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1 MULTI SCALE PHOTOACOUSTIC TOMOGRAPHY AND ITS APPLICATIONS TO CANCER RESEARCH By LEI XI 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 2012
2 2012 Lei Xi
3 To my wife, Hui J in; My mom and my dad; And all who have been supportive to me in my life
4 ACKNOWLEDGMENTS Firstly, I would like to thank Dr. Huabei Jiang for his support guidan ce and encouragement for me during all my years as a graduate student. Withou t his full financial support, constructive advice and discussions, I would not have the opportunity to make progress in my projects. Secondly, I wish to thank my committee membe rs Dr. Huikai Xie, Dr. Sihong Song and Dr. Rosalind Sadleir. Their comments and suggestions were very helpful for my research. I also appreciate their valuable time spent on reading my proposal and dissertation. Thirdly, thanks for help from Dr. Stephen G robmyer, an associate professor on Department of Surgery, on guidance and suggestions to our intraoperative photoacoustic tomography system. As our research partner, Dr. Huikai Xie provided indispensable MEMS devices for our imaging systems Additionally, I would like to thank Dr. Lily Yang providing nanoparticles for our photoacoustic molecular study and Dr. Jun Cai's valuable collaboration on anti angiogenesis medicine study using photoacoustic microscopy technique Last but not the least, I would like to take the opportunity to express my thanks to Qizhi Zhang, Yao Sun, Zhen Yuan, Lei Yao, Ruixin Jiang, Hao Yang, Xiaoqi Li, Lijun Ji, Bin He, Bo Wang, Jingjing Sun, Can Duan, Wenjun Liao and all members in our lab and our collaboration lab for their assis tance, suggestions and the great time we spent together.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 18 1.1 Background and Motivation s ................................ ................................ ............. 18 1.2 Overview of Photoacoustic Tomography for Cancer Research ........................ 19 2 OPTICAL RESOLUTION PHOTOACOUSTIC MICROSCOPY AND ITS APPLICATION ................................ ................................ ................................ ........ 23 2.1 Optical resolution Photoacoustic Microscopy ................................ ................... 23 2.2 Materials and Methods ................................ ................................ ...................... 23 2.3 In V ivo A nimal E xperiments D emonstration ................................ ...................... 24 2.4 Miniature Hybrid Probe Combining ORPAM and OCT for Endooscopic Vascular Visualization ................................ ................................ .......................... 24 2.4.1 M otivation s ................................ ................................ .............................. 24 2.4.2 Materials and M ethods ................................ ................................ ............ 2 5 2.4.3 Results and D iscussion ................................ ................................ ........... 27 3 ACOUSTIC RESOLUTION PHOTOACOUSTIC MICROSCOPY ........................... 33 3.1 Motivations ................................ ................................ ................................ ........ 33 3 .2 Materials and M ethods ................................ ................................ ...................... 33 3 .3 Quantitative ARPAM ................................ ................................ ......................... 34 3 .4 Phantom V alidation ................................ ................................ ........................... 37 3 .5 In V ivo A nimal E xperiments V alidation ................................ ............................. 38 4 APPLICATIONS OF ACOUSTIC RESOLUTION PHOTOACOUSTIC MICROSCOPY TO PRECLINICAL CANCER RESEARCH ................................ .... 44 4 .1 Photoacoustic I maging of T umor V asculature D evelopment for B reast C ancer R esearch ................................ ................................ ................................ 44 4 .1.1 Motivations ................................ ................................ .............................. 44 4 .1.2 Materials and M ethods ................................ ................................ ............ 47
6 188.8.131.52 Const ruction of scAAV2 vector for delivering siRNAs against the UPR proteins ................................ ................................ .......................... 47 184.108.40.206 scAAV2 infection of cells in vitro ................................ .................... 48 220.127.116.11 Microscopy and fluorescence activated cell sorting (FACS) analysis ................................ ................................ ................................ ... 48 18.104.22.168 Cell culture ................................ ................................ ..................... 48 22.214.171.124 In vitro angiogenesis ass ay ................................ ............................ 50 126.96.36.199 Apoptosis assay ................................ ................................ ............. 50 188.8.131.52 Proliferation assay ................................ ................................ ......... 51 184.108.40.206 Mice breast cancer xenograft models ................................ ............ 51 220.127.116.11 Photoascoustic (PA) imaging systems and noninvasive monitoring in vivo tumor angiogenesis ................................ .................... 52 18.104.22.168 Statistics analysis ................................ ................................ ......... 52 4 .1.3 Results ................................ ................................ ................................ .... 53 22.214.171.124 scAAV2 sept mut exhibits higher transduction effic iency in MMECs ................................ ................................ ................................ ... 53 126.96.36.199 Septuplet tyrosine mutations improve the transduction efficiency of scAAV2 mediated siRNAs infection in MMECs ................................ .. 54 188.8.131.52 siRNA knockdown of the UPR proteins decreased NeuT EMTCL2 induced in vitro angiogenic activity of endothelial cells ............ 54 184.108.40.206 siRNA knockdown of the UPR proteins down regul ated the survival of endothelial cells ................................ ................................ ..... 55 220.127.116.11 Different malignant mice breast cancer cells induced differential agngiogenic responses in breast cancer xenograft models were monitored by se rial photoacoustic (PA) imaging ................................ ..... 56 18.104.22.168 Knockdown of the UPR proteins significantly inhibited Neut EMTCL2 induced in vivo angiogenesis ................................ ................... 57 4 .1.4 Discussion ................................ ................................ ............................... 57 4 .2 uPAR T argeted M agnetic I ron O xide a s a C ontrast A gent for I n V ivo M olecular P hotoacoustic T omography of B reast C ancer ................................ ..... 62 4 .2.1 Motivations ................................ ................................ .............................. 62 4 .2.2 Material and M ethods ................................ ................................ .............. 64 22.214.171.124 Cell line ................................ ................................ .......................... 64 126.96.36.199 Preparation of NIR830 ATF IONP ................................ ................. 64 188.8.131.52 Animal preparation ................................ ................................ ......... 65 184.108.40.206 Photoacous tic microscopy imaging system ................................ .... 65 220.127.116.11 Near infrared planar fluorescence imaging system ........................ 66 18.104.22.168 Image processing ................................ ................................ ........... 67 22.214.171.124 Histologic analysis ................................ ................................ .......... 67 126.96.36.199 Statistical analysis ................................ ................................ .......... 67 4 .2.3 Re sults ................................ ................................ ................................ .... 67 4 .2.4 Discussion and S ummary ................................ ................................ ........ 69 4 .3 Photoacoustic a nd F luorescence Tomography o f HER 2/Neu Positive Ovarian Cancers Usi ng Receptor Targeted Nanoprobes In An Orthotopic Human Ovarian Cancer Xenograft Model ................................ ............................ 71 4 .3.1 Motivations ................................ ................................ .............................. 71
7 4 .3.2 Materials an d Methods ................................ ................................ ............ 73 188.8.131.52 Cell lines ................................ ................................ ........................ 73 184.108.40.206 HER 2/neu specific affibody conjugation to IONP .......................... 73 220.127.116.11 Orthotopic human ovarian cancer xenograft model ....................... 73 18.104.22.168 Fluorescence molecular tomography imaging System. .................. 74 22.214.171.124 In vivo and ex vivo planar near infrared fluorescence imaging ....... 74 126.96.36.199 Histolog ical analysis ................................ ................................ ....... 75 188.8.131.52 Statistical analysis ................................ ................................ .......... 75 4 .3.3 Results ................................ ................................ ................................ .... 75 4 .3.4 Discussion ................................ ................................ ............................... 77 5 INTRAOPERATIVE PHOTOACOUSTIC TOMOGRAPHY ................................ ...... 96 5 .1 M otivation s ................................ ................................ ................................ ........ 96 5 .2 Materials and M ethods ................................ ................................ ...................... 97 5.2.1 Animal P rotocol ................................ ................................ ....................... 97 5 .2. 2 Intraoperative P hotoacoustic T omography S ystem ................................ 97 5 .2. 3 Imaging P rocedure ................................ ................................ .................. 99 5 .2. 4 Histology ................................ ................................ ................................ .. 99 5 .3 Spatial R esolution of t he S ystem ................................ ................................ .... 100 5.4 Phantom E xperiments ................................ ................................ ..................... 100 5 5 Human Blood V essels E xperiments ................................ ................................ 101 5 6 Preclinical Evaluation o f Intraoperative Photo acoustic Tomography .............. 101 5 .7 Discussion ................................ ................................ ................................ ...... 103 6 CIRCULAR ARRAY BASED PHOTOACOUSTIC TOMOGRAPHY AND ITS APPLICATION TO BREAST CANCER DETECTION ................................ ........... 117 6.1 Motivations ................................ ................................ ................................ ...... 117 6.2 Materials and Methods ................................ ................................ .................... 118 6.2.1 System D escription ................................ ................................ ................ 118 6.2.2 Quantitative R econstruction M ethods of PAT and DOT ........................ 120 6 .3 Performance Evaluation and Phantom Experiments ................................ ....... 122 6 .3.1 Performance of PAT ................................ ................................ .............. 123 6 .3. 2 Phantom E xperiments ................................ ................................ ........... 124 6 .4 Ex Vivo Experiment ................................ ................................ ......................... 126 6 5 Multilayer Ultrasound Transducer w ith Improved Sensitivity a nd Bandwidth .. 126 6 .6 D isc ussion and F uture D irections. ................................ ................................ .. 130 7 CONCLUSION AND FUTURE WORK ................................ ................................ .. 146 LIST OF REFERENCES ................................ ................................ ............................. 147 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 163
8 LIST OF TABLES Table page 3 1 Comparison of exact and reconstructed values (absorption coefficient a nd size) in the target area for all 6 experimental cases ................................ ........... 43 6 1 Exact size (diameter), location (off center), depth, and absorption and reduced scattering coefficients of the five targets ................................ ............. 144 6 2 Exact and reconstructed target size (diameter), location (off center) and absorption and reduced scattering coefficients for the phantom experiments .. 145
9 LIST OF FIGURES Figure page 2 1 Schematic of ORPAM system ................................ ................................ ............ 30 2 2 MAP photoacoustic images of mice ear ................................ ............................. 30 2 3 Schematic and photograph of the hybrid probe ................................ .................. 31 2 4 Schematic of the integrated ORPAM and OCT system ................................ ...... 31 2 5 Resolution test for ORPAM and OCT ................................ ................................ 32 2 6 In vivo imaging of mouse ear by the integrated probe ................................ ........ 32 3 1 Our experimental PAM system ................................ ................................ ........... 40 3 2 Reconstructed absorption coefficient images (mm 1 ) from all 6 experimental cases ................................ ................................ ................................ .................. 40 3 3 Reconstructed absorption coefficient profiles ................................ ..................... 41 3 4 In vivo imaging of the blood vessels in a rat ear ................................ ................. 42 4 1 Comparative analysis of scAAV2 mediated transduction of MMECs .................. 80 4 2 scAAV2 encoding siRNAs against UPR proteins ................................ ................ 81 4 3 Pro angiogenic and s urvival role of the UPR proteins on endothelial cells ......... 82 4 4 Serial photoacoustic imaging of the developing tumor vasculature and quantitative analysis for mice breast cancer xenograft models .......................... 83 4 5 Knockdown of the siRNAs against the UPR protein resulted in decreased tumor growth and tumor vasculature in mice breast cancer xenografts .............. 84 4 6 Illustration of NIR 830 dye and ATF conjugated IONP probe ............................. 85 4 7 Schematic of NIR fluorescence imaging system ................................ ................. 85 4 8 In vivo and in vitro test of targeted nanoprobe using different wavelengths ....... 86 4 9 In vivo photoacoustic MAP and fluorescence images before and after injection ................................ ................................ ................................ .............. 87 4 10 Quantitative plot and comparison of photoacoustic and fluorescent signals ....... 88 4 11 In vivo photoacoustic images with adding chicken breast b etween detector and tumor ................................ ................................ ................................ ........... 89
