imaging modalities

(i.e., plain film, digital radiography, computed tomography, magnetic resonance imaging,

nuclear medicine, etc) to accomplish a complete diagnosis of human neurology.

Neuroradiology can be distinguished as conventional or interventional neuroradiology.

Conventional neuroradiology uses modalities such as plain film radiography, computed

tomography (CT), magnetic resonance imaging (MRI) and ultrasound (US) to diagnose

neurologic abnormalities. Interventional neuroradiology studies the vasculature and

blood kinetics of the brain by means of catheterization performed with the transfemoral

artery technique. Interventional neuroradiologic procedures can be further distinguished

as diagnostic angiographic or therapeutic embolization procedures. The imaging portion

of any interventional neuroradiologic procedure is accomplished by use of digital

subtraction angiography (DSA). In digital subtraction angiography, a mask image is

being subtracted from an image enhanced with injected iodinated contrast to isolate

vasculature structures from the rest of the anatomy as shown in Figure 1-1.

Interventional neuroradiologic procedures often involve long fluoroscopic

exposure times and the acquisition of a large number of radiographic images. As a result,

there is a possibility of induction of deterministic radiation effects such as skin erythema

and epilation. It is also important to determine the stochastic risks involved in such
1









procedures in both adult and pediatric patients. Modification of key imaging parameters

(i.e., tube voltage, input exposure to the image receptor and geometric magnification)

impact on image quality and patient doses from interventional neuroradiologic

procedures. The effects of these parameters on image quality and patient doses should be

quantified and optimized in order to ensure adequate diagnostic image quality and

reduced patient doses.




















FIGURE 1-1: Unsubtracted image (left) where anatomical details are mixed with
diagnostic information. Digitally subtracted angiogram (right) where
anatomical information has been subtracted to allow easier visualization of
vasculature.



Patient Dosimetry

The surface dose is the dosimetric quantity that measures the dose absorbed in the

surface of an irradiated region from radiation exposures. The surface dose accounts for

the energy absorbed in the skin and can predict the possibility of inducing deterministic

injuries from high dose interventional radiologic procedures (i.e., cardiac catheterization,

abdominal interventional or neurointerventional procedures). Deterministic injuries









associated with interventional neuroradiologic procedures primarily consist of injuries

induced to the skin of the patient such as skin erythemas and epilations. Knowledge of

surface doses may also provide information on the probability of deterministic injuries to

the lens of the eye from interventional neuroradiologic procedures.

The effective dose, E, is a dosimetric parameter, which takes into account the

doses received by all irradiated radiosensitive organs. The effective dose is able to

account for nonuniform irradiation of different organs and tissues in the body and can be

used as an indicator of the stochastic radiation risk associated with radiologic x-ray

examinations. Determining effective doses for radiologic examinations by measurement

or calculation is generally very difficult. By contrast, the energy imparted, e, to the

patient may be obtained from the x-ray exposure-area product incident on the patient. As

energy imparted is approximately proportional to the effective dose for any given x-ray

radiographic view, the availability of El/c ratios (Huda and Gkanatsios, 1997) for common

radiographic projections provides a convenient way for estimating effective doses. Such

ratios primarily depend on the projection employed (body region irradiated and x-ray

beam orientation) and secondarily on the tube potential and beam filtration.

The effective dose as a dose descriptor in diagnostic radiology enables a direct

comparison of the detriment associated with different radiologic procedures. Expressing

patient doses in terms of the effective dose provides a consistent method of reporting

doses from diagnostic radiologic examinations. Effective doses in interventional

neuroradiology can simply be compared to other radiologic doses (i.e., computed

tomography, nuclear medicine, cardiac catheterization procedures, etc.). The use of the

effective dose also permits an estimate of patient risk to be obtained by using current









stochastic risk factors (ICRP, 1991; UNSCEAR, 1993; NAS, 1990). Use of such

stochastic risk factors with the effective doses computed for interventional

neuroradiologic procedures will provide useful information on the stochastic risks to

patients undergoing such high dose procedures.




Dose Monitoring Systems

A radiation monitoring system which provides feedback of dosimetric information

could play a role in ensuring that patient exposures are as low as reasonably achievable

(ALARA, ICRP, 1982). The benefits of a radiation monitoring system include

identification of individual patients who may be at risk for the induction of deterministic

radiation effects (Wagner et al., 1994), provision of a formal record of the patient

exposure as well as an increase in the radiologist's awareness of potential high patient

doses. In addition, the radiation monitoring system can serve as a powerful tool to

empirically investigate the tradeoffs between patient dose and corresponding image

quality when radiographing appropriate phantoms.

Use of modem on-line dosimetry systems on today's advanced x-ray imaging

equipment provides the necessary tools for fast and accurate acquisition of dosimetry data

on patients undergoing complex radiologic procedures. A patient dosimetry system

(PEMNET*) was installed in 1995 in the neuroradiology suite at the Department of

Radiology at Shands Hospital of the University of Florida. The patient dosimetry system

monitored both frontal and lateral imaging planes and recorded the amount of radiation


* Clinical Microsystems, Arlington, VA









received by patients undergoing interventional neuroradiologic procedures along with

additional dosimetric information to help to compute effective doses.




Image Quality

The purpose of any radiographic image (analog or digital) is to provide the observer

with adequate diagnostic information to detect and identify or rule out an abnormality and

then to interpret its meaning and determine its cause. The ability of a radiographic image

to convey this information to the observer depends on the quality of the image, which can

be described in terms of contrast, noise and resolution. Image quality is very critical in

interventional neuroradiologic procedures. The ability to visualize small and low contrast

objects is of paramount importance, where neurovascular instruments may be as small as

200 grm and where vessel sizes are as small as 100 gim. The produced images require

high contrast, low noise and high resolution, which can be achieved with high radiation

doses. Any dose reduction strategy must always ensure that image quality is not

compromised and patients do not suffer any adverse clinical consequences as a result of

inadequate visualization of catheters or vasculature.

Ways to improve detection of small vessels during interventional neuroradiologic

procedures using digital subtraction angiography (DSA) include the variation of major

imaging parameters such as tube voltage and image intensifier input exposure, as well as

use of geometric object magnification. Although these parameters affect image quality,

they also influence patient surface doses and effective doses. Further study is necessary

to improve our understanding of how technique parameters affect patient doses and to

what extent they can improve image quality.









Purpose of This Work

Following the installation of the patient dosimetry system on the interventional

neurobiplane suite at Shands Hospital of the University of Florida, dosimetry data on

patients undergoing interventional neuroradiologic procedures were stored in a patient

database for later analysis and evaluation. Dosimetric information on 149 adult patients

and 26 pediatric patients who underwent interventional neuroradiologic procedures was

recorded in the database. Seventeen of 149 adult patients and ten of the 26 pediatric

patients recorded in the database underwent therapeutic embolization procedures.

In this work, the dosimetry data to the adult and pediatric patients recorded by the

patient dosimetry system are analyzed to compute surface doses to the patients' head

region from interventional neuroradiologic procedures. Surface doses are then considered

to assess the risk of deterministic effects to patients who undergo such interventional

procedures, as well as similar high dose radiologic procedures.

Information on the x-ray beam qualities (kVp and HVL) recorded by the patient

dosimetry system with patient thickness and the x-ray beam exposure area are used to

compute the energy imparted to these patients from recorded values of entrance skin

exposures. Values of energy imparted are then converted to patient effective dose, E,

using Els conversion factor corresponding to the projections and body regions irradiated

during interventional neuroradiologic procedures. Values of Els for the posterio-anterior

(PA) projections of the abdomen, chest and cervical spine and for the PA and lateral

(LAT) views of the head are obtained from radiation dosimetry data computed using

Monte Carlo calculations on an adult anthropomorphic phantom (Hart et al., 1994a).









This method is extended to determine effective doses to pediatric patients who differ in

mass from the adult sized phantoms used in current patient dose assessment procedures.

Manipulation of the tube voltage, input exposure to the image receptor and

geometric object magnification impact on patient doses and image quality, with the latter

being defined as the lowest concentration of iodine in a vessel that may be visually

detected in the radiographic image. The effects of these imaging parameters on signal

detection and the corresponding changes in patient doses are investigated in this work.

The results of this work provide the radiologic community with a variety of

information on patient surface doses, energy imparted and effective doses. Such

information will help to evaluate the risks of deterministic and stochastic effects to

patients undergoing interventional neuroradiologic or similar high dose radiologic

procedures. The results on how imaging parameters (i.e., tube voltage, image intensifier

input exposure and geometric object magnification) affect image quality will help to

improve image quality and reduce patient doses, thus providing improved patient care to

the healthcare community.














CHAPTER 2
LITERATURE REVIEW



Introduction




Interventional Neuroradiologic Procedures

During diagnostic neuroradiologic procedures, all initial angiograms performed

on a given vessel territory constitute complete coverage of arterial, capillary and venous

phases. Subsequent examinations of that vessel with various alterations in positioning

(projection), magnification, and contrast injection are performed to specifically evaluate

the visualized or anticipated pathology. As a result, these are limited to arterial phase for

aneurysms, capillary phase for tumors, and venous phase for study of venous patency. In

therapeutic neuroradiologic procedures, a complete diagnostic angiographic procedure is

followed by the introduction of embolic agents into the vasculature from a location next

to the vascular abnormality. Such embolic agents might consist of gelatin sponge or

polyvinyl alcohol for short-term occlusions or detachable balloons, metallic coils and

cyanoacrylates for long-term occlusions. Subsequent evaluation of the pathology during

embolization continues until a satisfactory occlusion of the vascular abnormality has been

achieved.

During all neuroradiologic procedures, frontal fluoroscopy is used in the truncal

and thoracic regions to visualize catheter placement. Biplane fluoroscopy of the head









region is used for target position verification. Most DSA image acquisitions are

performed using biplane acquisitions with only occasional use of single plane

acquisitions. Single (frontal) plane imaging is primarily used to evaluate aneurysm neck

origin with either standard DSA imaging or with rotational digital angiography (DA).

Due to the nature of neuroradiologic procedures, good image quality, long fluoroscopic

times and a significant number of angiographic images are required to evaluate any

visualized pathology. Thus, neuroradiologic procedures result in high patient doses,

primarily absorbed over the head region of the patient. This suggests the possibility of

induction of deterministic radiation effects such as skin erythema and epilation (Huda and

Peters, 1994; Shope, 1996).




Deterministic Radiation Effects

Deterministic or non-stochastic effects of ionizing radiation include the types of

injuries resulting from whole-body or local exposures to radiation that cause sufficient

cell damage or cell killing to substantial numbers or proportions of cells to impair

function in the irradiated tissues or organs (ICRP, 1977). Since a given number or

proportion of cells must be affected, there is a threshold dose below which the number or

proportion of cells affected is insufficient for the defined deterministic injury to occur

(ICRP, 1984). The threshold dose depends on the level of injury or the sensitivity of the

tissues or organs being irradiated (Field and Upton, 1985). Any increase in dose above

the threshold increases the level of injury, since fewer cells will survive at increased

radiation dose. The effect will also increase with increased dose rate. Increased dose rate









will accelerate cell damage without allowing enough time for more effective cell repair or

repopulation (ICRP, 1991).

The doses that result in the clinical appearance of deterministic effects are

generally of the order of a few Gray to tens of Gray. The time at which the effect

becomes noticeable may range from a few hours to some years after exposure, depending

on the type of effect and the characteristics of the irradiated tissue. The levels of

radiation exposure and the irradiated tissues involved in interventional neuroradiology

raise concern for deterministic effects of the skin and eyes. Table 2-1 lists the skin

effects, threshold doses and time of observation of the expected effect after irradiation

(Wagner et al., 1994). An early transient erythema may be observed a few hours after

irradiation at skin absorbed doses in excess of 2 Gy, with a main erythema appearing

about 10 days later, when skin doses exceed 6 Gy. A temporary epilation may be

observed three weeks after an absorbed dose of 3 Gy to the skin surface with a permanent

condition resulting at doses above 7 Gy. The concern to the eye involves small opacities

on the lens of the eye, which may occur at doses of the order of about 1-2 Gy (Merriam

and Focht, 1957; NAS, 1990). More severe cases of cataracts occur at thresholds above

5-6 Gy with a latent period of about a year after irradiation.

Deterministic effects will often have a more severe impact on children, since

tissues are actively growing in comparison to adults (UNSCEAR, 1993). Additional

deterministic effects that have been observed from irradiation during childhood include

effects on growth and development, hormonal deficiencies, organ dysfunctions and

effects on intellectual and cognitive functions. From current data available (UNSCEAR,

1993), there is no evidence that the threshold of deterministic effects to the skin and eyes









are any different for children or adults. Although the brain is most sensitive to radiation

insults during the first four years after birth when rapid growth and development of the

brain takes place, single doses in excess of 10 Gy have to be administered to the brain

during childhood before any deterministic effect of neurophysiologic or neuroendocrine

nature are observed.



TABLE 2-1: Deterministic Effects of the Skin after Single-Fraction Irradiation

Deterministic Dose Threshold Onset Time Peak Time
Efec ______(y)__ Onset Time Peak Time
Effect (Gy)
Early transient 2 hours -24 hours
erythema
Temporary epilation 3 -3 weeks --

Main erythema 6 -10 days -2 weeks

Permanent epilation 7 -3 weeks --

Dry desquamation 10 -4 weeks -5 weeks

Invasive fibrosis 10 .

Dermal atrophy 11 >14 weeks --

Telangiectasia 12 >52 weeks --

Moist desquamation 15 -4 weeks -5 weeks

Late erythema 15 -6-10 weeks --

Dermal necrosis 18 > 10 weeks --
Secondary 20 >6 weeks --
ulceration
SOURCE: Wagner et al., 1994.