10 4 1 2 Schematic and spectrum of targeted nanoprobe and FMT system description .. 90 4 13 Multi mode in vivo imaging and histolog ical validation ................................ ....... 91 4 14 Quantitative analysis based on different wavelengths ................................ ........ 92 4 15 Image depth abilit y evaluation ................................ ................................ ............ 93 4 16 3D results for both PAMT and FMT ................................ ................................ .... 94 4 17 Quantitative comparison of spatial resolution between photoacousti c and fluorescence molecular tomography images with increased imaging depth ....... 95 5 1 System description of the experimental platform ................................ .............. 106 5 2 Schematic representation of MEMS based photoacoustic imaging system ..... 107 5 3 Schematic and performance of a ring shaped ultrasound transducer .............. 108 5 4 Schematic of the miniaturized probe ................................ ................................ 109 5 5 Performance evaluation of the system ................................ ............................. 11 0 5 6 Phantom experiment with single target ................................ ............................. 111 5 7 Diagram of the phantom and imaging result with multiple targets .................... 111 5 8 Ex vivo experimen t with single target ................................ ............................... 112 5 9 In vivo experimental evaluation using human hand ................................ .......... 112 5 10 In vivo three dimensional (3D) tumor mapp ing in a mouse model .................... 113 5 11 Quantitative analysis of the photoacoustic slices and H&E stained sections .... 114 5 12 Quantitative a nalysis of photoacoustic images in correlation with H&E sections with increasing image depth ................................ ............................... 115 5 13 Design of future imaging probe ................................ ................................ ......... 116 6 1 System description ................................ ................................ ........................... 132 6 2 Performance evaluation of the hybrid system ................................ ................... 133 6 3 System resolution evaluation ................................ ................................ ............ 134 6 4 System performance evaluation by phantom experiments ............................... 135 6 5 Quantitative PAT and DOT images of ta rget s ................................ ................... 136
11 6 6 Reconstructed optical properties of ex vivo tumor ................................ ........... 140 6 7 Schematic of the multi layered transducer and performance evaluation systems ................................ ................................ ................................ ............ 141 6 8 Calibration of multi layered transducer ................................ ............................. 141 6 9 Frequency response of mult ilayer ed and single layered PVDF transducer ...... 142 6 10 In vivo experimental evaluation of multi layered transducer ............................. 143 6 11 In vivo photoacoutic imaging of mouse hea d using multilayer ed and single layered transducer ................................ ................................ ............................ 144
12 LIST OF ABBREVIATION S 1D One dimensional 2D Two dimensional 3D Three dimensional AAV A deno associated V irus ANOVA Analysis of Variance ARPAM Acoustic resolution Photoacoustic Micr oscopy ATF Amino terminal Fragment s ATF6 A ctivating T ranscription F actor 6 ATF IONP A mino terminal fragment s of uPA conjugated to iron oxide nanoparticles BCT Barker Coded Transducer BSA Bovine Serum Albumin bZIP Basic Leucine Zipper CBA CMV chicke n actin CO 2 Carbon Dioxide CT Computed Tomography CW Continuous Wave DAQ Data Acquisition DOT Diffuse Optical Tomography DRS Diffuse Reflectance Spectroscopy EDAC E thyl 3 dimethyl A mino P ropyl C arbodiimide EMS Electromagnetic Shield ER Endoplasm ic Reticulum ESS Elastic Scattering Spectroscopy FACS Fluorescence Activated Cell Sorting
13 FCS Fetal Calf Serum FD Frequency Domain FE Finite Element FEM Finite Element Method FMT Fluorescent Molecular Tomography FT Folded Transducer FWHM Full Widt h Half Maximum G1 Generation one G2 Generation two GFP Green Fluorescent Protein GRIN Graded Index H&E Hematoxylin and Eosin Hb Hemoglobin His6 Z HER2:342 Cys H istidine tagged HER 2 specific A ffibody IACUC Institutional Animal Care and Use Committee ICG Indocyanin Green IONP Magnetic I ron O xide N anoparticles iPAI Intraoperative Photoacoustic Imaging iPAT Intraoperative Photoacoustic Tomography IRE1 I nositol requiring P rotein 1 IUPUI Indiana University Purdue University at Indianapolis LOIS Laser based Optoacoustic Imaging System MAP Maximum Amplitude Projection MEMS Microelectromechanical System MMECs Mice Microvascular Endothelial Cells
14 MOI Multiplicity of Infection MRI Magnetic Resonance Imaging NIR Near infrared NIR 830 Near infrar ed 830 NIR830 ATF IONP Near infrared Dye Amino terminal Fragment s Iron Oxide N anoparticles NIR830 Z HER2:342 IONP N ear infrared D ye HER 2/neu specific A ffibody IONP NTA Nitrilotri acetic Acid OCT Optical Coherence Tomography ORPAM Optical resolution Ph otoacoustic Microscopy PAM Photoacoustic Microscopy PAT Photoacoustic Tomography PCR Polymerase chain reaction PERK PKR like ER Kinase PET Positron Emission Tomography PET Positron Emission Tomography PMN PT Pb(Mg1/3Nb2/3)O3 PbTiO3 PO Prophylactic Oophorectomy PVDF Polyvinylidene fluoride PZT Lead zirconate titanate RBC Red Blood Cell RSOD Rapid Scanning Optical Delay RTE Radiative Transfer Equation SBCT S witchable Barker C oded T ransducer scAAV Self complementary Adeno associated Virus scAAV2 Self complementary Adeno associated Virus Serotype 2
15 SEM Standard Error of the Mean SNR Signal to Noise Ratio SPECT Single Photon Emission Computed Tomography sulfoNHS S ulfo N H ydroxysu ccinimide TAT Thermoacoustic Tomography TD Time Domain uPAR Urokinase plasminogen receptor UPR Unfolded Protein Response UV Ultraviolet VEGF Vascular Endothelial Growth Factor XBP 1 X box binding P rotein 1 Y F Tyrosine to phenylalanine
16 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 MULTI SCALE PHOTOACOUSTIC TOMOGRAPHY AND ITS APPLICATIONS TO CANCER RESEARCH By L ei Xi December 2012 Chair: Huabei Jiang Major: Biomedical Engineering Photoacoustic tomography (PAT) is an emerging non ionizing, non invasive biomedical imaging modality combining high optical contrast with high ultrasound resolution. It offers a uniqu e ability of multi scale imaging from microscopic, mesoscopic, to macroscopic for biological tissues. This dissertation intends to develop a spectrum of strategies that aim to explore and demonstrate such multi scale imaging ability of PAT and its applica tions to cancer research. The first part of my dissertation discusses the development of optical resolution photoacoustic microscopy (ORPAM) owning high resolution and high sensitivity in vivo imaging. Here we first describe the conventional ORPAM system including the principle s system design, experimental procedures and a simple application of visualizing microvascular structure in a m ouse ear. Subsequently, a miniature hybrid probe combing ORPAM with optical coherence tomography (OCT) is proposed and de monstrated with in vivo animal experiments This hybrid ORPAM/OCT technique shows the potential for endoscopic cancer research. The second part of the dissertation focuses on developing acoustic resolution photoacoustic microscopy (ARPAM) that break s the l imitation of imaging depth associated with ORPAM. We
17 demonstrate the applications of ARPAM to anti angiogenesis medicine study of breast cancer and to molecular imaging of breast and ovarian cancers with targeted nanoprobes The third part of my dissertati on is concerned with intraoperative photoacoustic tomography (iPAT) miniature probe for image guided surgery of breast cancer. The probe is evaluated with phantom and in vivo experiments i n an animal mode l The fourth part of my dissertation focuses on int egrating circular array based photoacoustic tomography system with diffuse optical tomography system. Several sets of phantom experiments with embedded tumor mimicking targets and ex vivo tumor are used to evaluate the performance of this hybrid system. I n the last part of the dissertation conclusion s are made and future direction s are discussed
18 CHAPTER 1 INTRODUCTION 1.1 Background and M otivation s Photoacoustic tomography (PAT) referred to as thermoacoustic tomography (TAT) is a promising biomedical i maging technique based on light generated acoustic wave over last decade It combines the high contrast of pure optics imaging modalities with the high ratio of imaging depth to spatial resolution of ultrasound. The research history of photoacoustic phenom enon study is relatively long. In 1880, Alexander Graham Bell observed the generated acoustic wave in solid due to absorption of rapid interrupted sunlight 1 2 The same effect was observed in gases and liquids by other researchers in their following experi ments. The invention of lasers in the 1970s revived the applications of photoacoustic effect. 3 Until the mid 1990s, researchers began to employ photoacoustic effect for biomedical imaging. 4 8 From the beginning of 2000s, this field had a rapid growth in te rms of the development of ultrasound transducer 9 and imaging reconstruction algorithms 10 13 realization of functional imaging with multi wavelength excitation source 14 15 and mole cular imaging with molecular probes 16 17 as well as the in vivo clinical tr ies for human breast cancer detection 18 22 During photoacoustic experiments short pulsed laser is commonly used to generate ultrasound waves. For photoacoustic, optical wavelength s from visible to near infrared (NIR) are used, where NIR spectrum range ( 700 900nm) lies in optical transparent window provid ing the greatest penetration depth. 23 24 Owning to the use of visible or NIR light, there is no radiation issues compared with convent ional X ray imaging techniques. 25 26 From the fundamental of photoacou stic, absorption of light by specific tissue absorbers such as hemoglobin, melanin, water et al. results in a rapid
19 temperature rise that leads to an initial pressure which subsequently releases generating the emission of acoustic signals. Hence, PA imagin g is very sensitive to absorption from intrinsic contrast for optics which pr ovide the potential for excavat ing the functional parameters such as concentration of hemoglobin, oxygen saturation blood flow et al. 27 35 In addi ti on, exogenous absorbers such as gold nanoparitcles, iron oxide, indocyanin gree n (ICG) et. al. can provide external contrast enhancement for photoacoustic imaging providing the opportunities to recover molecular information 36 41 The motivation driv es us to investigate photoacoustic i maging due to some challenges for pure optics imaging modalities. There are two main challenges diffraction and diffusion. For diffraction, it limits the spatial resolution of ballistic imaging techniques such as confocal microscopy two photon microscopy and optical coherence tomography (OCT) 42 43 This has been overcome by super resolution techniques. 44 Diffusion limits the penetration of ballistic imaging techniques to be 1mm in human tissue due to its high scattering properties. Photoacoustic imaging h as the potential to overcome this limitation. In photoacous tic imaging, the photon s diffuse inside the scattering tissue too h owever, all the photons arriving at the targets are useful photons which will be absorbed by the targets T hen most part of the a bsorbed energy is convert ed to acoustic signals. A ddition ally the scattering of acoustic signal inside tissue is 100 times weaker than that of light. 1.2 Overview o f Photoacoustic Tomography for Cancer R esearch There are two main types of photoacoustic im aging modalities including photoacoustic microscopy and reconstruction based photoacoustic tomography. For photoacoustic microscopy, the photoacoustic imaging is obtained through mechanically scanning of a focused transducer or a focused laser beam. The a cquired
20 A lines are used to form a n image without any reconstruction algorithms. If a focused laser beam is used, it is called optical resolution photoacoustic microscopy (ORPAM). 45 If using a focused transducer without light beam focusing it is termed ac oustic resolution microscopy (ARPAM). 46 Typically, the resolutions for ORPAM is from sub micro to several micrometers while the imaging depth is limited to be less than 1mm. For ARPAM, the resolutions is various determined by the transducer's aperture, ban dwidth and focal length, usually from 30 micrometers to 1mm, meanwhile the depth is from seve ral millimeters to centimeters. 47 Currently, ORPAM is the rapides t developed photoacoustic technique among all photoacoustic modalities 48 55 However, the applicat ion of ORPAM to cancer research i s limited due to the shallow pene tration depth. The most common application is to visualize the n eovascularization inside mice or rat ears to investigate some anti angioge netic factors 54 55 This is very useful for study of some anti angiogenesis medicine during cancer treatment research. Another application of cancer related research is employing ORPAM to analyz e histology sections which is still under investigation The applications of ARPAM are much wider than those of O RPAM. 56 62 If a high frequency focused transducer is used, ARPAM can be used to visualize the neovascularization not only inside mouse/rat ear but also around the tumor in animal model because the imaging depth is deeper (up to 3mm if 50MHz transducer is u sed) compared with ORPAM If a low frequency focused transducer is used, this techniq ue can be used to imaging large target s such as lymph node, brain tumor, breast tumor located deep in tissues Some group s also reported their studies to employ ARPAM to
21 i mag e various targets with contrast agent such as ICG, blue dye, gold nanorod, iron oxide et al. Reconstruction based p hotoacoustic tomography is regarded as the traditional mode of photoacoustic imaging. It is also the most commonly used and least restri ctive photoacoustic imaging technique In PAT, a large diameter pulsed laser beam illuminate s the full imaging field and N IR wavelengths enable the deep penetration. Various methods for reconstructing the PAT image from the detected signals have been devel oped such as back projection, Fourier transform, P transform, k wave method and finite element method (FEM) based reconstruction algorithm. 63 65 Among all these reconstruction methods, FEM can recover accurate quantitative acoustic and optical properties. The first and most important clin ical application of PAT is breast cancer detection. Also t he detection and analysis of lymph node by reflection mode PAT has the great potential to be used in clinical. Meanwhile, some researchers also use d three dimensiona l (3D) PAT to monitor angiogenesis inside human breast. For endoscopic area, several endoscopic probes have been reported for ovarian canc er and colon cancer detection. 66 68 In this study, we developed several photoacous tic imaging systems covering from ma cro scale to micro scale area for various cancer applications. In Chapter 2, we will introduce ORPAM and its a pplication for endoscopic cancer research integrated with OCT In Chapter s 3 and 4, the ARPAM system and its applications on anti angiogenesis med icine studies on breast tumor, molecular imaging of breast cancer and ovarian cancer u sing targeted contrast agent Q uantitative ARPAM will be described in detail too In Chapter 5, we will move to pre clinical evaluation of intraoperative photoacoustic
22 t omography (iPAT) during breast cancer surger y on an animal model. In Chapter 6, we will introduce newest results of combining circular array based PAT integrated with diffuse optical tomography ( DOT ) for breast cancer detection. Finally, in Chapter 7, the summary of my dissertation and future directions are presented.