Stochastic Radiation Effects

Unlike the deterministic effects, stochastic effects are those for which the

probability of an effect occurring is a function of dose without threshold and its severity

of the effect is dose-independent (ICRP, 1977). Stochastic effects can be categorized as

somatic (carcinogenic) effects and hereditary (genetic) effects, which may occur from

injury to one or a small number of cells. Since a single cell may be enough to initiate the

effect, there is a finite probability that the effect will occur however small the dose.

Thus, stochastic effects are normally assumed to have no dose threshold below which the

effect cannot possibly occur.

Since a stochastic effect may occur at any level of radiation exposure, the

exposure should be kept as low as reasonably achievable (ICRP, 1977). Unnecessary

exposures should be avoided, necessary exposures should be optimized to provide the

maximum benefit to the patient, and the total doses should be limited to the minimum

amount consistent with the medical benefit to the individual patient (ICRP, 1982, 1983).

In the case of optimizing medical procedures for the best dose-benefit outcome, the main

concern should be the amount and type of information derived from the examination and

its diagnostic value.

Whole body irradiation or its equivalent as expressed by the effective dose

equivalent or effective dose can be converted to a stochastic risk estimate using a total

risk factor as determined by the ICRP (1977, 1978, 1991). From the ICRP (1991)

attempt to estimate absolute stochastic risks from whole-body irradiation, a risk

coefficient of 5x105O cancers and genetic abnormalities per mSv of radiation dose was

derived. Such a risk coefficient puts one out of 20,000 people who received a whole









body dose equivalent of 1 mSv to risk of developing a fatal cancer. This is a more

conservative value from the previously derived risk coefficient of 1.65x105 (ICRP,

1978), where one out of 60,600 people who receive 1 mSv will develop a fatal cancer. In

general, these risk factors need to be treated with great caution given the current

uncertainties associated with the extrapolation of radiation risks from high doses to those

normally encountered in diagnostic radiology (Fry, 1996; Puskin and Nelson, 1996)

Although knowledge of the pediatric effective dose associated with radiologic

procedure is helpful, it is important to note that any resultant detriment will depend on the

age of the exposed individual. The stochastic radiation risks of carcinogenesis and

genetic effects are generally greater for children than for adults to at least a factor of two

(ICPR, 1991; NCRP, 1985). These factors would need to be taken into account when

converting any pediatric effective doses into a value of risk or detriment. As a result,

direct comparisons of pediatric doses with those of adults need to be treated with

circumspection.




Dosimetry




Surface Dose

The surface dose is the simplest and most frequent method used to measure

patient doses from radiologic examinations because direct measurements on patients can

be performed easily at the skin surface. The surface dose can be obtained from

measurements of the skin exposure using an ionization chamber or specialized detectors

attached to the skin surface during the examination (i.e., thermoluminescent chips,









fiberoptic scintillarors). The surface dose may also be converted to organ doses (Jones

and Wall, 1985), although such an approach may result in errors of more than 20%

(Padovani et al., 1987).

Although simple to obtain, the surface dose is a poor indicator of the true

significance of radiation exposure to the patient because it overlooks a number of

important factors. For example, in a fluoroscopic exam the surface dose does not account

for changes in the depth of the radiosensitive organs, changes in the exposed field size,

changes in the position of the patient, and changes in the beam qualities, overlaying

exposure fields and partial exposure of organs (Wagner, 1991). More importantly, the

surface dose does not account for the area of exposure or the penetrating ability of the x-

ray beam as the energy of x-rays varies.

The above factors make surface dose a quantity of limited dosimetric value when

estimating stochastic risks. However, the surface dose is the quantity of choice when

trying to predict the occurrence of deterministic radiation effects of the skin during high

dose interventional radiologic procedures. In this case, the surface dose is the dose to the

organ, the skin. Vano et al. (1998) measured surface doses of 11-15 Gy resulting in

erythematous lesions and chronic radiodermatitis from procedures in interventional

cardiology. Huda and Peters (1994) computed an upper estimate of 6.6 Gy to the

occipital region of the skull resulting in temporary epilation from an embolization

neuroradiologic procedure. Other studies reported a range of surface doses for

neurointerventional procedures. Norbash et al. (1996) studied twelve typical

interventional neuroradiologic procedures and measured a range of 0.31-2.7 Gy to the

skin surface of the head with a mean value of 1.5 Gy. Bergeron et al. (1994) measured









0.13-1.3 Gy with a mean value of 0.62 Gy for eight patients undergoing

neurointerventional procedures. Chopp et al. (1980) reported an average surface dose to

the head of 0.16 Gy. Gkanatsios et al. (1997) measured surface doses to 114 patients

undergoing neurointerventional procedures and recorded doses ranging from 0.1-5.0 Gy

with median values of 1.2 Gy and 0.64 Gy for the frontal and lateral planes, respectively.

Although about 30 patients in the latter study exceeded the deterministic threshold of 2.0

Gy, no radiation-induced skin effects were noticed.




Energy Imparted

Although the surface dose or exposure has been popular when expressing patient

radiation doses, these parameter do not take into account the x-ray beam quality (i.e.,

half-value layer) or the size of the irradiated area. An alternative quantity that can be

used to assess patient dosimetry is the energy imparted, or integral dose (Wall et al.,

1979; Harrison, 1983; Huda 1984; Cameron 1992). Energy imparted is a measure of the

total energy deposited in a volume (i.e., head, chest, abdomen, etc.) from exposure to x

rays. The primary factors that affect energy imparted are the x-ray exposure, the area

exposed, the energy of the x-ray beam, and the thickness of the exposed volume

(Gkanatsios, 1995; Gkanatsios and Huda, 1997). Secondary factors affecting energy

imparted are the filtration, the voltage waveform ripple, and the target angle (Shrimpton

et al., 1984; Gkanatsios, 1995). Energy imparted may be used to compute the associated

risk from different types of radiologic examinations, optimize imaging techniques with

respect to patient dose, or even estimate the effective dose to the patient.









The computation of energy imparted can be carried out with accuracy and ease

(Carlsson, 1963; Huda, 1984; Shrimpton et al., 1984; Gkanatsios and Huda, 1997). A

number of approaches have been developed to obtain values of energy imparted from

radiologic procedures (i.e., Carlsson, 1963, 1965a, 1965b; Carlsson et al., 1984; Harrison,

1983; Shrimpton and Wall; 1982). Most methods calculate values of energy imparted

from depth dose data or from estimates of the incident energy to the irradiated volume.

Energy imparted generally depends on the x-ray beam quality, as well as the field size

and irradiation geometry, which makes depth dose data of limited value in the everyday

clinical setting. Monte Carlo techniques are another way to compute energy imparted,

given that photon interaction cross sections and x-ray energy distributions are well known

(Persliden and Carlsson, 1984; Boone 1992). However, these methods are computer

intensive, time-consuming, and relatively cumbersome to use. Other simplified methods,

such as the use of half-value thickness of tissue (Hummel et al., 1985), also can be used

to calculate values of energy imparted. The most practical approach developed to obtain

values of energy imparted is the use of transmission ionization chambers, which can

generate energy imparted data from an exposure-area, or air collision kerma-area product

(Shrimpton et al., 1984). Measurements of exposure-area product have been reported to

result in an accuracy of energy imparted between 10% and 20% (Shrimpton et al., 1984;

Berthelsen and Caderbland, 1991). However, exposure-area product meters do not take

into account patient thickness, and the incident beam may not totally irradiate the patient.

Although it may be possible to overcome both these limitations, an accurate and practical

method for estimating energy imparted to patients that does not rely on special

instrumentation would clearly be advantageous.









Recently, Gkanatsios and Huda developed a simplified method to compute energy

imparted from any radiologic procedure (Gkanatsios, 1995; Gkanatsios and Huda, 1997),

which may be used with the dosimetry equipment available in most radiology

departments. The method is based on Monte Carlo calculations of energy imparted from

monoenergetic photons (Boone, 1992) and makes use of published diagnostic energy x-

ray spectra (Tucker et at., 1991). The patient is modeled as a homogenous slab of water

with a specified thickness. The water equivalence of a given patient may be obtained by

direct measurement of the patient or by estimating the thickness of water which results in

the same x-ray technique factors when the imaging equipment is in automatic exposure

control (AEC) mode. Experimental measurements needed for this computation include

the entrance skin exposure, the x-ray beam qualities (kVp and HVL), as well as the

exposed area and thickness of the patient, all of which may be readily measured or

otherwise estimated. Gkanatsios and Huda compared this method with values of energy

imparted determined using Monte Carlo techniques and anthropomorphic phantoms for a

range of diagnostic examinations. At 60, 80 and 120 kVp, absolute values of energy

imparted obtained using this method differed by 3%, 10% and 22% respectively, from the

corresponding results of Monte Carlo computations obtained for an anthropomorphic

phantom.

The assumption that energy imparted to the head and trunk can determine

radiologic risk has been investigated by many researchers (Bengtsson et al., 1978; Huda,

1984; Carlsson and Carlsson, 1986; Le Heron, 1992; Chapple et al., 1994). It was found

that there may be a valid relationship between energy imparted and radiologic risk.

Although the radiosensitivities of different organs and tissues are ignored, the energy









imparted will predict associated radiologic risks as accurately as when computing doses

to individual organs (Wall et al., 1979; Harrison, 1983; Cameron, 1992). A reasonable

linear correlation within a factor of two or three (IPSM, 1988; Huda and Bissessur, 1990)

was also detected between total energy imparted and effective dose to the head and trunk.

Provided that the examining view (AP, PA, LAT, etc.) and the x-ray beam qualities are

known, the effective dose can be determined easily from values of energy imparted (Huda

and Gkanatsios, 1997, 1998).




Effective Dose

The effective dose, E, is a dosimetric parameter, which takes into account the doses

received by all irradiated radiosensitive organs. The effective dose is able to account for

nonuniform irradiation of different organs and tissues in the body. Thus, the effective

dose is considered a measure of the stochastic risk associated with radiologic

examinations by directly comparing partial body irradiation to whole body radiation

exposure (ICRP, 1977, 1991; Huda et al., 1991). Although the effective dose is an

occupational dose quantity based on an age profile for radiation workers, this dose

descriptor is being increasingly used to quantify the amount of radiation received by

patients undergoing radiologic examinations which use ionizing radiation (ICRP, 1987;

NCRP, 1989; UNSCEAR, 1993).

Measurement or computation of effective doses for any x-ray examination is

difficult and time consuming. An additional problem is that most measurements or

calculations make use of a standard phantom based on the reference man as defined by

the International Commission on Radiological Protection (ICRP, 1975). Although the









importance of patient size for medical radiation dosimetry has been recognized

(Lindskoug, 1992; Chappel et al., 1995), it is not obvious how to scale the effective dose

computed for standard man to different sized patients, such as pediatric patients, who

undergo similar examinations. These limitations impede the wider use of effective dose

in radiology. Huda and Gkanatsios (1997) proposed a method to determine the effective

dose, E, to patients undergoing any radiologic examination using the energy imparted to

the patient, -. Values of E/le were obtained from the radiation dosimetry data presented

for 68 x-ray projections computed using Monte Carlo calculations on an adult

anthropomorphic phantom (Hart et al., 1994a). The energy imparted to patients may be

determined from values of the exposure-area product incident on the patient and can be

combined with E/le ratios (i.e., 5.0 mSv/J for a head PA view) to yield values of the

patient effective dose. In addition, this method was extended to determine effective doses

to patients who differ in mass from the adult sized phantoms used in current patient dose

assessment procedures (Huda et al., 1989b; Le Heron, 1992).

Although the computation of effective dose is cumbersome in most cases, a range

of effective doses has been reported in the literature that pertain to neurointerventional

procedures. Feygelman et al. (1992) studied ten cases and reported values ranging from

1.6-14 mSv with a mean of 6.2 mSv. Bergeron et al. (1994) reported an average of 1.8

mSv with a range of 0.44-3.4 mSv for a limited number of eight patients undergoing

similar procedures. McParland (1998) reported a median of 7.0 mSv with a range of 2.1-

20 mSv when he computed effective doses to patients undergoing cerebral angiography.

A wider range was reported by Berthelson and Cederblad (1991), who computed effective

doses between 3.5 mSv and 25 mSv.









Despite its popularity, the effective dose introduces some problems when used in

diagnostic radiology. First, it does not account for differences between the age

distribution of workers and that of the general public with regard to the determination of

the appropriate organ weighting factors. The effective dose also excludes curable cancer

or hereditary harm beyond the second generation. Both these factors make the effective

dose a questionable quantity in risk assessment associated with diagnostic radiologic

procedures (UNSCEAR, 1988; Cameron, 1992). It should also be mentioned that the

effective dose applies only to low radiation doses, which generally is the case in

diagnostic radiology. However, in areas like cardiology and neuroradiology, where

extended diagnostic and therapeutic procedures may deliver local patient doses of several

Gray, the effective dose may not be an appropriate dosimetric quantity.

Another problem with the effective dose is the uncertainties involved with its

calculation. The calculation of the effective dose must include an analysis of the dose

distribution within the body, which is difficult to do for radiologic procedures,

particularly fluoroscopy. As an alternative, dose distributions are derived from Monte

Carlo techniques using mathematical phantoms (Gibbs et al., 1984; Jones and Wall,

1985; Huda et al., 1991; Le Heron, 1992) or from calculations of the average organ dose

in anthropomorphic phantoms (Faulkner and Harrison, 1988; Huda et al., 1989a, 1989b).

Such techniques, though, can only provide approximations of the true organ dose

distribution. Furthermore, the selection of the "remainder" organs is problematic in dose

distribution analysis and may vary for each examination. Effective dose also requires the

use of a dose equivalent, which is based on the quality factor, Q, of the type of radiation









involved (ICRP, 1977), and use of organ and tissue weighting factors, W, (ICRP, 1991).

Both these factors are considered to be biologically uncertain (Cameron, 1992).