23 CHAPTER 2 OPTICAL RESOLUTION PHOTOACOUSTIC MICROS COPY AND ITS APPLICA TION 2.1 Optical resolution Photoacoustic M icroscopy ORPAM uses optical confinement for localization purpose and is simil ar to many conventional optical microscopy techniques where the lateral resolution is determined by the dimensions of a focused light spot Due to high optical scattering in most tissues, the imaging depth is limited to be less than 1mm. However, it has th e highest lateral resolution of 5 m or even better compared with that of ARP AM and circular scanning photoacoustic tomography As a powerful optical absorption based microscopy technique and a valuable complement to the e xisting microscopy techniques, ORPA M has been demonstrated to be used in wide biomedical areas. In this chapter, we will introduce the ORPAM system and show the potential application for cancer research using ORPAM. 2 .2 Materials and M ethods As shown in Fig ure 2 1 the ORPAM system employs optical focus ed laser beam from a Nd:YAG pulsed laser to achieve micro level lateral resolution. Laser pulses with duration of 6ns and repetition rate of 10Hz are spatially filtered by a 50 m diameter pinhole. Then the laser beam is collimated by a convex lens and focused by a n objective lens to obtain a focal spot with a diameter of 5 m in lateral The laser energy on the tissue surface is 25mJ/cm 2 which is still under the reported skin damage threshold 69 A 50MHz focused transducer with 3mm activate area and 6mm focal length is confocal led with the focused light beam. Combination of depth resolved photoacoustic waves with a 2D raster scanning along the x y plane generates the volumetric images.
24 2.3 In V ivo A nimal E xperiments D emonstration To demonstrate the imaging abilit ies of our ORPAM system, we employed the ORPAM to visualize the microvasculature in an ear of a nude mouse (body weight: 25g ) in vivo with excitation source at 532nm wavelength All an imal procedures were conduct ed in conformity with the laboratory animal protocol approved by Animal Studies Committee of University of Florida. Before experiments, the mice were anesthetized with a mixture of Ketamine (85mg/kg) and Xylazine An imaging ar ea covering 11 mm 2 was scanned with a step size of 6 m without any signal averag e In Fig ure 2 2A a typical microvasculature of mice ear is shown. From the result, we can easily identify even the single capillary with size of less than 5 m. As indicated, red blood cell (RBC) can be seen a mong these capillar ies too In Fig ure 2 2B the top panel shows the result of ORPAM and bottom panel shows the photograph taken with a transmission optical microscope at a 4 magnification. These two blood vessels are clearly identified in ORPAM; however they cannot be seen b y a transmission optical microscope. 2.4 Miniature Hybrid Probe Combining ORPAM and OCT for Endooscopic Vascular V isualization 2.4.1 M otivation ORPAM is fundamentally sensitive to optical absorbers inside the tissue such as hemoglobin and melanin. Hence, O RPAM is well suited for imaging bloo d vessels However, it is hard for ORPAM to image tissues with low absorption contrast. Recently, studies from several research groups have shown the potential to combine photoacoustic imaging with other modalities such as space fluorescent microsco py 32 OCT 70 and ultrasound 66 These combinations enable the hybrid system to
25 provide more information than each single modality by itself. The major drawback of these combin ed systems is that the system is bulky and thus canno t be employed in endoscopic or intravascular visualization. Several groups have proposed single modality or multi modality miniature probes for endoscopic or intravascular purposes. Yang et al. reported an integrated photoacoustic and ultrasound endoscopic p robe with outer diameter of 3.5 mm. In this probe, they used a focused ultrasound transducer with a hole in the center for optical illumination and a micromotor to achieve internal scanning. 67 Wang et al, showed the possibility of combining ultrasound wit h photoacoustic imaging in an external rotati on based intravascular probe Due to the limited resolving abilities of ultrasound, conventional PAT and ARPAM it is hard for these probes to obtain as high as optical resolution. 65 In another study, Yang et al reported an endoscopic probe with a size of 5 mm in diameter for ovarian cancer detection where three separated miniature probes for PAT OCT and ultrasound, respectively, were used, making the co register of the images form th e three modalities difficul t 68 To overcome the aforementioned limitations, we propose a miniature GRIN lens based probe (2.3mm in diameter) integrating ORPAM with OCT. The common optical path of both the ORPAM and OCT is built using a single mode fiber, a miniature GRIN lens and tw o microprisms, enabling these two modalities to scan the same tissue area. The self focusing ability of the GRIN lens results in a highly focused light beam for OCT and ORPAM. As a result, both OCT and ORPAM yield a high lateral resolution of 15 2.4.2 M aterial s and M ethods Figure 2 3A shows the schematic of the probe. In this probe, both illumination beams for OCT and ORPAM are coupled into one tip of a single mode fiber (SMF 28e+,
26 Thorlabs) with 0.14 NA and 0.9 mm outer diameter. The other tip of the fib er is cut with an 8 angle to minimize back reflection. Optical UV glue is used to connect the fiber tip with the GRIN lens resulting in a 5 mm working distance. Two microprisms (one without coating and the other coated with aluminum) are glued together and a small unfocused ultrasound transducer with 10 MHz central frequency and 2 mm aperture is mounted on the top of the cubic prism group. The light beams are focused by the GRIN lens and reflected by the thin aluminum film to the tissue surface. The generated ultrasound waves transmit through the cubic prism group and are detected by the transducer. The back scattering photons from the tissue are reflected by the aluminum film and coupled into the same fi ber through GRIN lens. Figure 2 3B shows a photograph of the probe, where both the cubic pri sm group and GRIN lens have 0.7 mm in diameter. A stainless steel tube with 1.0 mm in diameter is used to protect the light path (i.e., the single mode fiber, GRIN lens and cubic prism group). The probe is glued to the tra nsducer and protected by another bigger stai nless steel tube (2.3 mm in diameter). The probe is mounted on a 3D linear stage shown i n Figure 2 4 The light beam for ORPAM is generated from a pulsed Nd:YAG laser with 532 nm wavelength and a 1 0 Hz repetition ra te. Two neutral density filters are used to attenuate the light energy and a small iris is utilized to select the homogeneous part of the light beam. The shaped spatial filteri ng and is then coupled into a 21 beam coupler. The detected acoustic waves are amplified by two wideband amplifiers and then digitized by a data acquisition board (NI5152, National Instrument) at a sampling rate of 250MS/s.
27 A broadband light source (Dens eLight, DL BX9 CS3159A) w ith a center wavelength of 1310 nm is employed for OCT. The light source has a full width half maximum (FWHM) of 75 nm, prov iding an axial resolution of 10 light is split into the reference arm and the sample arm by a beam splitter and coupled into the same 21 beam coupler used for ORPAM. T he depth scanning from 0 to 1.6 mm at the reference arm is realized by a rapid scanning optical delay line (RSOD) coupled wi th a galvanometer scanning at 1 kHz. The OCT signal is then detected by a balanced photodetector, whose output is acquired and stored by a DAQ card. The sensitivity of the system is measured to be 74 dB. 2.4.3 Results and D iscussi on The lateral resolution is determined by imaging a selected part of an USAF 1951 r esolution test target. Figure 2 5B shows the photograph of the resolution test target and red dashed line shows the pa rt selected for imaging. Fig ure 2 5A and C show one di mensional (1D ) profile as marked in Figure 2 5B where the smalle st resolvable bar spacing is 15 m (group 6, element 1). The three bars can be clearly indentified by both OCT and ORPAM, hence the later al resolution is better than 15 m. In tissue imaging, th e scattering will reduce the spatial resolution of this probe; however, when the sample is optically thin, the degradation of lateral r esolution is not significant To demonstrate the microscopic imaging ability of this dual mode probe, we chose to image the ear of a mouse. Before starting the experiments, the hair on the ear was gently removed using a human hair removing lotion. The mouse was placed on a homemade animal holder and was anesthetized with a mixture of Ketamine (85mg/kg) and Xylazine. After t he experiments, the mice were sacrificed using University of Florida Institutional Animal Care and Use Committee (IACUC) approved techniques. Strict
28 animal care procedures approved by the University of Florida IACUC and based on guidelines from the NIH, gu ide for the Care and Use of Laboratory Animals were followed. The laser exposure was 25 mJ/cm 2 at the optical focus point which is higher than the ANSI la ser safety limit (20 mJ/cm 2 ) 71 while it is still below the re ported skin damage threshold 69 The scann ing step size is 6m along X Y plane. Top rows of Fig ure 2 6A and B show the maximum amplitude projection (MAP) images of ORPAM and OCT. OCT and ORPAM visualize different targets of the tissues, where ORPAM clearly maps the microvasculature and OCT images the sebaceous gland with high resolution. The bottom rows ( cross section of tissue) of Fig ure 2 6A and B show the benefits of combining these two modalities more clearly. The ear's thickness in the OCT image is from 400m to 600 m. The dermal structure and the sebaceous gland are clearly observed. In the cross sectional OCT, we can identify epidermis, dermis, and cartilage as indicated with yellow arrows. We have observed that ORPAM is good at locating micro vessels in the ear with limited surroundi ng tissu e information. Figure 2 6C shows the 3D reconstruction of co registered OCT and ORPAM. The signal to noise ratio (SNR) of the ORPAM is 25 dB, which is lower than that of conventional ORPAM. There are several reasons contributing to the reduced SNR: 1) A 1 0MHz ultrasound transducer was used in this probe, while it is known that the strongest generated acoustic signal lies between 30MHz to 70MHz; 2) The transducer is flat resulting in reduced sensitivity compared with a focused transducer commonly used in co nventional ORPAM; 3) The size of the light spot a t the focal point was around 20 m which is much larger than that used in conventi onal ORPAM (2m to 5 m) ; and
29 4) During the experiments, we used an 8 bit resolution DAQ card which can only resolve signals la rger than 40 mV. As a result, we lost some signals from small capillaries. These limitations, however, can be easily overcome using a miniature focused high frequency transducer (>30MHz), which will in turn improve the axial resolution of ORPAM and make the whole probe smaller as well. Using a high resolution DAQ card will also help solve the aforementioned problems. For future improvements, we also need a suitable micro motor to implement an internal scanning mechanism and a faster laser to reduce the scann ing time. Finally, we plan to replace the current slow time domain (TD) OCT with fast frequency domain (FD) OCT. These are necessary steps towards the clinical evaluation of the ORPAM/OCT. In this paragraph, we will introd uc e our ORPAM system and it potent ial application for endoscopic cancer research combi ned with OCT. Due to penetration limitation of highly focused light beam, it is difficult to apply this technique for cancer research if the tumor lies deep under the surface. Hence, in the next paragraph we conduct a ARPAM for broad study of cancer.
30 Figure 2 1 Schematic of ORPAM system Figure 2 2 MAP photoacoustic images of mice ear A ) MAP photoacoustic image of microvascular ization of blood vessels of a mouse ear. B ) Comparison of ORPAM image ( top panel) with conventional transmission microscope image (bottom panel)
31 Fig ure 2 3 Schematic and photograph of the hybrid probe Figure 2 4 Schematic of the integrated ORPAM and OCT system
32 Figure 2 5 Resolution test for ORPAM and OCT. A ) 1D profile of OCT B ) Photograph of USAF 1951 resolution test target. C ) 1D profile ORPAM Figure 2 6 In vivo imaging of m ouse ear by the hybrid probe. A ) MAP image (top panel ) and cross section (bottom panel ) of ORPAM B ) MAP image (top panel ) and cross section (bottom panel ) of OCT C ) 3D rendering of the co registered ORPAM and OCT images of the mouse ear.
33 CHAPTER 3 ACOUSTIC RESOLUTION PHOTOACOUSTIC MICROSCOPY 3.1 M otivation s ARPAM employs a single mechanically translated or rotated focused transduce r to map the photoacoustic signals. Commonly, it comprises a focused transducer and weakly focused light beam where excitation light is delivered through a conical lens in order to eliminate the affect of tissue surface, termed as dark field illumination. However, to our experience, bright field illumination which is full field illumination without a hollow hole, is fine for ARPAM too. Although the lateral resol ution of ARPAM from 50 m to 1mm is not as high as ORPAM, it can detect target more deeper under surface than that of ORPAM. Hence it breaks the penetration limitation of ORPAM and has been widel y used for breast cancer detection lymph node detection and molecular imaging using targeted or non targeted contrast agent s 3 .2 Materia ls and M ethods Fig u re 3 1 A shows details of our PAM imaging system In this system, a short pulsed laser beam of 6ns duration at 10Hz repetition rate generated by a 532nm Nd:YAG pulsed laser was divided into two beams by a beam splitter and coupled into two fiber bundles. Wi deband photoacoustic waves induced as a result of thermoelastic expansion of target was collected by a focused high frequency ultrasound transducer (50MHz) with 3mm aperture and 6mm focal length. The fiber bundles were well adjusted to optimize the signal to noise rati o of the system as shown in Fig ure 3 1 B A water tank with a window sealed with an optically and ultrasonically transparent membrane was used. The imaging probe consisting of acoustic transducer and fiber
34 bundles was mounted on a two dimension al (2D) moving stage with a scanning step size of 60 m during experiments. 3 .3 Quantitative ARPAM When a short pulsed laser light irradiates biological tissues, photoacoustic waves are induced as a result of transient thermoelastic expansion, which is mode led by the following Helmholtz like photoacoustic wave equation 72 : ( 3 1) whe re is the pressure wave, is the acoustic velocity is the absorbed energy d ensity of the medium and represents the product of optical absorption coefficient and excitation light distribution, is the specific heat, is the thermal expansion coefficient, and J is the time and space de pendent light intensity. While acoustic speed, is practically a spat ially varying parameter 73 74 in this work we assume it a homogeneous constant for simplicity. The first order absorbin g boundary conditions used are 75 78 : ( 3 2 ) where is the unit normal vector. In our PAM model, photoacoustic signal is first collected at each location of transduc er, resulting in 1D depth (defined as Z direction) resolved image. Thus for the transducer which located on the surface plane, also named as X Y plane, at the position where M is the total number of the transducers, the acoustic pressure can be obtained by:
35 ( 3 3 ) The reconstruction approach we used to obtain the map of the absorbed energy density is an iterative Newton method with combined Marquardt and Tikhonov regularizations that can provide stable inverse solutions 73,79 The FE discretization of equation ( 3. 3 ) and the matrix equation capable of inverse solution can be stated as: ( 3 .4 ) ( 3 .5 ) where the eleme nts of matrix K, C, and M are expressed as respectively, and the element of vector B can be expressed as is the 1D FE basis function in Z axis and in equation ( 3. 3 ) are vectors representing observed and computed acoustic field data at the k th transducer location, respectively. is the update vector for the absorbed energy density, is the Jacobian matrix formed by at the boundary measurement sites; is the regularization parameter; and I is the identity matrix. Thus here the image formation task is to update absorbed energy density distribution for each transducer located at the position via iterative solution of equations ( 3. 3 ) and ( 3. 4 ) so that an object function composed of a we ighted sum of the squared difference between computed and measured acoustic data can be minimized.
36 A 3D image of absorbed energy density is produced by scanning the ultrasound transducer in the X Y plane and aggregated as: (3.6 ) T he second step is to recover the absorption coefficient from the absorbed energy density obtained earlier. We can obtain the distribution of the optical fluence through a m odel based finite element solution to the light propagation equation. It is generally believed that light propagation in tissues is best modeled by the radiative transfer equation (RTE) 80 82 : ( 3.7 ) where is the scattering coefficient; is the radiance; is the source term; denotes a unit vector in the direction of interest. The kernel, is the scattering phase function describing the probability density that a photon with an initial direction will have a direction after a scattering event. In this study we assume that the scattering phase function depends onl y on the angle between the incoming and outgoing directions; thus Here the commonly used Henyey Greenstein scattering function is applied : where is the angle between and and The optical fluence is related to the radiance by Thus the map of absorbed energy density can be obtained by
37 Base d on the FE solution to RTE 83 84 we can then determine the distribution by the iterative solution procedure 85 In this fitting procedure, and g are assumed as constant, while the incident laser source stren gth and the absorbed energy density are estimated in advance. 3 .4 Phantom V alidation In the first three phantom experiments, we embedded a 0.8mm diameter rounded target in the background. The absorption coefficient of the target wa s 0.07mm 1 while the depths are 0.0mm (beneath the surface), 5.0mm and 7.5mm, respectively. In the fourth experiment, the absorption coefficient of the target was 0.035mm 1 and the target was embedded at the depth of 2mm. In the fifth experiment, the abs orption coefficient of the target was 0.021mm 1 and the target was embedded at the depth of 1mm. We also present a multi target case in the last experiment. Three targets with different absorption coefficients (0.07mm 1 0.035mm 1 and 0.021mm 1 respectiv ely) were embedded at the depth of 2mm in the background. In all of above experiments, the absorption coefficient of the background was 0.007mm 1 and the reduced scattering coefficient of the background and targets were 1.0mm 1 Fig ure 3 2 shows recon structed absorption coefficient images from these phantom experiments. We see that the object(s) are clearly detected by our finite element based quantitative PAM algorithm for all six cases. Based on these results, we can make observations that the method ology outlined earlier is a feasible reconstruction algorithm that can provide both qualitative and quantitative information about the targets in terms of their location, size, shape and the optical property values. These conclusions are further confirmed by the results provided in Fig ure 3 3 which presents
38 reconstructed absorption coefficient profiles along transects that across the target(s) in these cases. We also summarize the comparison of exact and reconstructed values (absorption coefficient and si ze) in the target area for all these phant om experimental cases in Table 3 1 Based on these quantitative results, we find that the relative errors for the recovered absorption coefficient of the targets for all of the single target cases (case I~V) are al l less than 5%. It is worth noting that the values are better reconstructed for the higher contrast cases (case I~III with the contrast level 10:1) than for the lower contrast cases (case IV with the contrast level 5:1, and case V with the contrast level 3 :1). It is also better reconstructed for the shallow target cases (case I) than for the deeper target cases (case II~III). The results from the multi target case (case VI) also confirm this conclusion. Similar observations on the recovered size of the targ ets can be made based on these results 3 .5 In V ivo A nimal E xperiments V alidation A rat ear experiment was chosen to further validate the algorithm. All experimental animal procedures were forth by, IACUC of University of Florida, as based on the guideline s from the NIH guide for the Care and Use of Laboratory Animals. In Fig ure 3 4 A we present the maximum amplitude projection (MAP) images projected along the vertical direction (z axis) to the orthogonal plane, and seven orders of vessel branching can be ob served in the image, which indicated by numbers 1 7. The B scan image and the reconstructed absorption coefficient image obtained by the FE based quantitative PAM algorithm are shown in Fig ure 3 4 B and C respectively. These images are in the vertical plan e (x z plane) at the location in dicated by the dash line in Fig ure 3 4 A It can be clearly seen that all of the vessel branching can be reconstructed by using our FE
39 based quantitative PAM algorithm. Although both methods provide the images with almost th e same high spatial resolution, we obtain the absolute optical absorption coefficient values based on our quantitative PAM method. In next paragraph, we will show three applications on cancer research using ARPAM. Firstly, we used ARPAM to monitoring devel opment of neovasculation of breast tumor on an animal model with and without anti angiogenesis drugs. Second, ARPAM was used to image breast tumor with injection of targeted contrast agent which is very important to study the procedure of drug delivery. Th irdly, it was combined with fluorescence molecular tomography (FMT) to map the ovarian cancer with injection of targeted contrast both for photoacoustic and fluorescent imaging.
4 0 Fig ure 3 1 Our experimental PAM system A ) D iagr am of experimental install ation B ) T he optical illumination area of the experimental system Fig ure 3 2 Reconstructed absorption coefficient images (mm 1 ) from all 6 experimental cases A ) C ase I B ) C ase II C ) C ase III D ) C ase IV E ) C ase V F ) C ase VI.