Notwithstanding the fact that there are problems associated with converting

effective doses to a corresponding detriment (Huda and Bews, 1990), there are important

benefits to be gained by using effective dose to quantify patient doses in diagnostic

radiology. One advantage is that the effective dose attempts to measure the risk to the

patient, which is the motivation for all patient dosimetry studies in diagnostic radiology.

In addition, the effective dose to a patient undergoing any examination may be compared

to that of any other radiologic procedure, as well as to natural background exposure and

regulatory dose limits, which are increasingly expressed using effective dose values

(ICRP, 1991; NRC, 1995a, 1995b).




Image Quality

The extraction of adequate diagnostic information from radiographic images is

important in radiology in order to detect and identify an abnormality and then to interpret

its meaning and determine its cause. Thus the quality of the radiographic image is very

important in conveying diagnostic information to the observer. Image quality can be

described in terms of contrast, noise and resolution.




Image Contrast

Image contrast can be defined as the difference in the optical density (film) or

brightness (digital) in an image between an area of interest and its surrounding

background. Image contrast is determined by several factors including the characteristics









of the materials being imaged, the characteristics of the x-ray spectrum, the

characteristics of the detector and display media and physical perturbations such as

scattered radiation (Hasagawa, 1991). These dependencies separate image contrast into

radiographic (subject) contrast, detector contrast and display contrast.

Radiographic contrast. Radiographic or subject contrast characterizes the

differences in x-ray fluence emerging from different regions of the imaged object.

Radiographic contrast depends on differences in material thickness, atomic numbers,

physical density and electron density between different regions of the imaged object and

their interaction with radiation.

Detector contrast. Detector contrast on the other hand, can be expressed as the

ability of the imaging detector to convert differences in x-ray fluence emerging from an

object to differences in optical density (film detector) or brightness (digital detector). The

detector contrast can shape the radiographic contrast according to the detector's

characteristic response to x-rays. Thus, detector contrast depends on the properties of the

detector material, its thickness, atomic numbers, electron density and the physical process

by which the detector converts x-ray fluence into an image.

Display contrast. The third component of image contrast is the display contrast,

which refers to the digital display of images. Display contrast depends on the display

parameters (i.e., window and level) under which the image is viewed and can be

manipulated by the observer.

Other contrast dependencies. Image contrast in general, is also affected by

physical perturbations such as scattered radiation, image intensifier veiling glare and the

base and fog of film, all of which reduce image contrast.









Image Noise

Every radiographic image is degraded by noise superimposed on the image by

random processes occurring along the imaging chain. Detection of a signal that is

superimposed on noise depends on the relative magnitude of the noise compared to the

signal and the ability of the observer to differentiate between the brightness distribution

of the noise and that of the signal plus noise (Giger et aL, 1986b). The overall noise of an

image consists of various noise components. The statistical nature of x-ray production

and attenuation in the detector results in quantum mottle. Structure mottle, electronic

noise, quantization noise, time jitter and display device noise are additional noise

components in digital imaging detectors (Giger, 1985).

In digital imaging as in digital subtraction angiography, the noise components can

be categorized as static and non-static noise. Static noise is independent from one frame

to the next and always presents the same pattern. Thus, static noise is eliminated in

digital subtraction angiography. Structure mottle is the most important static noise

component in digital imaging. Non-static noise is frame dependent, which means that the

noise pattern varies from one frame to the next. Non-static noise sources are always

present in digital subtraction angiography. Significant no-static noise sources in digital

imaging are the quantum mottle and electronic noise.

The primary source of noise in digital imaging is usually quantum mottle, which

corresponds to random spatial fluctuations of the distribution of x-ray quanta absorbed by

the detector. Since the production and attenuation of x rays are Poisson statistical

processes, quantum mottle follows Poisson statistics, which makes it easily quantifiable.

Consequently, increasing the exposure to the imaging detector will improve a









radiographic image by decreasing quantum mottle. Improving the attenuation properties

of the imaging detector will also reduce quantum mottle of radiographic images.

Secondary sources of noise become important in radiographic imaging, when the

image receptor is exposed to high enough radiation to eliminate most of the quantum

mottle. Secondary noise sources in digital imaging consist of the structure mottle,

electronic noise, quantization noise and time jitter.

Structure mottle. Structure mottle is the second most important noise component

in single-frame digital imaging after quantum mottle, and it becomes the dominant noise

source in images acquired using high x-ray fluence (Giger et al., 1986b). The structure

mottle is introduced to the imaging line by the image intensifier. Structure mottle

depends on the physical structure of both the input and output phosphor layers. Since

structure mottle is a static component of image noise, its noise pattern is constant from

frame to frame. Therefore, structure mottle can be eliminated by the subtraction of two

image frames as done in digital subtraction angiography. Another characteristic of

structure mottle pertaining to its static nature is that structure mottle remains unchanged

after frame integration.

Electronic noise. Electronic noise arises from the video camera as a form of dark

current added to the exposure-dependent video signal. The magnitude of electronic noise

is inversely proportional to the dynamic range of the TV camera and is relatively

independent of video signal size. In order to minimize the perturbations added to a

digital radiographic image by electronic noise, the video signal should be maximized

when possible (Cohen et al., 1982). In general, the electronic noise in a digital imaging

system is quite small relative to the quantum and structure mottle (Roehrig et al., 1981;









Baiter et al., 1984). However, electronic noise becomes a significant noise source when

an object is imaged at low video levels and using low x-ray fluence. It was also

demonstrated by Geiger et al. (1986b) that electronic noise contribution becomes

substantial at spatial frequencies of about 1.0 cycles/mm.

Quantization noise. Another noise component of a digital imaging system is quantization

noise. Quantization noise is the error introduced into an analog signal (i.e., TV video

signal) when it is digitized. Quantization noise depends on the width of the quantization

step. In general, digital imaging systems are designed to minimize quantization errors,

which makes quantization noise insignificant in comparison to quantum mottle or even

electronic noise (Burgess, 1984; Boon et al., 1990; Rajapakshe and Shalev, 1994; Baxter

etal., 1997).

Time jitter. Another component of noise that may appear in digital imaging systems is

time jitter (Arnold and Scheibe, 1984; Esthappan et al., 1998). Time jitter is usually

caused by incorrect alignment of the scanning electron beam in the television camera

from one video frame to the next. Time jitter may also be caused by a variable

asynchrony between the video signal and the analog-to-digital converter. In general, time

jitter produces a variation in pixel position from one image frame to the next. The

importance of time jitter becomes significant in digital subtraction angiography, when

this spatial pixel shift changes the spatial pattern of static noise bringing up structure

mottle in a digitally subtracted image. Therefore, careful design and stable electronics are

required in digital imaging systems to avoid time jitter in order to eliminate structure

mottle completely from digitally subtracted images.









Spatial Resolution

The third parameter used to quantify image quality in addition to contrast and noise

is spatial resolution, frequently referred to as resolution. Although spatial resolution does

not have as much of an impact on image quality as contrast or noise, in applications of

neurointerventional imaging spatial resolution becomes somewhat more important.

During interventional neuroradiologic procedures, the need to visualize tiny

neurovascular instruments (i.e., catheters and guide wires) and vessels as small as 100

lim, requires high spatial resolution. The spatial resolution of an imaging system can be

characterized by its modulation transfer function (MTF) (Haus, 1979; Metz and Doi,

1979) which can be obtained from measurements of the point or line spread functions.

The determination of MTF of digital imaging systems, however, requires careful handling

to avoid aliasing effects caused by the discrete data sampling of digital systems (Giger

and Doi, 1984; Fujita et al., 1985). In general, the spatial resolution of an imaging

system depends on geometric, motion, detector and digitization unsharpness.

Geometric unsharpness. Geometric unsharpness refers to the loss of image detail

due to the finite size of the radiation source (i.e., focal spot) (Hasagawa, 1991). Heat

loading of the anode of an x-ray tube requires that the focal spot is large enough to

dissipate the generated heat. The finite size of the focal spot creates unsharpness called

penumbra at the edges of the imaged object. To limit the amount of geometric

unsharpness in neurointerventional imaging, x-ray tubes with steep anode angles (i.e., 9-

11 degrees) and small effective focal spots (i.e., 0.3 mm or 0.6 mm) are used. Another

practice often used in neurointerventional imaging is the use of magnification, which also

increases geometric unsharpness.









Motion unsharpness. Motion unsharpness refers to the loss of spatial resolution

due to motion of the x-ray source, detector and/or object being imaged (Hasagawa, 1991).

When one or more of these components move, motion unsharpness is introduced, which

degrades spatial resolution. Patient motion caused by discomfort and the continuous

moving of the patient's heart and diaphragm is usually the greatest concern, since source

and detector can be easily secured in place. Sedation or immobilization of the patient

during a radiographic procedure and short exposure times will help reduce the amount of

motion unsharpness.

Detector unsharpness. Detector unsharpness refers to the loss of spatial resolution

due to the finite resolving power of the detector (Hasagawa, 1991). In screen-film

systems, detector unsharpness is also caused by light diffusion in the intensifier screens.

Thicker intensifier screens will allow more light diffusion and create more unsharpness.

In digital imaging, spatial resolution depends on the TV bandwidth and pixel size. Thus,

TV systems with 1024 lines are used in neurointerventional applications. In addition, any

digitization will result in loss of spatial resolution due to the inherent pixellation of a

digital image in comparison to the original analog image.




Imaging Technique Factors

Patient doses and image quality are both influenced by the selection of imaging

techniques. Figure 2-2 shows some key parameters along the line of an x-ray imaging

system which can alter patient absorbed doses and image quality. Such parameters are

the tube voltage, tube filtration, input exposure to the imaging detector, magnification

and image processing. With the exception of image processing, an attempt to decrease








patient dose by altering one or more of these parameters will also degrade image quality.

Thus, tradeoffs between varying different imaging technique factors merit investigation

to find better ways to improve image quality while maintaining low patient doses.


Image Processing


Magnification Input Exposure

\q- Filtration


FIGURE 2-1: Key parameters that affect patient dose and image quality in x-ray imaging.


Tube Potential

Very early in the history of diagnostic radiology, tube voltage and the use of

specialized K-edge filters were studied extensively to optimize patient dose and image

quality (Trout et al., 1952; Koedooder and Venema, 1985; Shrimpton et al., 1988; Nagel,

1989). In general it was shown that an increase in tube voltage decreases patient

exposure and degrades image quality. The optimal tube voltage for detecting large-area,









low-contrast iodinated objects was determined to be between 50-60 kVp (Tapiovaara and

Sandborg, 1995). The same study also showed that for detecting thin, soft-tissue detail a

tube voltage between 70-100 kVp should be used. Also, Thompson et al. (1983)

concluded that high tube voltages between 100-110 kVp combined with increased

contrast agent concentration are the optimal techniques for detecting stones in operative

T-tube cholangiography.

The optimal tube potential depends on the imaging requirements of each imaging

procedure. In interventional neuroradiologic procedures, where both visibility of small

iodinated vessels and high spatial resolution are important, low tube voltage may be used

to maintain adequate image quality. As a consequence, low tube voltage will contribute

to high patient absorbed doses. As the tube voltage increases, both entrance absorbed

dose and energy imparted to the patient decrease for a constant input exposure to the

imaging detector. However, it should be noted that for a constant input exposure to the

patient, increase in tube voltage would increase the energy imparted to the patient

(Gkanatsios and Huda, 1997).



Input Exposure to the Image Receptor

The relationship between input exposure to the image receptor and patient

absorbed dose is linear. The input exposure to the image intensifier also affect image

quality. As the input exposure increases, the dose to the patient increases and the

significance of the quantum mottle in a radiographic image decreases. Since most

radiographic images are quantum limited, increasing the exposure to the image receptor

will always improve contrast-to-noise (CNR) and signal-to-noise (SNR) ratios by









reducing image noise. However, as the input exposure increases to the point that other

noise sources (i.e., structure mottle in singe-frame digital radiographs) become as

significant as quantum mottle, then any increase in input exposure will have a minor

effect on image quality.

Any increase in input exposure to the image receptor at a given tube voltage will

increase patient absorbed doses, proportionally. For a film-screen imaging system, where

the input exposure to the system is controlled by an optimum optical density, there is

negligible flexibility in varying the input exposure. In digital imaging systems, however,

the range of input exposure can vary considerably and still produce a useful, diagnostic

image. Thus, while operating in the range of input exposures where quantum mottle is

the dominant noise component, increasing the input exposure for the purpose of

improving contrast visibility is justifiable. However, if the input exposure to the image

receptor is already high enough so that quantum mottle is not the primary component of

radiographic noise, any increase in input exposure only increases patient absorbed doses.

Such practice lowers the standard of patient care by not following the ALARA principle.




Magnification

Magnification and its effects on image quality have been studied in both

conventional radiography and mammography (Doi and Rossmann, 1974; Wagner et al.,

198 la, 1981b; Sandrik and Wagner, 1982). In general, magnification improves visibility

of small, low contrast objects. As the magnification increases, the effective noise in the

image detector is reduced improving the signal-to-noise ratio, and visibility of small









structures improves (Doi and Imhof, 1977). Scatter radiation is also reduced with

increased magnification, which improved contrast detectability (Sandor and Nott, 1980).

In neurointerventional radiologic procedures, magnification is often used as a tool

to visualize small vasculature. Care should be taken, however, when magnification is

used, since the entrance absorbed dose to the patient increases significantly with

magnification. Energy imparted, on the other hand, is independent of magnification as

both distance from the x-ray source and area of exposure decrease equally as

magnification is employed. The choice between geometric-change of distance between

patient and x-ray source-and electronic magnification -changing the input diameter of

the image detector-should be considered every time magnification is required, and the

possibility of dose savings between the two methods should be investigated in any

imaging system.