41 Fig ure 3 3 Recons tructed absorption coefficient profiles A ) y=0.0mm for image shown in Fig.2 A. B ) y= 4.8mm for image shown in Fig.2 B C ) y= 7.6mm for image shown in Fig.2 C D ) y= 2.1mm for image shown in Fig.2 D E ) y= 1.1mm for image shown in Fig.2 E F ) y= 1.9mm for image shown in Fig.2F
42 Fig ure 3 4 I n vivo imaging of the blood vessels in a rat ear A ) T he MAP image of the photoacoustic signals projected on the orthogonal plane, with seven small vessels that can be observed in the image as indicated by numbers 1 7 B ) B scan image in the vertical plane at the location indicated by the dash line in A C ) R econstructed absorption coefficient image in the vertical plane at the location indicated by the dash line in A
43 Table 3 1 Comparison of exact and reconstructed values (absorption coefficient and size) in the target area for all 6 experimental cases. Case # Depth of the target (mm) a of target (mm 1 ) size of target (mm) Exact Recovered Exact recovered I 0 0.07 0.071 0.8 0.90 II 5.0 0.071 0.90 III 7.5 0.072 0 .95 IV 2.0 0.035 0.034 0.95 V 1.0 0.021 0.020 1.0 VI 2.0 0.07 0.068 0.9 0.035 0.033 1.0 0.021 0.019 1.1
44 CHAPTER 4 APPLICATIONS OF ACOUSTIC RESOLUTION PHOTOACOUSTIC MICROSCOPY TO PRECLINICAL CANCER RESEARCH 4 .1 Photoacoustic I maging of T um or V asculature D evelopment for B reast C ancer R esearch 4 .1.1 Motivations Tumors growth requires sustenance by nutrients and oxygen as well as an ability to evacuate metabolic waste and carbon dioxide (CO 2 ). To address these needs, an permanently activated, causing new vessel growth from the adjacent host vasculature. The vasculatures within tumors are typically aberrant, controlled by a complex biological stress that involves both the cancer cells and stromal microenvironment. 86 Susta ined aberrant tumor angiogenesis plays a central role in breast cancer carcinogenesis and metastatic potential. 87 Members of vascular endothelial growth factor (VEGF) family are well known angiogenesis activators. Despite the promising activity of anti ang iogenic drugs in preclinical tumor models, targeting VEGF signaling appears to be insufficient for permanently inhibiting tumor angiogenesis in patients with breast cancers. The reasons for this are likely to be multiple and complex. For example, animal st udies revealed that some tumor vessels induced by VEGF require continuous VEGF expression for their maintenance and undergo apoptosis if VEGF levels fall below threshold level. 88 Others, once induced by VEGF, persist indefinitely in the absence of exogenou s VEGF 89 suggesting that tumor vasculature is heterogeneous and an endogenous intracrine VEGF signaling may be crucial for vascular homeostasis. 90 Nevertheless, there remains an urgent and unmet need for novel targeted therapies for patients with resistan ce to current anti angiogenic agents. An ideal anti angiogenic model for cancer treatment should consist of three
45 elements: i) identification and validation of rationale based anti angiogenic targets; ii) an adequate drug form and delivery route; iii) adva nced imaging modalities allowing noninvasive detection and monitoring tumor response to anti angiogenic agents. In rapidly growing tumors, vascular endothelial cells face a hostile microenvironments characterized by hypoxia, nutrient deprivation and acidos is. These environmental stressors induce endoplasmic reticulum (ER) stress, and cells respond by activating the unfolded protein response (UPR) pathway. 91 The UPR relieves ER stress by inducing genes i) to increase the protein folding capacity of the ER, i i) to enhance the clearance of unfolded proteins from the ER, and iii) to inhibit general protein translation in the ER. 92 Our recent study demonstrated that under the stress conditions caused by tumor microenvironment or/and anti VEGF therapies, tumor end othelial cells adopt the up 1 and ATF6 97 which may contribute to the up regulation of molecular chaperone and in turn increase the protein folding capacity for misfolded/unfolded VEGF and maintain VEGF intracrine signaling for end othelial cell survival. In the past decades, remarkable progress has been made in using siRNA as a promising new class of drugs by targeting the mutant or overexpression oncogenes. Several siRNA cancer therapies have entered early clinical trials. 98 In th e present study, we investigated whe 1 or ATF6 can serve as novel antiangiogenic therapeutic targets for breast cancer treatment. Also, we have been pursuing that further development of alternative vector systems, such as self complementary adeno associated virus (scAAV) vec tors for their potential to transduce microvascular
46 endothelial cells with high efficiency, given that proven safety of AAV vectors in several clinical trials. 106 107 Several current non invasive imaging modalities have differing limitations for monitoring vasculature development. For instance, X ray computed tomography (CT) needs extrinsic contrast agent and exposures patients to ionization radiation 108 positron emission tomography (PET) screening often involves extrinsic contrast agents and magnetic reso nance imaging (MRI) is limited by its low temporal/spatial resolution. 109 Pure high resolution optical imaging modalities such as single photon, multi photon fluorescence microscopy suffer from limited imaging depth (<1mm) and repeated fluorescent dye inje ction 110 Adequate non invasive imaging can help physicians to determine whether to start and when to start anti angiogenic therapies. In particular, such imaging is essential for monitoring the tumor response to anti angiogenic therapies because tumor shr inkage may not occur within a short period of time even when anti angiogenic treatment is effective. One potential non invasive imaging modality is photoascoustic imaging (PA) consisting of the advantages of rich optical contrast in optical imaging and hig h ratio of imaging depth to spatial resolution in ultrasound imaging. In the present study, we have identified that scAAV2 septuplet mutant vector, in which seven surface exposed tyrosine residues of the capsid were changed to phenylalanine, was able to in fect mouse microvascular endothelial cells with high 1 or ATF6 were effective in decreasing breast cancer induced angiogenesis in vitro. Serial non invasive photoacoustic imaging further conf irm that intratumoral delivering the siRNAs against
47 1 or ATF6 by scAA2 vectors resulted in a significant decreased in tumor growth and tumor angiogenesis in breast cancer xenograft models. These data have generated a proof to concept model with important implications for the development of novel anti angiogenic targeted therapies for patients with breast cancer. 4 .1.2 Materials and M ethods 4 .1.2.1 Construction of sc AAV 2 vector for delivering siRNAs against the UPR proteins Self complementary AAV serotype 2 (scAAV2) and their corresponding single and multiple tyrosine to phenylalanine (Y F) mutants containing a ubiquitous, truncated chimeric CMV ch actin (smCBA) promoter 116 were generated and purified by previous described methods. 117 Vectors were titered by quantitative real time PCR and re suspended in balanced salt solution (BSS: Alcon, Forth Worth, TX). 118 siRNA has been widely used to kn ock down target gene expression for a variety of purpose. However, siRNAs can provoke unspecific degradation of all cellular RNAs through the induction of the interferon response. 119 We utilized three pre validated sequences of the siRNAs against mice IRE1 1 and ATF6, respectively, with a significant knockdown in our previous study 120 (Fig ure 4 2 D GGATGTAAGTGACCGAATA CAGCTTTTACGGGAGAAAA GGATCATCAGCGGAACCAA leotide ones using Notl and Sall (F igure 4 2B and C ). The integrity of the siRNA coding region was confirmed by sequence analysis (data not shown).
48 4 .1.2.2 scAAV2 i nfection of cells in vitro Infectio n of scAAV2 vectors were carried out as previous described. 121 Briefly, cells were plated in 14 mm microwells in a 35 mm Petri dish (MatTek, Ashland, MA) and reached up to 60 70% confluence. scAAV2 vectors were diluted in serum free medium to achieve the d esired multiplicity of infection (MOI). MOI was defined as the number of genome containing vector particles per targeting cell. The cells were rinsed in PBS and the virus dilutions were added. After 1 hour, complete medium was added to the cells. Infected cells were maintained at 37C in 5% CO 2 for 3 days. All infections were performed in triplicate. Each vector was used to infect 24 wells in total. Each 4 .1.2.3 Microscopy and flu orescence activated cell sorting (FACS ) analysis Three days post infection, cells were observed using bright field microscopy to ensure 100% confluence. Cells were then analyzed by an OlympusI81 DSU Spinning Disk Confocal microscopy (Olympus America,Inc., Center Valley, PA) and digital photomicrographs were captured by imaging software Innovations, Denver, CO). For FACS analysis, the cells were dissociated with Accutase solution (MP Biomedicals. Solon, OH) and collected by c entrifugation (200g for 5 min). 10,000 cells per sample were counted and analyzed using a BD LSR II flow cytometer. Uninfected control cells were also counted and analyzed to establish transduction efficiency of baselines. 4 .1.2.4 Cell culture Mice micr ovascular endothelial cells (MMECs) were isolated from mouse brain tissue using modified methods as previous described. 122 Briefly, freshly isolated mouse brains were homogenized, and after trapping on an 83
49 transferred into and 200 for 30 min. The resultant vessel fragments were trapped on a 53 Island, NY) at 37C in 5% CO 2 for 3 days. Purification of MMECs was achiev ed as previous described. 123 Briefly, the cells col lected from primary culture T25 cm 2 flask by trypsinization were reacted with rat anti VE cadherin antibody (Biolegend, Inc, San Diego, CA ) for 10 min at 4C. After washing, the cells were mixed with pan rat IgG beads ) for 20 min at 4C with gentle tilting and rotation. 15, Life ) to immunoadsorbed cells for 2 min at room te mperature. The bead bound cells were then re sus pended in release buffer for 15 min. The purified cells were collected by cen trifugation and seeded into T25 cm 2 flask in complete medium. Cells were subculture at a ratio of 1:2 on reaching confluence. The cel ls were used within three passages. NeuT (a mice breast cancer cell line) was isolated from MMTV c neu transgene mice as described. 124 NeuT EMTCL2 served as a malignant variant of NeuT and was developed by consecutive serial implantation of NeuT cells in athymtic nude mice. The mouse breast cancer cells were grown in RPMI1640 supplemented with 10% fetal bovine serum and gentamycin (Gibco BRL, MD) to 70 80% confluence subjected to sequential experiments or sub cultured at ratio of 1:3 in fresh complete medi um supplemented with 10% FBS.