CHAPTER 3
SURFACE DOSES



Neuroradiologic Imaging

Clinical Practice

Interventional neuroradiologic procedures are performed on patients suspected to

have vascular anomalies in the brain (i.e., aneurysm, vasculitis or arteriovenous

malformations), patients that have brain tumors, patients who have had a stroke episode

or patients requiring certain types of psychological evaluation. A neurointerventional

procedure may be a diagnostic angiographic or therapeutic embolization procedure. In

diagnostic angiographic procedures, the vasculature and blood dynamics of certain parts

of the brain are studied by imaging the kinetics of radio-opaque media injected in the

vasculature of the brain. In therapeutic neurointerventional procedures, corrective action

is taken to occlude vascular anomalies by injecting embolic agents such as gelatin

sponges or metallic coils. Usually, a therapeutic embolization procedure is preceded by a

diagnostic angiographic procedure. In both types of neurointerventional procedures, x-

ray imaging is used extensively in the forms of fluoroscopy, conventional film and digital

radiography.

The transfemoral artery technique is used to perform neurointerventional

procedures, where a catheter is inserted into the common femoral or deep femoral artery

from where it is driven to the vascular network of the brain. Limited amount of frontal









plane fluoroscopy is used on the trunk and thoracic regions to guide the catheter up to the

vertebral or carotid arteries. Once there, further use of fluoroscopy in both imaging

planes, frontal and lateral, is used to position the catheter at the appropriate site to be

studied. Although biplane fluoroscopy is used in this stage, most of the fluoroscopy is

still done using the frontal plane. Once the catheter is in place, radio-opaque contrast is

injected to that location and a series of radiographic images are acquired in plain film or

in digital format. In diagnostic angiographic procedures, the acquisition of radiographic

images is done in biplane mode almost exclusively. In therapeutic embolization

procedures, both biplane and single plane imaging, either frontal or lateral are used

during different stages of the embolization progress evaluation. During each radiographic

acquisition, the frame rate and number of frames may vary from 1-3 frames per second

and 10-50 frames per acquisition, respectively.




Imaging Equipment

The x-ray imaging system used in this study consisted of a biplane Toshibat

KXO-80 high voltage diagnostic x-ray generator and the Toshiba DFP-2000A/A3 digital

fluorography system configured for neuroradiologic procedures. The configurations of

the two imaging planes, frontal and lateral, were identical. The frontal plane was built

around the Toshiba KXO-80C high frequency x-ray generator. The lateral plane was

based on its sister generator, the KXO-80D. Both generators were interfaced together to

function as a biplane unit suited for neurointerventional applications.


t Toshiba America Medical Systems, Tustin, CA









Tri-focal metal Toshiba ROTANODE x-ray tubes having nominal focal spot sizes

of 0.3 mm, 0.6 mm and 1.0 mm and inherent filtration of about 3.0 mm aluminum were

used as the x-ray sources. The collimator assembly provided an almost circular x-ray

field using a multi-blade collimating iris matched tightly to the size of the image

intensifier input area. The collimator assembly provided total collimation with the help

of four metal blades or partial collimation using wedge shaped, transparent filters. A

support table with a comfort pad totaling an equivalent filtration of 3.0 mm aluminum at

80 kVp were placed between the x-ray beam and the patient.

Two image receptors were available. The first receptor was a biplane screen-film

system rated as 600-speed and 400-speed for the frontal and lateral imaging planes,

respectively. The second image receptor was a digital radiography detector. The digital

radiography detector consisted of a CsI image intensifier tube with three effective input

diameters of 31 cm, 23 cm and 15 cm. A carbon fiber interspaced grid with a ratio of

10:1 was used to remove scatter radiation to the input phosphor of the image intensifier.

An automatic iris control adjusted the amount of light reaching the TV camera. The TV

camera consisted of a high-resolution CCD head (1024 lines) and 10-bit analog to digital

converter. Digital information was passed from the TV camera to the digital image

processor. Analog video signals of 1024 lines at 60 Hz interlaced were passed to the live

fluoroscopic high-resolution monitors. The digital image processor was a Toshiba DFP-

2000A/A3 digital fluorography system capable of split display fluoroscopy, roadmap

fluoroscopy, digital angiography and digital subtraction angiography.









Operation

The x-ray imaging system was capable of continuous fluoroscopy or pulsed

fluoroscopy at 15 or 30 frames/sec. Pulsed fluoroscopy could operate at low or high kVp

ranges when a high or low tube current (mA) was selected. Pulsed fluoroscopy at 30

frames/sec and high mA setting was primarily used as the default fluoroscopic technique

during most neurointerventional procedures. Targeted input exposures to the image

intensifier in fluoroscopy were measured at 1.9 iR/frame, 3.4 gR/frame and 4.7

pR/frame for the 31 cm, 23 cm and 15 cm input diameters, respectively, using a 2.0 mm

copper filter.

Limited amount of frontal plane fluoroscopy was used on the trunk and thoracic

regions to drive the catheter to the head region. On average, about thirty seconds (34

11 sec) of fluoroscopy were spent along the trunk region. An additional two minutes

(133 77 sec), on average, were spent along the upper thoracic, lower neck region to

enter the vertebral or carotid arteries. The remaining use of fluoroscopy was allocated to

the head region during placement of the catheter in the appropriate arterial branch to be

imaged. During this time, the majority of fluoroscopy was performed in the frontal plane.

Lateral fluoroscopy was used in those cases where frontal imaging does not contain

adequate information to help in catheter manipulation. Biplane fluoroscopy was used to

verify target positioning prior to each contrast injection and imaging.

Digital subtraction angiography (DSA) was the primary imaging method during

interventional neuroradiologic procedures and was mainly performed at a rate of 3.0

frames/sec. Rates up to 6.0 frames/sec were used to evaluate high flow dynamics. The

input exposure to the image intensifier in digital subtraction angiography was user









selected and it could vary from 50 tR/frame to 1000 gR/frame with 500-700 pR/frame

being the default value.

In digital subtraction angiography, most diagnostic radiographic procedures used

biplane imaging with the occasional use of single plane imaging during the evaluation of

aneurysms of neck origin. In therapeutic embolization procedures, on the other hand,

single plane radiography may provide enough information to evaluate the progress of the

embolization during the intermediate stages of vessel occlusion. Thus, embolization

procedures made extensive use of single plane radiography. Biplane radiography was

still required to make definitive evaluation of the embolization result at the more critical

stages of the procedure.




Imaging Techniques

In fluoroscopy, the automatic brightness control (ABC) adjusts the x-ray tube

voltage to yield the appropriate amount of light at the output of the image intensifier.

Figure 3-1 shows the relative frequency at which different tube voltages were used during

fluoroscopy of a typical interventional neuroradiologic procedure. Relative frequencies

were computed by determining the fraction of surface dose delivered to the patient at

each kVp interval. In the frontal plane, the tube voltages mostly used during fluoroscopic

imaging were distributed between 66 kVp and 95 kVp, most frequently in the 81-85 kVp

range. In the lateral plane, tube voltages between 61 kVp and 85 kVp were equally used

during fluoroscopy with a more frequent use of the 71-75 kVp range. In general, the tube

voltages used in the frontal plane were shifted about 10 kVp higher to those of the lateral









plane. The difference in physical thickness of the head region between frontal and lateral

views explains such differences.


30%

0



M
CL

.20%
0
0
0



S0
0
I.
0-
U_ 10%
4)



0%


56-60


71-75 86-90 101-105
X-Ray Tube Voltage (kVp)


FIGURE 3-1: Histogram of surface dose contribution at different x-ray tube voltages from
frontal plane (black bars) and lateral plane (gray bars) fluoroscopy for an
average interventional neuroradiologic procedure.



In digital radiography, the tube voltage is determined from the associated

fluoroscopic techniques. Figure 3-2 shows the relative frequency at which different tube

voltages were used during radiography of a typical interventional neuroradiologic

procedure. Similarly to fluoroscopy, the distribution of radiographic tube voltages in the

frontal plane was shifted about 10 kVp higher to that of the lateral plane. The most

frequently used voltages in the frontal plane were located at the 76-80 kVp range. Tube

voltages at the 81-95 kVp range were also used extensively during radiography in the









frontal plane. In the lateral plane, voltages between 61-75 kVp and 86-90 kVp were most

frequently used. The range of 66-70 kVp signifies the radiographic tube voltages

primarily used in the lateral plane.


30%


U)
0.

S20%
0
0

0r
0

I- 10%

o



0%


56-60


71-75 86-90 101-105
X-Ray Tube Voltage (kVp)


FIGURE 3-2: Histogram of surface dose contribution at different x-ray tube voltages from
frontal plane (black bars) and lateral plane (gray bars) radiography for an
average interventional neuroradiologic procedure.



The Patient Dosimetry System

System Description

A patient dosimetry system (PEMNET ) was installed in April 1995 on each of

the two x-ray imaging planes of the Toshiba neurobiplane KXO-80C/D unit. The


SPEMNET: Patient Exposure Monitoring Network. Clinical Microsystems Inc., Arlington, VA.









PEMNET unit is a microprocessor-based system running its own on-board software.

Eight units can be networked to a single PC server via RS-121 interfaces, through which

they transfer patient dosimetric data to the PC server for storage and analysis, or receive

calibration information from the PC. The PEMNET system does not measure surface

doses directly, as may be the case of dose area product meters (Shrimpton and Wall,

1982). Instead, the system is passively hardwired to the x-ray generator to acquire the

input signals listed in Table 3-1. These input signals permit the computation of surface

doses that patients would receive, if it were assumed that the same skin area is continually

exposed to the x-ray beam.

The PEMNET dosimetry system computed patient surface doses by using the x-

ray tube radiation output at the selected technique factors (kVp and mA) together with

information about the patient location relative to the x-ray tube and measured exposure

times. The patient location was determined from the height of the x-ray table relative to

the x-ray tube or by using an ultrasonic sensor at orientations where the position of the

table was not relevant, as in lateral views. When the x-ray table intercepted the x-ray

beam, x-ray attenuation by the table was taken into account. The surface dose was

computed in digital radiography, whereas the surface dose rate was determined in

fluoroscopy. In both digital radiography and fluoroscopy, the patient dosimetry system

calculated surface dose rates by sampling the radiation technique factors every 5 ms, and

by computing an average exposure rate every 800 ms. The surface skin exposure rate and

the cumulative surface skin exposure were displayed in real time for each imaging plane









on two panel displays adjacent to the image display monitors and were readily visible by

the neuroradiologic staff.



TABLE 3-1: List of the Input Signals Interfaced to the PEMNET Dosimetry System from
the Toshiba Neurobiplane Imaging Unit

Signal Description Comments

Tube Potential (kV) Radiographic or fluoroscopic

Tube Current (mA) Radiographic or fluoroscopic

Pulsed Fluoroscopy Current (mA) 20 mA or 50 mA

Table Height (cm) Relative to the floor plane

C-Arm Height (cm) Relative to the floor plane

C-Arm Angulation () RAO and CAU rotations

Ultrasonic Distance Measurement (cm) Active after a C-arm rotation of 15

NOTE: RAO = right anterior oblique; CAU craniocaudal.



Calibration

The x-ray tube radiation output was detmined from exposure measurements

obtained using an MDH 1015CO radiation monitor with a 10x5-6 ionization chamber

attached to the surface of an RSD RS-235** anthropomorphic head phantom as depicted

in Figure 3-3. For exposure calibrations, the ionization chamber was located at the

isocenter of each C-arm and in direct contact with the head phantom. For frontal (PA)

exposures, the ionization chamber was located at the occipital area of the



Radcal Corporation, Monrovia, CA
" Radiology Support Devices Inc; Long Beach, CA








anthropomorphic phantom, while for lateral exposures the chamber was located next to

the temporal bone of the phantom. All measurements of entrance skin exposures

included the contribution of backscatter radiation. The entrance skin exposure was

converted to the surface dose using the expression


D 87.7 Cz c J
D 1=- mGy (3.1)



where D is dose to muscle in mGy for monoenergetic photons, and X is the exposure in

roentgens; (ip),nuce is the mass energy absorption coefficient of muscle, and (,1/p,)a, is

the mass energy absorption coefficient of air. The ratio of mass energy absorption

coefficients of muscle to air does not change significantly with energy (about 4%

between 30 keV and 100 keV x rays) and can be taken to be equal to 1.06 for

polyenergetic, diagnostic x-ray spectra (Johns and Cunningham, 1983; Jones and Wall

1985; Wall et al., 1988). The dose D to muscle from polyenergetic x-ray spectra then is

given by

87 7
D = 87.7 x-xl.06xX=9.30xX mGy (3.2)
10

where both the dose in muscle and the exposure in air include contribution from

backscatter radiation.

X-ray generator signals fed to the patient dosimetry system were calibrated to read

the correct technique factors, source-to-patient distance and tube orientation. The

ultrasonic sensors attached to the side of each x-ray tube collimator were calibrated to

measure the x-ray source-to-patient surface distance directly. Measured surface doses

were entered into the patient dosimetry system and transferred to the PC server along









with kVp, mA, and exposure time information. A calibration program on the PC server

generated corresponding surface dose curves (third and fourth degree polynomials) as a

function of the applied kVp and mAs at different modes of operation (i.e., radiographic or

fluoroscopic) and transferred the curve coefficients back to the system's microprocessor.

Two calibrations were performed separately for each imaging plane, with and without the

presence of the x-ray table, in order to derive the table attenuation coefficients.




44











FIGURE 3-3: Calibration setup of the frontal plane (left) and lateral plane (right) using an
RSD RS-235 anthropomorphic head phantom.