50 For in vitro co culture systems, NeuT or NeuTEMTCL2 cells were seeded onto 6 well Transwell inserts with 0.4 well plate for 72 hr s MMECs were cultured in a separate 6 well plate. Confluent breast cancer cells on Transwell inserts were then transferred on top of MMECs and placed at 37C for 48 hr prior to sequential exp eriments. 4 .1.2.5 In vitro angiogenesis assay In vitro angiogenesis was measured by in vitro tube formation assay which reflects a combination of proliferation, migration and tube formation of microvascul ar endothelial cells Briefly, MMECs were plated sp arsely (2.510 4 /well) on 24 well plates coated with 12.5% (v/v) Matrigel (BD, Franklin Lakes, NJ) and left overnight. The medium was then aspirated and 250 s to allow its polymerizati on, followed by addition of 500 fetal calf serum (FCS) for 48 hr s The culture plates were observed under a phase contrast microsc ope and photographed at random in five fields (10). The tubule length (mm/mm 2 ) per microscope field was quantified. 4 .1.2.6 Apoptosis assay Apoptosis was evaluated using FITC conjugated annexin V/propidium iodide assay kit (R&D System, Minneapolis, MN) ba sed on annexin V binding to phosphatidylserine exposed on the outer leaflet of the plasma membrane lipid layer of cells entering the apoptotic pathway. Briefly, MMECs were collected by EDTA detachment and centrifuged (200g for 5 min), washed in PBS and re suspended in the annexin V incubation reagent in the dark for 15 min before flow cytometric analysis. The analysis of samples was performed by flow cytometric analysis on BD lSRII flow cytometer (BD Biosciences, MD). An excitation wavelength of 48 8 nm was used with
51 fluorescenc e emission measured at 530 15 nm through fluorescence channel one. A minimum of 10,000 cells per sample were collected using log amplification for fluorescence channel one and linear amplification for forward light scanner and 90 lig ht scatter before being analyzed using in house software. 4 .1.2.7 Proliferation assay Crystal violet assay was conducted as previously described. 125 Briefly, MMEC well plates at 110 5 cell/ml. The cells were fixed in 4% paraformaldehyde in PBS for 15 min. After being washing with distilled water, the plates were stained with 0.1% crystal violet solution for 20 min. The plates were washed with water and allowed to be air dry. Acetic acid (100 was added to each well for extraction of dye. Absorbance of the staining was measured by an automatic microtiter plate reader at 590 nm. 4 .1.2.8 Mice b reast cancer xenograft models All animals used in this study were maintained at the animal facility of the Un iversity of Florida and handled in accordance with institutional guidelines. Athymic female nude mice (nu/nu) at 5 to 8 weeks were purchased from Charles River Laboratories (Charles River Laboratories, Inc., Wilmington, MA) and caged in groups of 5 or fewe r. Mice breast cancer xenografts were established by subcutaneous injection of 110 5 NeuT or NeuT EMTCL2 cells into the mammary fat pads of the mice. Tumor volume was calculated from caliper measurements of the large (a) and smallest (b) diameters of each tumor using formula ab 2 0.4. Three days after inoculation, most tumors had grown to ~30mm 3 All mice were euthanized when the tumor volume in the non treated group reached ~1000mm 3
52 For in vivo siRNA knockdown of the UPR proteins, mice with similarly si zed tumors were divided into two groups (siRNA treatment and scrambled control). Mice were anesthetized with a mixture of ketamine (85mg/kg)/xylazine (4 mg/kg) and were mut vectors encoding siRNAs against or XBP 1 or ATF6 or scrambled siRNA at a titer of 210 13 genome copies per ml. 4 .1.2.9 Photoascoustic (PA) imaging systems and noninvasive monitoring in vivo tumor angiogenesis In this study, photoacoustic microscopy (PAM) system was used to monitor vascul ature change in breast cancer xengraft models. The system is th e same as shown in Figure 3 1A In this applications, a focused ultrasound transducer (50MHz, 3 mm aperture and 6 mm focal length) was used to receive induced photoacoustic waves. PA signals ampl ified by two d ifferent amplifiers (one was 17dB from 100kHZ to 1GHz and the other was 20dB from 20MHz to 3 GHz) were digitized by a 8 bit data acquisition board (NI5152, National Instrument, Aust in, TX) at a sample rate of 250 MS/s. The two dimensional trans verse scanning combined with the depth resolved ultrasonic detection generated 3D PA images displayed in maximum amplitude projection (MAP). 3D PA signals were processed b y Hilbert transform normalized to the same scale (0 256) and applied with a threshol d ( 6dB level). 126 The volumes of the blood vessels were calculated by integrating the corresponding image voxels (1 for blood vessel and background being set to 0). Entropy was calculated from normalized MAP of each PA image at the same scale (0 256). 4 .1 .2.10 Statistics analysis All experiments were repeated at least 3 times. Analysis of variance (ANOVA) was used to assess the transduction efficiency of AAV2 vector infection. The unpaired
53 Student t test was to assess statistic significant for the rest dat a obtained from in vitro studies (in vitro angiogenesis assay, apoptosis assays and crystal violet assays) as well as in vivo animal experiments including tumor volume, entropy and normalized vessel volume. The data were expressed as meanSEM. Statistical analysis was conducted by GraphPad Prism (version 5.01; GraphPad Software, Inc., La Jolla, CA) with p<0.05 considered statistically significant. 4 .1.3 Results 4 .1.3.1 sc AAV2 sept mut exhibits higher transduction efficiency in MMECs AAV2 based vectors ha ve, to date, exhibited higher transduction efficiency when to targeting transgene expression to vascular endothelium than other native AAV. 127 A actin promoter/CMV promoter (smCBA) driving gr een fluorescent protein (GFP) was packaged into scAAV2 containing combinations of up to 7 surface exposed tyrosine residues (252, 272, 444, 500, 700, 704 and 730F). Combinations tested for transduction of mouse microvascular endothelial cells (MMECs) incl uded: triple (Y444+500+730F), quadruple (Y272+444+500+730F) and septuplet (Y252+272+444+500+700+704+730F). The transduction efficiency of each of the combination tyrosine mutant vector at MOIs ranging from 100 to 10,000 was analyzed (as measured by GFP exp ression) and compared with unmodified scAAV2 in MMECs 72 hr later by flow cytometry. We present data generated from cells infected only at an MOI of 10,000 as it is representative of trends seen across all MOIs. We demonstrated that the transduction effici ency of the all tyrosine mutant vectors was significantly higher compared with the unmodified scAAV2 (Fig ure 4 1 A ). Specifically, the transduction efficiency of the septuplet mutant vectors was maximal, ~173 fold higher than the unmodified vector (Fig ure 4 1 B ). Similarly, the
54 triple mutant and the quadruple mutant also had significant enhancement in GFP expression (~2 and 6 fold, respectively) (Fig ure 4 1 B ). 4 .1.3.2 Septuplet tyrosine mutations improve the transduction efficiency of sc AAV2 mediated siRNA s infection in MMECs Our previous study showed a crucial role of the unfolded protein response pathway in breast tumor angiogenesis 97 Base on the observation (Fig ure 4 1) that scAAV2 septuplet mutant (Y F) exhibited the most transduction efficiency in MME Cs, we constructed a septuplet mutant AAV2 vector containing three siRNA sequences 1, ATF6, respecti vely and a scrambled siRNA (Fig ure 4 2 A and B ). To verify that the inserted siRNA sequence transduced MMECs, scAAV2 sept mut containing t he scrambled siRNA was used to infect MMECs at MOIs ranging from 400 to 12,000. As shown in Fig ure 4 1 E mCherry expression was detected by fluorescence microscopy 72 hr post infection. The transduction efficiency of sc AAV2 sept mut vector was not affecte d by siRNA insertion and elevated with the i ncreased MOI of the vector (Fig ure 4 2C ). 4 .1.3.3 siRNA knockdown of the UPR proteins decreased NeuT EMTCL2 induced in vitro angiogenic activity of endothelial cells The balance between protein synthesis and pro tein proper folding in ER is essential for cellular homeostasis and survival. Many disease conditions that affect protein folding tip this balance and trigger an ER membrane bound protein stress pathway known as the unfolded protein response (UPR). Our rec ent study demonstrated that malignant breast cancer cells significantly up regulated three UPR proteins including IRE 1 and ATF6 in the endothelial cells, implicating possible pro angiogenic roles for UPR. 97 To address the possible involvements of the UPR proteins for pro angiogenic activity, MMECs were co cultured with NeuT EMTCL2 as
55 described in Experimental Pro cedures. Since late stage of angiogenesis requires morphologic alterations of endothelial cells, leading to lumen formation, NeuT EMTCL2 cell induced angiogenesis in MMECs was studied by measuring a network of capillary like tubes in an in vitro 3 D Matrig el model with reflects a combination of proliferation, migration and tubule formation of endothelial cells. 125 MMECs pre treated with the scrambled siRNA exhibited a significant in vitro angiogenesis in contrast to non treatment (F ig ure 4 3 A and B ). As exp ected, the siRNAs against the UPR proteins markedly reduced the angiogenic responses of MMECs to the NeuT stimulation (Fig ure 4 3 A and B ). When compared with scrambled siRNA, the siRNA 1 caused a ~50% reduction of in vitr o angiogenesis wile ATF6 siRNA caused a ~30% reduction. 4 .1.3.4 siRNA knockdown of the UPR proteins down regulated the survival of endothelial cells 1 or ATF6 on the survival of MMECs was analyzed by apoptosis and cr ystal violet assays. The knockdown of XBP 1 elicited a maximal apoptotic response in MMECs, as evidenced by a significant 8 fold increase in the apoptotic cells compa red to the scrambled siRNA (Fig ure 4 3 C and D ). siRNAs er, yet still significant apoptotic response in MMECs (4 fold increase) ( Fig ure 4 3 C and D ). Consistently, crystal violet assay showed NeuT EMTCL2 stimulation significantly increased MMECs proliferation, which was inhibited by the knockdown of XBP 1 at max ima l reduction of ~70% (Fig ure 4 3 E ). Both EMTCL2 in duced survival of MMECs (Fig ure 4 3E ).
56 4 .1.3.5 Different malignant m i ce breast cancer cells induced differential agngiogenic responses in breast cancer xenogr aft models were monitored by serial photoacoustic (PA) imaging To test the feasibility of noninvasively monitoring in vivo tumor vasculature development, we performed PA imaging for two mice breast cancer xenograft models. In one model, mice were injected with NeuT cells, while the other model was generated via injection of NeuT EMTCL2 cells. Serial PA imaging were carried out on day 3, 5, 7 and 9 after tumor inoculation, respectively. The inoculation of NeuT cells only caused a minimal tumor growth compare d to a rapid tumor growth in NeuT EMTCL2 model which tumor volume reached ~250 mm 3 by day 11 (Fig ure 4 4 A and D ). Additionally, NeuT EMTCL2 cells induced a significant vasculature development evidenced as gradual splitting large host blood vessels to small ones. This was noticeable on day 5 and peaking on day 9. Using PA resolution and depth section capability, we determined the kinetics of the vessel density changes surrounding tumor mass using the Entropy method. 128 Entropy is a statistical measure of rand omness that can be used to characterize the texture of the input image. In NeuT model, Entropy did not reveal a significant increase in vessel density. However, Entropy clearly indicated that NeuT EMTCL2 inoculation resulted in a increased vessel density b y 20 % compared with NeuT implantation (Fig ure 4 4 B ). Another PA image extraction analysis was normalized vessel volume changes that closely e cho Entropy data. As shown (Fig ure 4 4 C ), NeuT EMTCL2 implantation resulted in a steep increase in vessel volume, whereas there was no significant change in tumor vessel volume detected in the NeuT model (Fig ure 4 4 C ).
57 4 .1.3.6 Knockdown of the UPR proteins significantly inhibited Neut EMTCL2 induced in vivo angiogenesis To confirm the purported anti angiogenic activ ities of the siRNAs against the UPR proteins (Fig ure 4 5 A and B ), mice of NeuT EMTCL2 xenograft models were divided into four groups including one scrambled treatment, three siRNA treatm XBP 1 and ATF6) delivered by intratumoral injection of scAAV2 sept mut vectors encoding the siRNAs against the UPR proteins The tumor volume reached ~250 mm 3 after 11 day of NeuT EMTCL2 inoculation in the scrambl ed siRNAs treatment group (Fig u re 4 5 D ). Figure 4 5 D 1 both exhibited significant decreased tumor growth to a similar extent (~45% on day 11) and the ATF6 siRNA was even more effective on inhibition of tumor growth (~54% by day 11). Serial PA imaging was employed to monitor and evaluate the development of tumor vasculature in the NeuT EMTCL2 xenograft models on day 3 after NeuT EMTCL2 inoculation followed on day 5, 7, 9 and 11. Entropy method delineated a significant decrease in vessel densi ty (25%) by day 9 and sustained at that level thereafter in the scrambled group (Fig ure 4 5 B ). The NeuT EMTCL2 xenograft models treated with the siRNAs against XBP 1 or ATF6 evidenced a steady decrease in vesse l density with statistic significant in comparis on with the scramble group (Fig ure 4 5 B ). All three siRNAs exhibited significantly steady decrease in vessel volume compared with the scrambled siRNA treatment (Fig ure 4 5 C ). 4 .1.4 Discussion The UPR is gener ally considered to involve 3 signaling pathways from the ER
58 and PERK (PKR 1 (X box binding protein 1) mRNA, resulting in a frame shift and the translation of the spliced form of XBP 1, a 41kDa basic leucine zipper (bZIP) family transcription factor that induces genes involved in UPR response. 93 the load of proteins in the ER, in favor of restoration of ER homeostasis. 94 During ER stress, ATF6 is transported to the Golgi apparatus where it is cleaved and release a 50kDa bZIP to the nucleus to induced expression of ER chaperones. 95 ATF6 also supports ER biogenesis independently of XBP 1 96 1, ATF6 and PERK are aberrantly expressed in many types of 1 pathway is important for tumor growth under deteriorating conditions of microenvironment. The levels of XBP 1 correlate with glucose starvation. 129 XBP 1 splicing can be detected even in relati vely small tumors in several genetic models for breast cancer, implicating that ER stress may occurs from the early stage of tumor development. Indeed, XBP 1 knockout caused cancer cell death and XBP 1 knockout cells produced smaller tumors in xenograft mo dels. Over expression of 1 restored tumor growth under these conditions. The significance of ATF6 in tumor development is less well characterized. However, ATF6 was found to be over expressed and activated alone with increased XBP 1, suggesting a coordination and 1 dependent and ATF6 dependent systems. 130 ATF6 served as stress survival factor for dormant but not proliferative squamous carcinoma cells via GTPase and mTOR. 131 A role for ATF6 pathway is further suppo rted by an in vitro study in which activation of ATF6 resulted in apoptosis resistance of melanoma cells. 132 The UPR pathway is a general mediator of vascular endothelial cell
59 dysfunction in inflammatory disorders. 133 Furthermore, our recent studies sugges ted 1 and ATF6 is closely related to angiogenic activity of endothelial cells, whereas endothelial PERK functions were not drastically affected by angiogenic stimulation. 97 In the current study, we confirmed that selective knoc kdown of 1 or ATF6 can lead to inhibition of tumor angiogenesis and breast cancer growth using in vitro and in vivo models. A challenge for RNAi based therapies is the efficiency of delivery for the siRNA to the target cells. Delivery of siRN A into mammalian cells is usually achieved via the transfection of double strand oligonucleotides or plasmid encoding RNA polymerase III promoter driven small hairpin RNA. 99 Recently, retroviral and lentiviral vectors have been used for siRNA delivery, whi ch have overcome some of the problems of poor transfection efficiency seen with the plasmid based systems. 100 However, both retroviral and lentiviral vectors undergo random integration into host chromosomal DNA, and there is persistent risk for insertion m utagenesis. 101 102 Non integrating adenoviral vector has also been tested for vascular endothelial gene transfer 103 but many cell types express the adenoviral receptor preventing selective endothelial cell infection and precluding clinical use. 104 Further more, adenoviral vectors are highly immunogenic and are therefore unsuitable for long term gene expression in vivo. 105 In this study, we ultilized scAAV2 vectors containing seven surface exposed tyrosine to phenylanine capsid mutations to deliver siRNAs ag 1 and ATF6 into endothelial cells. AAV vectors have historically exhibited rather poor transduction efficiency in endothelial cells in comparison with other cells. Key steps in AAV transduction have been identified as cell surface receptor and co receptor mediated viral binding,
60 intracellular trafficking, nuclear transport, uncoating, and viral second strand DNA synthesis. 135 137 Of these, intracellular trafficking is considered a major impediment for efficient transgene expression. Tyrosin e phosphorylation of AAV2 can trigger ubiquitination dependent degradation of AAV2. 138 Based on these observations, we replaced tyrosine residues with phenylalanine to generation several multiple mutations of the seven surface exposed tyrosine residues on scAAV2 capsids. 121 In this study, we identified scAAV2 sept mut vector that can transduce mouse microvascular endothelial cell ~173 fold more efficiently than wild type scAAV2. Intriguingly, our current data is different from our previous published data w hich showed that the quadruple mutant was the most efficacious in bovine retinal microvascular endothelial cells 139 suggesting that tyrosine mutants of scAAV2 may have different levels of efficacy in different cell/tissue types. It is noteworthy that anot her group has also reported a similar discrepancy for scAAV2 transduction efficiency in the other cell types, possibly due to the differences in the intracellular milieus in different species and/or conformation dependent binding and accessibility to host cellular factors. 140 The chimeric CMV actin promoter (CBA) has been utilized extensively as a promoter that supports expression in a wide variety of cells due to a broad tropism. However, CBA is ~1700 base pairs in length, too large in most cases to be used in conjunction with A AV vectors. As the result, we selected a truncated CBA promoter globin intron is greatly shortened. The most significant advantage of the modification is that smCBA can initiate transgene expression soo ner and for a longer time period, compared to expression using other
61 promoters. Because smCBA is truncated, it permits packaging of more polynucleotides into the vector, which is particularly relevant for double stranded viral vectosr. Our results demonst 1 or ATF6 exerts robust anti angiogenic activities and suppresses tumor growth via local intratumorally administration of scAAV2 sept mut vectors encoding the corresponding siRNAs. Although there are controversies surro unding the approaching of local intratumoral gene therapy, this strategy may offer several advantages for anti angiogenic therapy: 1) intratumoral gene therapy can reduce the risk of widespread anti angiogenesis resulting from systemic administration of an anti angiogenic agent; 2) gene transfer can lead to a local accumulation of the anti angiogenic agents; 3) anti angiogenic gene therapy does not require that genes be transferred to all target cells. Photoacoustic microscopy (PAM), a hybrid imaging tech nique combining high optical contrast and high ratio of imaging depth to spatial resolution of ultrasound, has multiple advantages. 111 First, it can identify red blood cell perfused microvasculature supplying oxygen for tissues through the endogenous hemog lobin contrast. Second, unlike conventional optical microscopic, PAM is 100 time less acoustic scattering in biological tissues, thus greatly improving tissue transparency. Thirdly, the hig h contrast of hemoglobin at 532 nm (>100:1) enables using of low lev el laser exposure. Recently, a study showed noninvasively label free serial imaging of red blood cell perfused vasculature in ear of small mice by using ORPAM 112 The authors demonstrated that the spatia l resolution is high enough (<2 capillary and even a single red blood cell. Howeve r, the imaging depth of 400 700 m may limit ORPAM application for monitoring neovascularization during tumor growth. In contrast, acoustic resolution
62 PAM with imaging depth reaching 3mm is more adequate for monitoring tumor vascular development along with tumor growth. 141 Using our established acoustic resolution PAM systems, we were able to serially image dynamics of tumor neovascular network development in mice breast cancer xenograft models. More importa ntly, it was first time that we successfully monitored and determined differential tumor response to siRNAs 1 and ATF6. These were delivered using AAV2 septuplet mutant vectors by intratumoral injection in mice breast cancer xenograft mo dels. The acoustic resolution PAM systems system used here equips a lower repetition rate (10Hz) pulse laser resulting in re lative longer scanning time (20 min) in comparison with some commercial high repetition rate (>100Hz) pulsed laser enabling compl etin g each scanning within 2 min. To apply PA imaging in the clinic in the future, it will be greatly beneficial if we can develop quantitative PAM systems enabling extraction of more functional information, such as oxygen saturation, concentration of hemoglobi n correlating to tumor growth and anti angiogenic therapy. 4 .2 uPAR T argeted M agnetic I ron O xide a s a C ontrast A gent for I n V ivo M olecular P hotoacoustic T omography of B reast C ancer 4 .2.1 Motivations Breast cancer affects 1 in 4 women in the U.S. 143 and in 2011 there were 230480 new breast cancer cases and 39520 breast cancer death s 143 144 Conventional breast imaging techniques such as X ray, ultrasound and magnetic resonance imaging (MRI) have limitations. X ray mammography is the most widely used and rema ins the imaging standard for breast cancer diagnosis, but exposes patients to ionization radiation is not ideal for imaging dense breast s, which are typically associated with youn ger female patients (<50 yr) 145 Although ultrasound is very useful in chara cterizing breast lesions in
63 dense breasts, it is not used as a primary screening method due to its low co ntrast level and specificity Breast MRI shows high sensitivity but low specificity producing a high false positive rate 146 Therefore there is an urg ent need to develop noninvasive, and high ly sensiti ve specific techniques with a high resolution for early detection, accurate diagnosis and precise resection of breast cancer. Molecular imaging techniques using target specific probes for positron emissi on tomography (PET) or single photon emission computed tomography (SPECT) have been demonstrated for breast cancer diag nosis and treatment monitoring These imag ing methods have show n improved specificity and sensitivity in cancer detection compared with conventional mammography. However, PET and SPECT have low spatial resolution in determining anatomic location of the tumor. Additionally, their dynamic and time resolved imaging ability is limited because of the long half life of the radiotracers PET and SPECT both involve ionization radiation. 147 148 PAT is an imaging technique that detects wide band acoustic waves that are generated when biological tissue absorbs short laser pulses. This imaging m ethod has the advantages of high optical contrast as well as increased ratio of imaging depth to spatial resolution The image resolution and maximum imaging depth can be adjusted with the ultrasonic frequency and the penetration of diffuse photons. The optical to acoustic conversion efficiency represents how ma ny incident photons will be absorbed and converted to heat and how fast this heat can diffuse from the target during thermoelastic expansion and wave generation As such, this conversion efficiency will determine the contrast intensity of photoacousic imag ing. Although an increase in hemoglobin (Hb) content in breast tumors results in enhanced absorption of photon
64 energy that leads to 2 to 4 times higher photoacoustic signals, compared to the healthy breast tissue, the signal is not strong enough to detect breast tumors located deep below the skin To enhance the p hotoacoustic signal s, contrast agent s are used to generat e acoustic transients. Several r ec ent studies have reported the use of targeted and non targeted contrast agents in imaging lymph nod es me lanomas, angiogenesis, the cerebral cortex and brain tumors. IONP has been widely used to enhance the image contrast for MRI in mice experiments and has recently been approved for pilot studies on human s uPAR targeted nanoparticles ( ATF IONP ) have been us ed successfully in in vivo magnetic resonance imaging o f mouse mammary tumor s 149 In th is study tumor cells selectively bound and internalized the ATF IONP and provided high contrast for MRI. Ekaterina et al. also demonstrated the use of IONP as a contras t agent to detect circulating tumor cells. 150 In our study we used near infrared dye labeled amino terminal fragment s of uPA conjugated to iron oxide nanoparticles (NIR830 ATF IONP) to specifically bind to uPAR, a cellular receptor highly expressed in man y types of human cancer tissues, including breast cancer. 4 .2.2 Material and M ethods 4 .2.2.1 Cell line Mouse mammary carcinoma cell line 4T1 was used. Cells were cultured at 37C and 5% CO 2 s upplemented with 10% fetal bovine serum and antibiotics. Cells were harvested at 80% confluence. 4 .2.2.2 Preparation of NIR830 ATF IONP Recombinant amino terminal fragment s (ATF) were generated using from pET101/D TOPO expression vector s containing a mous e ATF of the receptor binding
65 domain of uPA cDNA sequence and expressed in E. coli BL21 ( Invitrogen, Carlsbad, CA ) ATF peptides were purified from bacterial extracts with Ni 2+ nitrilotri acetic acid (NTA) agarose columns (Qiagen, Valencia, CA) using manuf instructions. IR 783 (Sigma Aldrich, St. Louis, MO) was used to synthesize near infrared dye (NIR 830) as described by Lipowska. 151 The schematic and spectral characterization ( e xcitation wavelength: 800nm and e mission wavelength: 825nm ) of NIR 830 dye were shown in Figure s 4 6A and C F ree thiol groups on the ATF were labeled with NIR 830 dye and then conjugated to amphiphilic polymer coated 10nm magnetic iron oxide nanoparticles (IONP s ) via cross linking of caboxyl groups of the amp hilphilic polymer to the animo side groups of the peptides (Figure 4 6B ). Unconjugated peptides w ere removed by washing with 100 k spin columns for three times 149 4 .2.2.3 Animal preparation Mouse mammary tumor 4T1 cells (2 10 6 ) were implanted into mammary fat pads of 6 8 week old BALB/C mice. Tumors were allowed to grow for 6 10 days to a size of 0.5 0.8 cm. Animals were anesthetized with a mixture of Ketamine (85mg/kg) and Xylazine and were sacrificed using University of Florida Institutional Animal Care a nd Use Committee (IACUC) approved techniques. Strict animal care procedures approved by the University of Florida IACUC and based on guidelines from the NIH guide for the Care and Use of Laboratory Animals were followed. 4 .2.2.4 Photoacoustic microscopy imaging system The schematic of photoacoustic microsco py system is shown in Figure 3 1A Two pulsed lasers were used in this study: 1) a tunable Ti:Sapphire laser (LT 2211A, LOTIS
66 TII) with 8 30 nanosecond (ns) pulse duration and 10Hz repetition rate for m acroscopic imaging of tumor s; and 2) a Nd:YAG laser ( NL 303HT from EKSPLA, Lithuania ) with 6ns pulse duration and 10Hz repetition rate for microscopic imaging of the blood vessels. The laser beam was spit and coupled into two optical fiber bundles separate ly which were both mounted and adjusted to allow optimal illumination in the imaging area. Induced photoacoustic waves were collected by a focused ultrasound transducer (50MHz or 3.5MHz). The 50MHz transducer with 3mm aperture and 6mm focal length yields 1 5 m resolution in axial and 60 m resolution in lateral at the focal point. The 3.5MHz transducer (V383, Olympus) with 15mm aperture and 35mm focal length yields axial and lateral resolution of 200 m and 820 m. The imaging probe transducer and optical fibe r bundles w ere mounted on a two dimension (2D) moving stage. One dimensional depth resolved images (A line) at each transducer location we re acquired and additional scanning along a transverse direction produce d the 2D images referred to as B scans. Furthe r raster scanning along the other transverse direction enable d the reconstruction of 3D images. All photoacoustic images were displayed in maximum amplitude projection (MAP) form 4 .2.2.5 Near infrared planar fluorescence imaging system As shown in Figur e 4 7 two laser beams from sep arate 785nm CW lasers (M5 785 0080, Thorlabs) were coupled into optical fiber bundle III and optical fiber bundle IV which were both fixed and adjusted for homogeneous illumination. A high performance fluorescent band pass fil ter (NT86 381, Edmund Optics) was mounted onto the front of a fast charge coupled device camera (CoolSNAP EZ, Photometrics) which was used to collect the fluorescence signal. All experiments were conducted using the same power illumination and camera expos ure time s.