Evaluation

The accuracy of the patient dosimetry system was evaluated in all fluoroscopic

and radiographic modes of operation. In fluoroscopy, the Toshiba neurobiplane unit may

be operated either in continuous or pulsed (15 or 30 frames/sec) mode. In the

radiographic mode, the unit may be operated either in cut film (CF) mode or in digital

subtraction angiography (DSA) mode. Each acquisition mode was investigated using the

geometry of a typical patient setup. The ionization chamber was attached to the

anthropomorphic phantom as shown in Figure 3-3. For the frontal plane system









evaluation, table attenuation and positioning were taken into consideration. There was no

table attenuation during testing of the lateral plane. The source-to-patient distance was

measured directly by the ultrasonic sensor in the lateral plane.

Measured surface doses were compared to the corresponding values computed by

the patient dosimetry system for the experimental arrangements listed in Table 3-2. Table

3-3 shows ratios of the measured, X., to calculated, Xc, surface doses obtained with the

patient dosimetry system. Average Xl/Xc ratios ( one standard deviation) over clinical

kV and mAs ranges are given in Table 3-3, together with the total number of individual

data points recorded. Changing the source to patient distance or the electronic

magnification during continuous fluoroscopy resulted in an average XJ/Xc ratio of

1.040.03. Simulation of a non-standard examination performed in the frontal plane,

with "maximized" changes made to all possible imaging parameters, resulted in a surface

dose computed by the patient exposure system of 0.98 Gy whereas the measured value

was 0.93 Gy (5% difference).

In general, these results demonstrated that the patient dosimetry system would

normally generate surface doses, which are within 5% of the true surface dose. The

uncertainties of threshold radiation doses for the induction of deterministic effects such as

skin erythema or epilation are considerably larger than 5% (Wagner et al., 1994; Rubin

and Casarett, 1968; UNSCEAR, 1988) due to factors such as the anatomical location and

size of the irradiated region, tissue vascularity and oxygenation, as well as the patient age,

genetic background and hormonal status. Thus the accuracy of the patient dosimetry

system is adequate for measuring surface doses to patients undergoing interventional

neuroradiologic procedures.









TABLE 3-2: Experimental Arrangements for Evaluating the Patient Dosimetry System

Arrangement Variables Purpose

Tube voltage

Tube current

Exposure time Evaluate the system
Imaging techniques response to technique
Frame rate changes

Electronic magnification

Geometric magnification

Continuous fluoroscopy

Pulsed fluoroscopy Evaluate the system
Image Acquisition Modes response in different image
Cut film acquisition modes
Digital subtraction
angiography
Image acquisition modes

Imaging techniques
Complete techniques Simulate a complete patient
neurointerventional Table height and location examination maximizing
procedure changes which are
procedure Source-to-image receptor technically possible to
(-1.0 Gy) distance determine an upper limit of
C-arm rotation the accuracy of the system
C-arm rotollimation
Collimation









TABLE 3-3: Summary of the Ratios of the Measured to Calculated Surface Doses, XA/Xc,
Obtained During Testing of the Accuracy of the Patient Exposure System
Operating Number Frontal Lateral Comments
-ivr i i .Comments
Mode of Tests Plane Plane
Continuous
Continuous 28 0.99 0.03 1.04 0.02
Fluoroscopy

Pulsed 13 0.96 0.02 0.96 0.02 kV and mAs techniques
Fluoroscopy we re.02 0.96 v-e0.02 wd

Radiography 10 0.94 0.05 1.01+ 0.01
(CF and DSA)
Continuous ,
Continuous min 1.03 1.01
Fluoroscopy

Pulsed 10 min 0.93 1.03 Automatic brightness
Fluoroscopy control (ABC) was used

Radiography 70
(CF and DSA) frames 0


NOTE: CF cut film acquisition; DSA = digital subtraction acquisition
SOURCE: Gkanatsios et al., 1997.


Data Acquisition

Following the introduction of the patient dosimetry system into clinical practice,

dosimetry data were obtained for 175 patients undergoing interventional neuroradiologic

examinations. At the end of each patient examination, the recorded surface dose data

were automatically uploaded to the PC server for subsequent analysis. A database with

information shown in Figure 3-4 was built. Dosimetry data were analyzed to provide

cumulative doses for each imaging mode on both imaging planes for the complete patient

neurointerventional procedure. In addition, dosimetry data were also obtained for

discrete kV intervals, as well as for discrete dose rate intervals.











Additional information made available by the patient dosimetry system included

the total fluoroscopic time, the number of times fluoroscopy was engaged, and the total


number of radiographic (cut film and DSA) images acquired.


Patient ID: Attending Radiologist j

Type of Exam E,.'3S M "l Fellow:. 3M

Date of Exam Resident

Age: W D BiplaneP"
MaleFN FemaeF


FRONTAL PLANE


f IMMUS
M)DROS~aili-


Time: -
Exposme: -
Engage.:


R<10:
R<20:
R>20:


Exposer
FRamer.


R00t
R<1ft
R<20:
R>20t


Time: -
Exposume:



R
R<20:
M-20: 3S


6O9kV:
65 kV:
70 kV:
75 kV:
80 kV:
05 kY:
90 kV:
QE Li.


(G0 kV:
60 kV:
65 kV:
70 kV:
75 kV:
80 kV:
85 kV:
90 kV:
ttl.f


-J W.. z;1 KYV
100kV: 100 V:
105 kV: 105 kV:
110 kV: 110 kV:

Record: "jI|J| 3a4 Ij |ml DI 114


<(60 kY:
60 kV:
65 kV:
70 kV:
75 kV:
80 kV:
85 kV:
90 kV:
95 kV:
100 LkV:
105 kV:
110 kV:


FIGURE 3-4: Sample page from the PEMNET database showing all recorded information
for the frontal imaging plane.


Time:
Expose:
Engage.


R R<20:
R>20:


DM*ITSAHH
StIiTsACiIONn









Fluoroscopyv

Dosimetric data including the surface dose received by the patient from use of

fluoroscopy, the total time of fluoroscopy, and the rate at which dose was delivered to the

patient were recorded by the patient dosimetry system. Additional recorded information

included the number of times fluoroscopy was engaged and the x-ray tube voltages used

in fluoroscopy during the course of a neuroradiologic procedure (seen in Figure 3-4).




X-Ray Beam Localization

During neurointerventional procedures, fluoroscopy was used to position the

catheter next to the vessel anomaly in the brain in order to inject contrast and

subsequently image the anomaly. Since the transfemoral artery technique was used to

guide the catheter to the vertebral or carotid arteries, some fluoroscopy was performed

over the truncal and thoracic regions of a patient. After studying the use of fluoroscopy

for ten patients, it was determined that, on average, about thirty seconds (34 11 sec) of

frontal plane fluoroscopy were spent on the truncal region and an additional two minutes

(133 77 sec) at the upper thoracic, lower neck region.

The amount of fluoroscopy performed over the truncal and thoracic regions was

relatively independent of the patient and the type of neurointerventional procedure.

Therefore, the surface dose corresponding to 2.5 minutes of fluoroscopy was subtracted

from the dose contributed by use of frontal plane fluoroscopy. The remaining dose was

considered to be absorbed in the head region of the patient. To subtract this fraction from

the surface dose to the head, the average dose rate was computed for each patient,









multiplied by 2.5 minutes and subtracted from the total surface dose corresponding to

frontal plane fluoroscopy.

During interventional neuroradiologic procedures, a 20-30 rotation of the x-ray

source in the sagittal plane of the patient may be used when acquiring radiographic

images. Although the central axis of the x-ray beam changes position on the surface of

the head with rotation of the x-ray source, there are parts of the x-ray beam, which

overlap before and after rotation. Such overlaps indicate that there are areas that will

always be exposed to radiation regardless of the applied x-ray source rotation. Thus, any

rotation of the x-ray source could be ignored when computing surface doses from

radiographic exposures, since the maximum surface dose to any given area of the head is

of interest.




Surface Doses

Figure 3-5 shows the histogram distribution of the patient surface doses received

from fluoroscopy alone. The median values of the fluoroscopic surface doses were 0.32

Gy and 0.11 Gy for the frontal and lateral imaging planes, respectively. Maximum

surface doses were computed at 2.4 Gy for the frontal plane and 2.7 Gy for lateral plane.

The data shown in Figure 3-5 do not differentiate between diagnostic and therapeutic

procedures.

The distribution of surface dose in frontal plane fluoroscopy was mainly spread

over the range of 0.0-0.8 Gy. In the lateral plane, the majority of patients (70%) received

less than 0.2 Gy with some patients (17%) receiving between 0.2-0.4 Gy. The lateral

plane was mainly used for catheter position verification and less for catheter









manipulation, which kept the surface doses in the lateral plane low in comparison to the

frontal plane. Surface doses at the tail of the dose distribution for each plane (above 0.6-

0.8 Gy) corresponded to embolization neuroradiologic procedures. Such procedures

require use of additional fluoroscopy for catheter positioning and verification at the site

of occlusion. Twenty-seven (15%) out of 175 patients recorded underwent cerebral

embolization.


150


120


90


60


30


0
0.


00-0.20


0.81-1.00 1.61-1.80
Surface Absorbed Dose (Gy)


2.41-2.60


FIGURE 3-5: Histogram distribution of surface doses for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) fluoroscopy.



Surface Dose Rates

Figure 3-6 shows the histogram distribution of the rate at which surface doses

were delivered to the patient during fluoroscopy. The median values of the fluoroscopic


FRONTAL
Median = 0.32 Gy
(Maximum) = 2.4 Gy

LATERAL
Median = 0.11 Gy
(Maximum) = 2.7 Gy







*'~ rJ i-B ^ _









surface dose rates were 37 mGy/min for the frontal plane and 43 mGy/min for the lateral

plane. The maximum skin dose rates recorded by the patient dosimetry system were

approximately 100 mGy/min for both planes. Since patient thickness is smaller in the

lateral dimension of the head, the automatic brightness control selects a lower tube

voltage (also seen in Figure 3-1), which increases the surface dose rate.

The histogram distribution of the surface dose rate in the frontal imaging plane

presents a normal distribution shape, but is widely spread over the range of mGy/min to

55 mGy/min. The dose rate distribution in the lateral plane is more concentrated at the

16-30 mGy/min and 46-65 mGy/min. In general, fluoroscopic imaging may vary

significantly from patient to patient due to variations in source-to-surface distance and the

selection of imaging techniques (i.e., kVp/mA).



30
FRONTAL
Median =37 mGy/min
(Maximum) = 99
20 mGy/min
4c 2
L -LATERAL
SMedian = 43 mGy/min
0 (Maximum) = 101
E10 niGy/min
z



0
o n E! n: : .


0.0-5.0 30.1-35.0 60.1-65.0 90.1-95.0
Surface Absorbed Dose Rate (mGylmin)
FIGURE 3-6: Histogram distribution of surface dose rates for 175 patients from frontal
plane (black bars) and lateral plane (gray bars) fluoroscopy.









Fluoroscopic Times and Intervals

Other useful information recorded by the patient dosimetry system included the

total time of fluoroscopy and the number of times fluoroscopy was engaged during an

interventional neuroradiologic procedure. Figure 3-7 shows the histogram distribution of

total fluoroscopic times in each imaging plane. The frontal plane was most frequently

used with a median value of 12 minutes per patient examination compared to a median

value of 3.0 minutes in the lateral plane. The fluoroscopic times in the frontal plane were

more spread over the range of 5-20 minutes in comparison to the lateral plane, which

used less than 5 min of fluoroscopy for the majority (73%) of the patients. The data

shown in Figure 3-7 include both diagnostic and embolization neuroradiologic

procedures with the higher values corresponding to the latter. For embolization

procedures, the duration of fluoroscopy may be extended well beyond the median values

to times as high as 70 minutes and 41 minutes for the frontal and lateral planes,

respectively.

Figure 3-8 shows a histogram distribution of the number of times fluoroscopy was

engaged on each imaging plane. The median number of times that the operator initiated

fluoroscopy was 62 in the frontal plane and 26 in the lateral plane. This difference clearly

indicates the extensive use of fluoroscopy in the frontal imaging plane during

interventional neuroradiologic procedures. Increased number of fluoroscopic instances

also indicate higher surface doses (seen in Figure 3-5) and longer fluoroscopic times

(Figure 3-7) between the frontal and lateral imaging planes.










150
FRONTAL
120 Median = 12 min
(Maximum) = 70 min


0 LATERAL
C. Median = 3.0 min
4m
0, (Maximum) =41 min
I-
S60
E
z
30


0 ,1 T, I I
0.0-5.0 20.1-25.0 40.1-45.0 60.1-65.0
Fluoroscopic Time (min)

FIGURE 3-7: Histogram distribution of fluoroscopic times to 175 patients from frontal
plane (black bars) and later plane (gray bars) fluoroscopy.


Catheter positioning primarily done using the frontal imaging plane varies widely

from patient to patient. This difference between frontal and lateral imaging planes

introduced a wider spread to the histogram distribution of fluoroscopic intervals

corresponding to the frontal plane, as shown in Figure 3-8. Both distributions, however,

show long tails with a maximum of 226 fluoroscopic intervals in the frontal plane and

170 intervals in the lateral plane. Such tails on the distribution may account for the need

to use additional fluoroscopy during embolization procedures.










60-
FRONTAL
Median = 62
(45 (Maximum) = 226
e-
*LATERAL
C. Median = 26
'`30 (Maximum)= 170
I-
E

0





0.0-15.0 90.1-105.0 180.1-195.0
Fluoroscopic Intervals

FIGURE 3-8: Histogram distribution of fluoroscopic intervals for 175 patients from
frontal plane (black bars) and lateral plane (gray bars) fluoroscopy.



Radiography

Dosimetric data on the surface dose received by the patient from use of

radiographic imaging (i.e., cut film and DSA images), the number of radiographic frames,

and the dose per frame were recorded by the patient dosimetry system. Additional

recorded information included the radiographic tube voltages used during the course of a

neuroradiologic procedure (seen in Figure 3-4).




X-Ray Beam Localization

Radiographic image acquisitions are performed almost exclusively on the head

region during interventional neuroradiologic procedures. The majority of them is









performed employing the digital subtraction angiography (DSA) technique. As discussed

under x-ray beam localization for fluoroscopy, the main goal is to compute the maximum

doses delivered to any surface of a patient's head. Similarly to fluoroscopy, radiography

may also be considered unaffected by the small degree of x-ray source angulation usually

applied to the frontal imaging plane in the sagittal plane of the patient.




Surface Doses

Figure 3-9 shows the histogram distribution of the patient surface doses received

from radiographic acquisitions. The median values of the radiographic surface doses

were 0.80 Gy and 0.50 Gy for the frontal and lateral planes, respectively. The maximum

radiographic dose recorded in the frontal plane was 4.8 Gy, twice the maximum dose

recorded in fluoroscopy for the same plane. In the lateral plane the maximum

radiographic surface dose was 3.8 Gy, about 30% higher than the maximum dose from

fluoroscopy in the same plane.

Although the histogram distribution of the radiographic surface doses in the

frontal plane has a longer tail and a higher median value than the dose distribution in the

lateral plane, both distributions are very similar. This supports the fact that frontal and

lateral plane radiography are equally utilized during any type of interventional

neuroradiologic procedure.




Surface Dose Rates

Figure 3-10 shows the histogram distribution of the surface dose per frame in

radiographic imaging with median values of 2.5 mGy/friame and 1.8 mGy/frame for the








frontal and lateral planes, respectively. Maximum doses of 5.6 mGy/frame were recorded

in the frontal plane and 4.9 mGy/frame in the lateral plane. In general, the size and

densities of the head region do not vary significantly among patients. Similar tube

voltages would be used for all radiographic imaging acquisitions. Therefore, the

distribution of doses per frame depends mostly on changes to the source-to-surface

distance (i.e., use of different degrees of magnification among patients). In the frontal

plane where geometric magnification is more frequently used, the dose per frame

distribution approaches that of a wide normal shaped distribution. In the lateral plane

where magnification is not uses as often, the distribution is steeper (less variability).



75 -
FRONTAL
Median = 0.80 Gy
(Maximum) = 4.8 Gy
50
._50 LATERAL
W. rMedian = 0.50 Gy
S(Maximum) = 3.8 Gy
L.
S25
Zo



0 C*

0.00-0.30 1.21-1.50 2.41-2.70 3.61-3.90
Surface Absorbed Dose (Gy)
FIGURE 3-9: Histogram distribution of surface doses for 175 patients from frontal plane
(black bars) and lateral plane (gray bars) radiography.









Radiographic Frames

Figure 3-11 shows the histogram distribution of the number of radiographic

(DSA) frames acquired during diagnostic and therapeutic neuroradiologic procedures.

Median values of 353 frames and 316 frames were recorded for the frontal and lateral

plane, respectively. Due to the complexity of some embolization procedures, however,

the number of frames acquired to evaluate the progress of an occlusion may run as high

as 1400 in the frontal plane and 1000 in the lateral plane.




45
FRONTAL
Median = 2.5 mGy/frame
(Maximum) = 5.6 mGy/frame
30
0 ~LATERAL
C. Median = 1.8 mGy/frame
'Maximum) = 4.9 mGy/frame


z



0

0.00-0.30 1.51-1.80 3.01-3.30 4.51-4.80
Surface Absorbed Dose per Frame (mGylframe)
FIGURE 3-10: Histogram distribution of surface dose rates for 175 patients from frontal
plane (black bars) and lateral plane (gray bars) radiography.


As shown by the distribution of radiographic frames in both planes, the number of

imaging frames required in each procedure is variable. Depending on the degree of

difficulty of extracting diagnostic information from the acquired images, as well as the









type of anomaly to be diagnosed, the number of radiographic images acquired is normally

between 100 and 500 frames. Embolization procedures, on the other hand, may require

two to four times the number of radiographic images to complete the associated tasks.




45
FRONTAL
Median = 353 frame
(Maximum) = 1388 frame
30
(I)_
LATERAL
.. Median = 316 frame
|(Maximum) = 999 frame

E15

z

.4!

0*
0-100 401-500 801-900 1201-1300
Number of Radiographic Frames
FIGURE 3-11: Histogram distribution of the number of radiographic frames for 175
patients from frontal plane (black bars) and lateral plane (gray bars)
radiography.



Conclusions

Figure 3-12 shows the histogram distributions of the total surface dose to the

patient from the use of fluoroscopy and radiography during an interventional

neuroradiologic procedure. The medians of the total surface dose were 1.2 Gy and 0.64

Gy for the frontal and lateral plane, respectively. The maximum surface dose received by

a patient was of the order of 5 Gy for both imaging planes. The majority of the doses









were concentrated between 0.2 Gy and 1.2 Gy for both imaging planes. A significant

number of higher doses, however, was indicated by the tails of the two histogram

distributions. Most of the dose was contributed by radiography, which accounts for the

67% of the total surface dose in the frontal plane and 78% of the total dose in the lateral

plane. Fluoroscopy only accounted for the 33% and 22% of the total surface dose in the

frontal and lateral plane, respectively.

Although 28% of the patients in this study may have exceeded the nominal

threshold absorbed dose to the skin for the induction of deterministic effects (2 Gy), there

were no cases of epilation or skin erythema observed in this series of 175 patients. This

is not surprising since any erythema would be fleeting and faint. Epilation would only be

identified by a slightly different amount of hair loss, as perceived when combing one's

hair, and would not require total loss of hair. For acute radiation exposures, observable

effects such as total epilation are more likely to occur at doses in excess of 6 Gy (Huda

and Peters, 1994).

Several factors need to be considered in predicting the likelihood of deterministic

effects to patients undergoing neuroradiologic examinations. One factor is the fact that

radiation doses are delivered over an extended time period, which may be as long as

several hours. Of great importance is also the fact that the radiation field is varied over

the patient. For individuals with the highest radiation exposures, the neuroradiologist

generally makes a concerted effort to either move the relative orientation of the x-ray

beam or to utilize the orthogonal imaging plane in so far as these options do not adversely

impact the required imaging information. Many of the neuroradiologic imaging

procedures also make use of wedge shaped transparent filters which serve to reduce the









radiation doses at the periphery of the x-ray field of view whilst maintaining image

quality within the central region. All these factors reduce the likelihood of deterministic

injuries to patients and should be practiced during extended neuroradiologic procedures.


60




to
1 40
'I
a.
4-
0
k.

E 20
z


0.00-0.30 1.51-1.80 3.01-3.30 4.51-4.80

Surface Absorbed Dose (Gy)
FIGURE 3-12: Histogram distribution of the total surface doses to 175 patients from
frontal plane (black bars) and later plane (gray bars) fluoroscopy and
radiography combined.














CHAPTER 4
ENERGY IMPARTED AND EFFECTIVE DOSE IN NEURORADIOLOGY



Introduction

The effective dose, E, is a dosimetric parameter which takes into account the doses

received by all irradiated radiosensitive organs and may be taken to be measures of the

stochastic risk (ICRP, 1977, 1991). Although the effective dose is an occupational dose

quantity based on an age profile for radiation workers, this dose descriptor is being

increasingly used to quantify the amount of radiation received by patients undergoing

diagnostic examinations which use ionizing radiation (ICRP, 1987; NCRP, 1989;

UNSCEAR, 1993). Notwithstanding the fact that there are problems associated with

converting effective doses to a corresponding detriment (Huda and Bews, 1990), there are

important benefits to be gained by using effective dose to quantify patient doses in

diagnostic radiology. One advantage is that the effective dose attempts to measure the

stochastic risk to the patient, which is the motivation for all patient dosimetry studies in

diagnostic radiology. In addition, the effective dose to a patient undergoing any

examination may be compared to that of any other radiologic procedure as well as natural

background exposure and regulatory dose limits (ICRP, 1991; NRC, 1995a, 1995b).

Measurements or computations of effective doses from x-ray examinations are

difficult and time consuming. An additional problem is that most measurements or

calculations make use of a standard phantom based on the reference man as defined by

the International Commission on Radiological Protection (ICRP, 1975). Although the

60









importance of patient size for medical radiation dosimetry has been recognized

(Lindskoug, 1992; Chappel et al., 1995), it is not obvious how to scale the effective dose

computed for standard man to different size patients, such as pediatric patients, who

undergo similar examinations. These limitations impede the wider use of effective dose

in radiology. Huda and Gkanatsios (1997) developed a more practical approach to

compute effective doses from energy imparted for a variety of radiologic examinations

and different size patients including infants and children. This method was used in this

chapter to compute effective doses from computed values of energy imparted to patients

undergoing interventional neuroradiologic procedures.

Exposure-area products to different regions of the body and at different tube

voltages were used to compute values of energy imparted, e, from interventional

neuroradiologic procedures (Gkanatsios, 1995; Gkanatsios and Huda, 1997). Values of

energy imparted were converted to patient effective dose, E, using Ele conversion factor

corresponding to the projections and body regions irradiated during interventional

neuroradiologic procedures. Values of Ele for the posterio-anterior (PA) projections of

the abdomen, chest and cervical spine and for the PA and lateral (LAT) views of the head

were obtained from radiation dosimetry data computed using Monte Carlo calculations on

an adult anthropomorphic phantom (Hart et al., 1994a). This method was extended to

determine effective doses to pediatric patients who differ in mass from the adult sized

phantom.









Method




Energy Imparted

The energy imparted, e, to a patient undergoing any radiologic x-ray examination

can be estimated by modeling the phantom as a slab of water with thickness z using the

expression

e= o x ESE x A J (4.1)


where Co is the energy imparted per entrance exposure-area product, ESE is the exposure

measured free-in-air at the beam entrance plane of the phantom, and A is the area of

exposure also measured at the entrance plane (Gkanatsios, 1995; Gkanatsios and Huda,

1997).

The parameter oC depends on the water phantom thickness z, the x-ray tube

voltage and x-ray beam half-value layer (Gkanatsios, 1995; Gkanatsios and Huda, 1997)

Values of co can be computed from

co = a x HVL + 0 J R1 cm-2 (4.2)

where a and fl are coefficients that depend on tube voltage and phantom thickness, and

HVL is the half-value layer of the x-ray beam at a given tube voltage in mm of

aluminum. Figure 4-1 shows the behavior of co as a function of water phantom thickness

for x-ray tube voltages of 60 kVp, 80 kVp and 100 kVp as apply to the x-ray tube and

table filtration of the neuro-biplane Toshiba imaging system. Examples of a and f8

coefficients and half-value layers of the x-ray beams at different tube voltages are given

in Table 4-1.






63


200
0 100 kVp (HVL = 5.3 mm AI)
x80 kVp (HVL =4.3 mm AI)
150 A 60 kVp (HVL = 3.2 mm Al)

E
= J..-- -.- -

u y^ .,---..
h100 A.



50




0 10 20 30 40
Water Phantom Thickness (cm)

FIGURE 4-1: Values of co as a function of water phantom thickness for tube voltages of
60 kVp, 80 kVp and 100 kVp.
NOTE: The values of co were computed for constant voltage
waveforms, an x-ray tube anode angle of 11 and 6.0 mm Al
filtration (x-ray tube filtration plus table filtration of the Toshiba
frontal imaging plane).


The free-in-air entrance exposures to the patient, ESE, were obtained from the

patient exposure data recorded by the frontal and lateral patient exposure meters at 5 kVp

intervals (Figure 3-4). The recorded exposures included the contribution of backscatter

radiation from an RSD RS-235t anthropomorphic head phantom. Therefore, backscatter

fractions measured using the same phantom were subtracted from the recorded exposures.

Table 4-2 lists measured backscatter fractions for the RSD RS-235 anthropomorphic head

phantom as a function of tube voltage for the frontal and lateral imaging planes.


t Radiology Support Devices Inc; Long Beach, CA









TABLE 4-1: Computed a and ft Coefficients and Half-Value Layers for X-Ray Beams as a
Function of Tube Voltage

Tube Voltage a coefficient Coefficient HVL-Frontal HVI--Lateral
a Coefficient C c(mm Al) (Cmm Al)
(kVp)_______'_____( m l __ ( m l
50 2.275E-05 1.300E-05 2.64 1.85

60 2.229E-05 1.895E-05 3.23 2.23

70 2.147E-05 2.521E-05 3.77 2.59

80 2.031E-05 3.215E-05 4.32 2.96

90 1.899E-05 3.910E-05 4.85 3.35

100 1.771E-05 4.557E-05 5.34 3.74

110 1.654E-05 5.145E-05 5.80 4.12

120 1.549E-05 5.673E-05 6.23 4.51

NOTE: a and f8 coefficients were computed for a water phantom
thickness of 20 cm. The half-value layers were determined for constant
voltage waveforms, an x-ray tube anode angle of 11, 6.0 mm Al
filtration for the frontal imaging system (x-ray tube filtration plus table
filtration) and 3.0 mm Al filtration for the lateral imaging system (x-ray
tube filtration).



Energy imparted values were computed separately for fluoroscopy and

radiography. In frontal plane fluoroscopy, the abdominal, upper thoracic, lower neck and

head regions were irradiated. As determined in Chapter 3, about thirty seconds of frontal

plane fluoroscopy were spent on average on the abdominal region and an additional two

minutes at the upper thoracic, lower neck region. The exposures corresponding to these

fluoroscopic times were used to compute the energy imparted to the abdomen and upper

chest, lower neck body regions. The remaining fluoroscopic exposure was focused over

the head region and was used to compute the energy imparted to the head. In lateral









fluoroscopy, frontal radiography and lateral radiography all exposure was taken to be

incident on the head region.



TABLE 4-2: Backscatter Fractions of Radiation Exposure at Different Tube Voltages
Tube Voltage Backscatter Backscatter
(kVp) Factor (Frontal) Factor (Lateral)
50 0.056 0.119

60 0.073 0.130

70 0.084 0.135

80 0.096 0.140

90 0.107 0.143

100 0.110 0.147

110 0.116 0.154

120 0.121 0.157

NOTE: Backscatter fractions were determined using the RSD RS-235
anthropomorphic head phantom.



For the purpose of computing energy imparted, the water equivalent thickness of

the irradiated region as well as the area of exposure at the x-ray beam entrance surface

were required. Table 4-3 lists the water equivalent thickness and exposure area of the

head regions corresponding to different age groups used to compute energy imparted to

the head. Table 4-4 lists the water equivalent thickness and exposure area for different

age groups used to compute energy imparted to the abdominal and upper thoracic, lower

neck regions.









TABLE 4-3: Patient Thickness and Area of Exposure Corresponding to the Head Region
of Different Age Groups

Patient Head PA LAT PA Area of
SDensity Thickness Thickness Exposure
Age (g/cm3) (cm) (cm) (cm2)

Newborn 1.057 12.3 9.51 86.7

1-yr-old 1.071 16.7 13.1 160

5-yr-old 1.090 19.8 15.5 221

10-yr-old 1.095 20.6 16.2 239

15-yr-old 1.104 21.6 17.2 265

Adult 1.112 22.2 17.8 279


NOTE: PA thickness and LAT thickness represent the equivalent
thickness of a water phantom computed from the physical dimensions
and density of the head. The area of exposure for each patient group in
the PA projection was computed using the physical dimensions of the
head. The area of exposure in the LAT projection was estimated to be
equivalent to 1.2 of the corresponding areas in the PA projection.
SOURCE: Densities and physical diameters of the head region at
different age groups were taken from Huda et al., 1997.


Adult Effective Doses

The National Radiological Protection Board (NRPB) have performed a

comprehensive series of Monte Carlo dose calculations for the most common x-ray

projections (Hart et al., 1994a). The Monte Carlo runs made use of a hermaphrodite

anthropomorphic phantom with a mass of 70.9 kg and a height of 174 cm, which included

the female breasts, ovaries, uterus and testes. Each Monte Carlo run tracked the pattern

of energy deposition in the anthropomorphic phantom from primary and scattered

photons for total 4,000,000 photons used with each x-ray projection. A total of 68

separate views were obtained using x-ray spectra generated between 50 kVp and 120 kVp









with added filtration ranging from 2 mm Al to 5 mm Al. X-ray spectral data were

obtained using an updated version of a computer program published by Iles (1987).



TABLE 4-4: Patient Thickness and Area of Exposure Corresponding to the Trunk Region
of Different Age Groups

Trunk Abdomen Chest/Neck PA Area of
Patient Densit Thickness Thickness Exposure
Age (em -PA- -PA- (cm')
(glcm3) (cm) (cm)

Newborn 0.995 9.75 9.00 175

1-yr-old 1.002 13.0 11.2 175

5-yr-old 1.000 15.0 13.2 175

10-yr-old 1.005 16.9 13.8 175

15-yr-old 1.030 20.2 14.6 175

Adult 1.018 20.4 15.0 175

NOTE: The PA thickness represents the equivalent thickness of a water
phantom computed from the physical diameters and density of the
trunk.
SOURCE: Densities and physical diameters of the trunk region at
different age groups were taken from Huda et al., 1997.



For each x-ray examination, the Monte Carlo dosimetry data generated by the

NRPB permitted the computation of the effective dose, E, as defined by the International

Commission on Radiological Protection (ICRP, 1977, 1991). The phantom breast dose

and the mean of the testes and ovary doses were used to determine the contributions to

the effective dose from the breast and gonads, respectively. The Monte Carlo dosimetry

data also provided the mean doses to three body regions consisting of the head, Dh, trunk,









D, and legs, D,. Mean doses to these three body regions were used to compute the mean

energy imparted to the patient, e, using the equation

S= Dh x 5.8 +D, x 43.0 +D, x 22.1 J (4.3)


where the mass of the head is 5.8 kg, the mass of the trunk, including the arms, is 43.0

kg and the mass of the legs is 22.1 kg.

The complete dosimetry results of these Monte Carlo simulations have been made

available in a software format (Hart et al., 1994b) and were used to obtain the values of

effective dose and energy imparted for specific projections as applied to radiation

exposures during interventional neuroradiologic procedures. These projections were the

posterio-anterior projections of the abdomen, chest, cervical spine, and head regions, as

well as the right lateral projection of the head region. For each x-ray projection, values of

El/ were computed at eight tube voltages ranging between 50 kV and 120 kV and

generated at 10 kV intervals with a beam filtration equivalent to 3.0 mm aluminum

(lateral plane) and 6.0 mm aluminum (frontal plane). The effective dose per unit energy

imparted, Ele (mSv J'), for the projections of the trunk and head regions are given in

Table 4-5. The average Ele ratios of the chest and C-spine projections at each kVp were

used to determine effective doses from irradiation of the upper thoracic, lower neck

region.


I Wall BF. Private communication (1996).








Pediatric Effective Dose

By definition, 1 Gy of uniform whole body irradiation to x-rays results in an

effective dose of 1 Sv and is independent of the mass of the exposed individual. For a

70.9 kg anthropomorphic adult subject to uniform whole body irradiation, energy

imparted can be directly converted into effective dose with one joule corresponding to an

effective dose of 14.1 mSv. For uniform whole body irradiation, the effective dose E(M)

to an individual with a mass M (Table 4-6) who absorbs a total of e J is given by


E(M) = c x 14.1 x 70--9 mSv (4.4)
M

Figure 4-2 shows how the effective dose varies with the patient mass for uniform whole

body irradiation with a total of one joule imparted to the individual.

For nonuniform exposures normally encountered in diagnostic radiology, the

relative radiosensitivity of the irradiated region needs to be taken into account when

obtaining the effective dose. The relative radiosensitivity of any body region remains

approximately constant with age (ICRP, 1991; Almen and Mattsson, 1996). For instance,

if the head accounts for x% of the total stochastic risk in adults uniformly exposed to x-

rays, this body region will also account for approximately x% of the total stochastic risk

for any other age group. As a result, the effective dose to a patient of mass M kg for a

given x-ray projection i who absorbs joules of energy is obtained using


E'(^\ E} 70.9
E{M)=ex 7. mSv (4.5)
i M









where (E/le), is the ratio of effective dose to energy imparted (mSv J') obtained for the

same projection i in the adult anthropomorphic phantom with a mass of 70.9 kg.

Standard masses of patients of different ages are given in Table 4-6.



TABLE 4-5: Values of Effective Dose per Unit Energy Imparted, Ele in mJ/Sv, for
Different Body Projections as a Function of Tube Voltage

SAbdomen Chest C-Spine Head Head
kVp (PA) (PA) (PA) (PA) (LAT)

50 10.7 12.8 4.19 4.06 4.08

60 12.0 13.6 4.67 4.62 4.56

70 13.2 14.1 5.04 5.00 4.94

80 13.9 14.6 5.40 5.40 5.29

90 14.5 15.0 5.66 5.68 5.61

100 14.9 15.4 5.92 5.90 5.87

110 15.4 15.6 6.11 6.17 6.06

120 15.8 15.8 6.27 6.32 6.24

NOTE: The values of Ele corresponding to PA views were computed
for 6.0 mm Al filtration (frontal imaging plane). The values of Ele
corresponding to the Head LAT view were computed for 3.0 mm Al
filtration (lateral imaging plane).


TABLE 4-6: Standard Patient Mass for Different Age Groups

Age Newborn 1-yr-old 5-yr-old 10-yr-old 15-yr-old Adult
Group
Patient
Mat 3.4 9.8 19 32 55 70.9
MassSOURCE: Huda e(kg) al., 1997.
SOURCE: Huda el al.. 1997.









1000


3.0 kg for a new born


Cl)
IuJ

o 100







10

10 100

Patient Mass (kg)

FIGURE 4-2: Effective dose as a function of patient mass for one joule of uniform whole
body irradiation.



Adult Patient Doses

The following sections summarize the energy imparted and effective doses to adult

patients from interventional neuroradiologic procedures. One hundred and forty nine

adult patients, 132 of them underwent diagnostic angiographic and seventeen underwent

therapeutic embolization procedures, were studied. Fluoroscopy and radiographic

acquisitions were reviewed separately.




Energy Imparted

Figure 4-3 shows the histogram distribution of the energy imparted to patients

from use of fluoroscopy during interventional neuroradiologic procedures. The median









value of energy imparted was 1.77 J with energy imparted in the frontal plane being the

major component of fluoroscopy. The distribution of energy imparted in fluoroscopy was

mainly spread over the range of 0-5 J. Fourteen (9%) of 149 adult patients received more

than 5 J with three (2%) patients receiving more than 10 J of energy imparted from

fluoroscopy with a maximum value of 12.6 J. Although there was no separation done in

the distribution between diagnostic angiographic and therapeutic embolization

procedures, the median value of energy imparted to patients who underwent

embolizations was 3.48 J. Six of the seventeen embolization patient exceeded the value

of 5 J.


IiL.


BIPLANE FLUOROSOPY
Median = 1.77 J
Maximum = 12.6 J

FRONTAL FLUOROSOPY
Median = 1.33 J
Maximum = 9.67 J

LATERAL FLUOROSOPY
Median = 0.31 J
Maximum = 8.98 J


4-5 8-9
Energy Imparted (J)


12-13


FIGURE 4-3: Histogram distribution of energy imparted to patients from use of
fluoroscopy during interventional neuroradiologic procedures.


60



U,
40
CL
N.0.
(.
o
0

E 20
z



0


I


mm m









Figure 4-4 shows the histogram distribution of the energy imparted to patients

from radiographic acquisitions during interventional neuroradiologic procedures. The

median value of energy imparted was 4.30 J. The distribution of energy imparted from

radiographic acquisitions was mainly spread over the range of 0-10 J. Sixteen (11%) of

149 adult patients received between 10 J and 15 J. A maximum value of 21.2 J was

recorded. The median value of energy imparted to patients who underwent therapeutic

embolization procedures was 8.40 J. Seven of the seventeen embolization patients

received energy imparted from radiographic acquisitions greater than 10 J.



30
BIPLANE RADIOGRAPHY
Median = 4.30 J
Maximum = 21.2 J
(a
S20 |FRONTAL RADIOGAPHY
E2 Median= 2.71 J
Maximum = 16.6 J
0
LATERAL RADIOGRAPHY
E 10 Median= 1.26J
Z Maximum = 10.9 J





0-1 5-6 10-11 15-16 20-21
Energy Imparted (J)

FIGURE 4-4: Histogram distribution of energy imparted to patients from radiographic
acquisitions during interventional neuroradiologic procedures.


Figure 4-5 shows the histogram distributions of the total energy imparted to adult

patient from diagnostic angiographic and therapeutic embolization neuroradiologic









procedures. The median value of the total energy imparted was 6.69 J. The maximum

energy imparted received by a patient was 26.9 J. The majority of the patients who

underwent interventional neuroradiologic procedures received up to 14 J of energy

imparted. Fifteen (10%) of the adult patients shown by the tail of the distribution in

Figure 4-5 received energy imparted between 14 J and 27 J. The median value of the

total energy imparted to patients who underwent therapeutic embolization procedures was

13.3 J. Eight of the seventeen embolization patients exceeded the 14 J value of total

energy imparted. The largest fraction of energy imparted was produced by radiographic

acquisitions. The average fraction of energy imparted from radiographic acquisitions was

about 66% of the total energy imparted. Only one third of the total energy imparted was

accounted for use of fluoroscopy.




Effective Doses

Figure 4-6 shows the histogram distributions of the total effective dose to adult

patient from diagnostic angiographic and therapeutic embolization neuroradiologic

procedures. The median value of the total effective doses was 36 mSv. The majority of

the patients who underwent interventional neuroradiologic procedures received between

10 mSv and 70 mSv of effective dose. The tail of the histogram distribution shown in

Figure 4-6 corresponds to nineteen (13%) patients who received effective doses greater

than 70 mSv. The median value of the total effective dose to patients who underwent

therapeutic embolization procedures was 74 mSv. Ten of the seventeen embolization

patients exceeded the 70 mSv value of total effective dose. As in surface doses and

energy imparted, radiographic acquisitions accounted for the largest fraction of the










effective dose to adult patients. On average, about 64% of the effective dose

corresponded to radiographic acquisitions. Use of fluoroscopy accounted for only one

third of the total effective dose received by patients during interventional neuroradiologic

procedures.


0 ,JU
4.2
0
4'
0.
o 20
,I-

E

Z10



0


0-2 8-10 16-18 24-26
Energy Imparted (J)


FIGURE 4-5: Histogram distribution of the total energy imparted to patients undergoing
diagnostic angiographic and therapeutic embolization neuroradiologic
procedures.



Pediatric Patient Doses

The following sections summarize the energy imparted and effective doses to

pediatric patients from interventional neuroradiologic procedures. Twenty-six pediatric

patients (younger than 20 years of age), sixteen of them underwent diagnostic









angiographic and ten underwent therapeutic embolization procedures, were studied.

Fluoroscopy and radiographic acquisitions were reviewed separately.


Sou
(I) L




S3
0
0U
o 20
L.
0
.0
E
z10




0


0-10 60-70 120-130
Effective Dose (mSv)


FIGURE 4-6: Histogram distribution of the total effective dose to patients from biplane
neuroradiologic examinations.



Energy Imparted

Figure 4-7 plots the energy imparted to pediatric patients from fluoroscopy as a

function of patient mass. The mass of each patient was interpolated from Table 4-6

according to the age of the patient. As Figure 4-7 shows, there is no significant

correlation of energy imparted to patient mass. The median value of energy imparted

from fluoroscopy to all pediatric interventional neuroradiologic procedures was 1.04 J.

Pediatric patients who underwent therapeutic embolization procedures had a median of









1.62 J. The median value of energy imparted from fluoroscopy to adult patients who

underwent interventional neuroradiologic procedures was 1.77 J.


A
S4-
(0

E

I-l
0 2-
C


40
Patient Mass (kg)


FIGURE 4-7:


Energy imparted as a function of patient mass from fluoroscopy during
interventional neuroradiologic procedures on pediatric patients. Line
shows the linear fit between energy imparted and patient mass.


Figure 4-8 plots the energy imparted to pediatric patients from radiographic

acquisitions as a function of patient mass. As was the case for fluoroscopy, there was no

significant correlation between the energy imparted from radiographic acquisitions and

patient mass. The median value of energy imparted from radiographic acquisitions to all

pediatric interventional neuroradiologic procedures was 2.01 J. Pediatric patients who

underwent therapeutic embolization procedures had a median value of energy imparted of


BIPLANE FLUOROSOPY
Therapeutic
x Diagnostic




UX
*X
x


m m x=.0 ______

5V
A N i i , , , t , , I x t , , t N t i









2.61 J. The median value of energy imparted from radiographic acquisitions to adult

patients who underwent interventional neuroradiologic procedures was 4.30 J.


BIPLANE RADIOGRAPHY
Therapeutic
12 X Diagnostic


9 U

x
6 X
r'= 0.17 .--''

3-X
3 X
,., x x
m X X X
n I I If l i i i i i i i i i i !- i 1 1 1 i 1 i- j 1 1 1 1 1- I i i i i
0
n . .. . . . .


40
Patient Weight (kg)


FIGURE 4-8: Energy imparted as a function of patient mass from radiographic
acquisitions during interventional neuroradiologic procedures on
pediatric patients. Line shows the linear fit between energy imparted and
patient mass.


Figure 4-9 plots the total energy imparted to pediatric patients during

interventional neuroradiologic procedures as a function of patient mass. As Figure 4-9

shows, there was no significant correlation between total energy imparted from

interventional neuroradiologic procedures and patient mass. The median value of total

energy imparted to all pediatric interventional neuroradiologic procedures was 3.45 J.

Pediatric patients who underwent therapeutic embolization procedures had a median

value of energy imparted of 4.09 J. The median value of energy imparted from









radiographic acquisitions to adult patients who underwent interventional neuroradiologic

procedures was 6.69 J.


20



-15
"O

E 10
C


'U


40
Patient Mass (kg)


FIGURE 4-9:


Energy imparted as a function of patient mass from interventional
neuroradiologic procedures on pediatric patients. Line shows the linear
fit between energy imparted and patient mass.


Effective Doses

Figure 4-10 plots the total effective dose to pediatric patients from interventional

neuroradiologic procedures as a function of patient mass. Although the pediatric data of

the total effective doses are widely scattered (r2 = 0.3), a linear correlation between

effective dose and patient mass is evident. The median value of total effective dose to all

pediatric interventional neuroradiologic procedures was 44 mSv and was higher

compared to the median value of 36 mSv effective dose to adult patients. Pediatric


FLUOROSCOPY
&
RADIOGRAPHY
Therapeutic
x Diagnostic



t
x
,X





: M X X
. . . . . . . . . . . .








patients who underwent therapeutic embolization procedures had a median value of 66

mSv effective dose.


250


200


150-


100-


50 -


40
Patient Mass (kg)


FIGURE 4-10: Effective dose as a function of patient mass from interventional
neuroradiologic procedures on pediatric patients. Line shows the linear
fit between effective dose and patient mass.



Discussion

Major errors in determining energy imparted to patients result when estimating the

equivalent water phantom thickness, z, and due to the implicit differences between a

(finite) heterogeneous patient and a semi-infinite homogeneous water phantom. Figure

4-1 shows the energy imparted per unit exposure-area product, c, as a function of

phantom thickness for a range of x-ray tube voltages. The largest increase of co with

phantom thickness is expected at small thicknesses, given that the mean free path of


FLUOROSCOPY
&
X RADIOGRAPHY

Therapeutic
x Diagnostic

x
X

Adult median

X r' 00. 3xK


X K1


t









monoenergetic photons in water ranges from 4.4 cm at 50 keV to 6.6 cm at 140 keV.

Once the phantom thickness reaches about three or four mean free paths, most of the x-

ray photons will have been absorbed and any further increase of the phantom thickness

will have little affect on Cw.

Figure 4-1 shows that at 80 kV, the thickness of the water phantom used to simulate

a patient for the purposes of estimating energy imparted generally will not be a critical

parameter for applications with phantom thicknesses greater than 15 cm. Since the water

equivalent size of an adult head is between 18 cm (lateral view) and 22 cm (frontal view),

small deviations from the average sizes given in Table 4-3 will have a minimal effect on

the computation of energy imparted. A difference between a 20 cm and 22 cm phantom

thickness at 80 kVp is of the order of 2%. Even for pediatric patients where the size of

the head is smaller (13 cm to 17 cm for 1-yr-olds), a 2 cm error in estimating the water

equivalent thickness will result in a maximum error of about 5% when computing energy

imparted.

Minor errors in computing energy imparted to patients arise from the use of

diverging x-ray beams in clinical applications and the presence of nonuniformities in x-

ray beam intensity due to the heel effect. The former is likely to be of negligible

importance whereas the latter could easily be accounted for by experimentally obtaining

an average entrance skin exposure over the beam area. Measuring the exposure at the

centerline of the x-ray beam is also a good approximation of the average exposure over

the entire field. Another error in determining energy imparted from irradiation to the

head region results from occasional use of wedge shaped transparent filters which serve

to reduce the radiation doses at the periphery of the x-ray field of view whilst maintaining









image quality within the central region. Such filters are used during imaging of the

frontal view of the head and can attenuate the entrance exposure by about 50% at 80 kVp.

As these filters cover an area between 10% and 20%, an overestimate of the energy

imparted from frontal imaging plane fluoroscopy of the order of 5% to 10% can occur.

Use of Equation (4.4) permits the determination of the approximate values of

effective doses to pediatric patients who undergo radiologic examinations. The NRPB

has recently published dosimetric data on pediatric patients ranging from the newborn to

15 year olds (Hart et al., 1996). Figure 4-11 shows a comparison between the Ele values

obtained using Equation (4.4) (continuous line) with the NRPB data (solid circles), which

were determined by performing Monte Carlo calculations in a range of anthropomorphic

phantoms of different age. Differences between these two data sets, when averaged over

the five ages investigated, were 17% with the largest differences shown for the 1-yr-old

(31%) and 5-yr-old (36%) phantoms. Such differences may be due to pediatric heads

accounting for a markedly larger fraction of the total body masses in these ages compared

to adults. It is of interest to note, however, that use of different types of anthropomorphic

phantoms to determine pediatric effective doses in planar radiography can result in

differences in effective dose of the order of 30% (Hart et al., 1996b).

In general, the effective doses computed in this work compare three to six times

higher to values published by others for similar interventional neuroradiologic procedures

(Feygelman et al., 1992; Bergeron et al., 1994; McParland, 1998; Berthelson and

Cederblad, 1991). However, all other reported values refer to limited number of 8-28

procedures, and none of them made use of means of recording radiation exposures in real

time. Different imaging equipment, setup and imaging procedures among institutions









play a major role to how different effective doses may be among institutions. The fact

that Shands hospital at the University of Florida is an academic institution that trains new

neurointerventional radiologists may also account for some of the differences between the

recorded effective doses in this work and others.




Conclusions

Values of energy imparted from interventional neuroradiologic procedures were

high due to the demands and complexity of these procedures. The median value of the

total energy imparted to adult patients who underwent interventional neuroradiologic

procedures was 6.69 J. Pediatric patients received a median value of energy imparted of

3.45 J from interventional neuroradiologic procedures. In the case of therapeutic

embolization procedures, additional use of fluoroscopy is required for catheter

manipulation and positioning at the site of occlusion, as well as extensive radiographic

acquisitions to evaluate the progress of the occlusion. Such demands increased the

median values of energy imparted to adults undergoing therapeutic embolization

procedures to 13.3 J. Pediatric embolizations received a median value of 4.09 J. Overall,

radiographic acquisitions accounted for two thirds of the total energy imparted, with

fluoroscopy contributing only one third. There was no significant correlation between

energy imparted from interventional neuroradiologic procedures and patient mass.










1000-

S/ Equation (4.4)


Newborn
S100-



^1 -yr
-to

S5-yr 10-yr Adul
:15-yr




1 10 100
Patient Mass (kg)

FIGURE 4-11: Comparison of Ele values vs. patient age as determined by Equation (4.4)
and by using the dosimetry data from Hart et al. (1996a).
NOTE: Values of Ele were computed for the right lateral
projection of the head.





Effective doses computed for the 149 adult patients who underwent interventional

neuroradiologic procedures had a median value of 36 mSv. Pediatric patients received a

median effective dose of 44 mSv from interventional neuroradiologic procedures. The

median effective dose to adults undergoing therapeutic embolization procedures was 74

mSv. Pediatric embolizations received a median effective dose of 66 mSv. As was the

case for energy imparted, radiographic acquisitions accounted for two thirds of the total

effective dose, with fluoroscopy contributing only one third. Unlike energy imparted,

effective doses showed a good linear correlation with patient mass.









The use of the effective dose permits an estimate of stochastic risk to be obtained

by using current stochastic risk coefficients (ICRP, 1991; UNSCEAR, 1993; NAS, 1990).

At the last attempt of the ICRP (1991) to estimate absolute stochastic risks from whole-

body irradiation, a risk coefficient of 5x10.5 cancers per mSv of effective dose was

derived. Using this risk coefficient, the median effective dose of 36 mSv to adult patients

would result in one fatal cancer for every 555 interventional neuroradiologic procedures.

An effective dose of 74 mSv to adults undergoing therapeutic embolization procedures

would result in one fatal cancer for every 270 such procedures. The immediate, life

saving benefits of interventional neuroradiologic procedures, however, far outweigh the

risk of distant stochastic effects associated with these procedures. Also, such risk

coefficients need to be treated with great caution given the current uncertainties

associated with the extrapolation of radiation risks from high doses to those normally

encountered in diagnostic radiology (Fry, 1996; Puskin and Nelson, 1996).

In the case of pediatric patients undergoing interventional neuroradiologic

procedures, it is important to note that any resultant stochastic detriment will depend on

the age of the exposed individual. The stochastic radiation risks of carcinogenesis and

genetic effects are generally greater for children than for adults to at least a factor of two

(ICPR, 1991; NCRP, 1985). These factors would need to be taken into account when

converting any pediatric effective doses into a value of risk or detriment. As a result,

direct comparisons of pediatric doses with those of adults need to be treated with

circumspection.














CHAPTER 5
IMAGE QUALITY



Image Acquisition

A phantom made of acrylic incorporating 1.0 mm diameter cylindrical vessels

filled with iodinated contrast was used to investigate signal detection during digital

subtraction angiography (DSA). The detection of signal from iodinated vessels was

evaluated by studying the threshold iodine contrast concentration detected in images

acquired under specified parameters using digital image subtraction.




Phantom Description

Figure 5-1 illustrates the phantom used to simulate 1.0 mm diameter vessels for

the purpose of evaluating image quality in neuroradiology. The phantom consists of

stacked acrylic blocks with dimensions of 30 cm x 30 cm x 1.3 cm. An insert holder

made of acrylic with a thickness of 1.3 cm is positioned at the center of the phantom to

accommodate a vessel insert. A blank and a vessel insert measuring 30 cm x 9.0 cm x

1.3 cm were made out of acrylic. The blank insert was used to acquire mask images

during digital subtraction angiography. The vessel insert had thirty cylindrical vessels

1.0 mm in diameter and 35 mm in length drilled along its midplane at intervals of 8.0 mm

apart. The total phantom thickness was 16.5 cm of acrylic, which was taken to be






87

equivalent to about 20 cm of water taking the density of acrylic to be 1.19 g/cm3 (Shleien,

1992).


Acrylic
H' I er


it~


-4------


Vessel
Insert


Blank
Insert


Insert Slide


Acrylic
Blocks


FIGURE 5-1: Schematic diagram of the acrylic phantom with the vessel and blank inserts
used to simulate small vessels for the purpose of evaluating image quality
in neuroradiology.


I-


----i









The vessels on the acrylic insert were filled with iodinated contrast prepared from

Ultravist" 300 iopromide solution diluted in heparin solution. The iodine concentrations

in the contrast medium used to fill each vessel ranged from 50 mg/cc iodine in contrast

solution to about 5.0 mg/cc as given in Table 5-1. The iodine concentration in each

vessel was made to be 92% of the previous concentration.




Acquisition of Digitally Subtracted Images

The general experimental setup shown in Figure 5-2 used an x-ray source to

image receptor distance (SID) of 105 cm (maximum SID) with the acrylic phantom

positioned so that the geometric magnification of the vessel insert was 1.2. A 10x5-6

ionization chamber of an MDH 1015C*** exposure meter was attached to the beam

entrance surface of the phantom to record entrance exposure. A 10x5-60tt1 ionization

chamber of a second MDH 1015C exposure meter was attached to the surface of the

image intensifier behind the grid to record the input exposure to the image receptor. Both

ionization chambers were positioned carefully not to overlap with the vessels of the

vessel insert as shown in Figure 5-3.

The 23 cm diameter image intensifier mode was used for all image acquisitions.

The generator was set to manual techniques allowing fine adjustments of the tube voltage

(kVp), tube current (mA) and exposure time (ms). The optical gain was electronically

adjusted by changing the diameter of the iris located between the image intensifier output

phosphor and TV camera lens to produce a constant video level. All digital subtraction


H Berlex Laboratories, Wayne, NJ
**" Radcal Corporation, Monrovia, CA.