67 4 .2.2.6 Image processing Photoacoustics were reconstructed by a program implemented in Matlab 7.0 and merged with Amira 5.3.3. Fluorescent images were collected by RS Image (Roper Scientific, Inc) provided by the manufacturer and processed by Matlab 7.0 and fused through Amira 5.3.3. 4 .2.2.7 Histologic analysis Tumor s collected from mice were preserved in 10% neutral buffered formalin for 10 hours at room temperature. Histological sections were stained with Prussian blue staining and analyzed using standard procedures to confirm the presence of iron oxide nanoparticles. 4 .2.2.8 Statisti cal analysis All data obtained from the experiments were summarized using mea ns standard error of the mean (SEM). 4 .2.3 Results The absorbance of light by IONP s is much stronger compared to blood at wavelength longer than 650nm The absorbance of IONPs decreases with increasing wavelength (Figure 4 8B ). Although the strongest absorption for IONP s lies near 650nm, the appropriate wavelength for medical imaging is within the transparent window of 700 1000nm which increas es the penetration depth. W e measured the photoacoustic signal of NIR830 ATF IONP between wavelengths of 730 870nm (Figure 4 8B ) and our findings agreed well with IONP absorption spectra presented by Galanzha 150 T umor bearing m ice were imaged at 24 hours after injection of NIR830 ATF IONP and 730nm was chosen as the best wavelength for in vivo photoacoustic imaging From the MAP photoacoustic images shown in Figure 4 8A NIR830 ATF
68 IONP targeted tu mor s displayed the highest absorption at 730nm compared to 800nm and 850nm. From the quan titative plot shown in Figure 4 8C the in vivo photoacoustic signal at 730nm is 21% and 49% greater compared to 800nm and 870nm respectively Similarly, the absorpt ion of NIR830 ATF IONP at 730nm was 18% and 47% higher compared to 800nm and 870nm respectively Th is in vivo data is consistent with in vitro signal enhancement studies To investigate the feasibility of photoacoustic contrast enhancement with targeted NIR830 ATF IONP, we performed in vivo PA imaging of 4T1 mouse mammary tumor s in the following three groups of mice: 1) one group (n=3) received an uPA R targeted IONP targeting agent ( 100 p mol NIR830 ATF IONP, 2) one group (n=2) received a non targeted cont rol that conjugated with bovine serum albumin (BSA) IONP ( 100pmol NIR830 BSA IONP ) and 3) the third control group injection (n=2). Nanoparticle probes were injected via the tail vein. As shown in Figure 4 9B F J the contrast between tumor to normal tissue is low before injection. At 24 hours post NIR830 ATF IONP injection the contrast increased significantly while there was no significant contrast increase using the non tar g eted NIR830 BSA IONP or in animals without injection ( Figure 4 9D H L ). To verify the delivery of NIR830 ATF IONP to tumor cell s NIR fluorescent images were collected before each ph o toaocustic experiment. W e observed the strongest NIR signal in the tumor site at 24 hours post injection for NIR830 ATF IONP (Figure 4 9A ) and the strongest NIR signal for NIR830 BSA IONP was in the spleen (Figure 4 9E ). We plotted the photoacoustic enhancement in the tumor as a function of time in Figure 5 A The photoacoustic signals with NIR830 ATF IONP increased 3 and 10 times
69 at 5 and 24 hours post injection compared to non injection control mice. This result indicates that NIR830 ATF IONP accumulated in the tumors as expect ed Figure 4 10B shows the quantitative comparisons of photoacoustic signal s in the tumor using different con centrations of NIR830 ATF IONP Compared with non injection control mice the photoacoustic contrast enhancement for 50pmol, 100pmol and 170pmol is 300%, 1000% and 1200% respectively (Figure 4 10B ). The trend of photoacoustic signal enhancement over conce ntration was significantly different. Specifically, t he photoacoustic signal increased steeply from 50pM to 100pM and then slowly from 100pM to 170pM. T umor s w ere resected and imaged by the NIR fluorescent system ( Figure 4 10C ) T he NIR signal inside the t umor is strong and no NIR signals from the surrounding tissue or organs. To evaluate the clinical utility of this technique, the imaging depth was investigated by adding biological tissues (chicken breast) to th e top of the mice skin. Figure 4 11A (left) s hows the MAP of the tumor in m ice injected with NIR830 ATF IONP at 24 hours post injection without adding chicken breast. T he dashed red line represents the position of B scan images shown in Figure 4 11C Even a fter adding 31mm of chicken breast ( Figure 4 11A right), the t umor is clearly seen in Figure 4 11C (d6). As the imaging depth increased, the signal to noise ratio (SNR) decreased from 4 0dB to 18dB as shown in Figure 4 11B 4 .2.4 Discussion and S ummary This work represents the first report on in viv o molecular photoacoustic tomography of breast cancer with receptor targeted magnetic iron oxide i n a tumor bearing animal model The photoacoustic signal enhancement f or NIR830 ATF IONP
70 was 3 times higher compared to that for non targeted NIR830 BSA IONP and 10 times higher than the non injection c ontrols T h e uPA R targeted IONP nanoparticle probe used in these studies address some of the challenges for the use of targeting specific tumor imaging agents Our methodology utilizes stable and high affinity t argeting ligands and produce s strong imaging contrast for multi image modalities. From prior extensive studies, human breast cancer and tumor stromal cells have a higher level of uPA receptor compared with other normal breast tissues. 152 153 In addition, t he highest level of uPA receptor expression is detected in the invasive edge of the tumor region usually enriched in blood vessels making it accessible for uPA receptor targeted IONP in this area. 154 155 The high quality and uniformly sized IONPs used in t his study were coated with a thin ampiphilic copolymer and have a relatively small particle complex (18nm) which is more suitable for in vivo delivery of the imaging probe compared with other receptor targeted or non targeted nanoprobes for photoacoustic i maging It also has been shown that polymer coated IONPs have more than 8 hours of plasma retention time compared with other molecular imaging agents for PAT which are usually less than 2 hours. The delivery of NIR830 ATF IONP to the tumor was verified by planar NIR fluorescent imaging. This shows the potential for us to combine fluorescenc e molecular tomography (FMT) with photoacoustic tomography While FMT has lower spatial resolution than PAT, it has higher specificity than photoacoustic imaging techniq ues and the combination of PAT and FMT would provide complementary information for breast cancer detection The demonstrated imaging depth of 31mm indicates the potential of our method for tumor imaging in humans. While the photoaocustic system as describe d here is not
71 yet suitable for clinical application s, it can be improved to be a clinical prototype using a commercial ultrasound array and/or a high frame rate of laser pulses with a high speed scanning system By using high frequency focused ultrasound t ransducer (>50MHz), the microvasculature inside and around the tumor can be imaged as shown in Figure 4 9 indicating the potential of understanding the interplay between the tumor microvasculature and delivery of targeted contrast agents We are also inve stigating m ulti wavelength and quantitative PAT reconstruction methods to calculate functional parameters and nanoparticle concentration s in tissue This will allow us to quantitatively study the delivery mechanism of NIR830 ATF IONP through microvasculatu re to tumor tissue. 4 .3 Photoacoustic a nd F luorescence Tomography o f HER 2/Neu Positive Ovarian Cancers Using Receptor Targeted Nanoprobes In An Orthotopic Human Ovarian Cancer Xenograft Model 4 .3.1 Motivations Ovarian cancer has a low survival rate due t o the lack of specific early symptoms. 156 157 Although prophylactic oophorectomy (PO) can reduce the risk of ovarian cancer by more than 50% in BRCA mutation carriers, PO also increases the mortality rate in women undergoing this procedure prior to the age of 45. 158 Several imaging diagnostic technologies developed in the past decades for early detection of ovarian cancers has some limitations due to their low specificity and sensitivity. 159 161 More recently, the use of imaging probes that specifically tar get ovarian cancer showed promises for enhanced sensitivity and specificity in tumor imaging. Positron emission tomography (PET) and single photon emission tomography (SPECT) using targeted cancer probes with high specificity and sensitivity have been used clinically to detect
72 cancers. However, their resolutions are generally poor and they are not able to accurately locate tumors. 162 163 Fluorescence molecular tomography (FMT) is an inexpensive and fast optical imaging modality that has been widely used i n molecular imaging. FMT is able to sensitively detect low molecular concentration within the tissue. In contrast to planar fluorescence molecular imaging it provides depth information. However, similar to PET and SPECT, FMT has poor spatial resolution 16 4 Photoacoustic tomography ( PAT ) is a hybrid imaging method that combines the rich optical contrast with the high resoluti on of ultrasound imaging PAT detects wide band acoustic waves that are generated through transient thermoelastic expansions due to th e absorption of short pulses by biological tissue. PAT has been used in vascular biology, ophthalmology, dermatology, gastroenterology, and breast imaging. A variety of PAT contrast agents have been used to image brain tumors, angiogenic processes melano mas, lymph nodes, and the cerebral cortex. Recently, Ekaterina et al. demonstrated the use of targeted IONP as a contrast agent to detect circulating tumor cells using the photoacoustic effect. Here, we firstly report the use of a new near infrared dye lab eled HER 2/neu specific affibody conjugated IONP (NIR830 Z HER2:342 IONP) for PAT and FMT imaging of orthotopic ovarian cancers in a human ovarian cancer xenograft model in nude mice. This nanoparticle probe provided photoacoustic signal enhancement and flu orescent signal as well as tumor targeting capabilities.
73 4 .3.2 Materials and Methods 4 .3.2.1 Cell lines The human ovarian cancer cell line SKOV3, stably expressing a firefly luciferase gene (SKOV3 Luc) was kindly provided by Dr D. Matei at Indiana Unive rsity Purdue University at Indianapolis (IUPUI). SKOV3 luc cells were cultured in McCoy's 5A (Cat # 10 050 CV, Cellgro, Mediatech Inc. VA, USA) supplemented with 10% fetal bovine serum (FBS Cat # SH30396.03, Hyclone from Thermo scientific) and 1% penicilli n streptomycin (Cat # SV30010, Hyclone, Logan, Utah). Cultured cells were maintained at 2 Growth medium was replaced every other day. When appropriate, cells were detached with trypsin ( C at # 25 050 C1, Cell gro, Mediatech Inc. VA, USA) and transferred to single cell suspension s. 4 .3.2.2 HER 2/neu specific affibody conjugation to IONP A histidine tagged HER 2 specific affibody (His6 Z HER2:342 Cys), was provided by Dr. J Capala at the National Institute of Bio medical Imaging and Bioengineering (NIBIB) Bethesda, Maryland. The NIR dye NIR830 maleimide, was treated with Tris (2 carboxyethyl) phosphine (TCEP) for 30 minutes and then conjugated with HER 2/neu specific affibody. Amphiphilic polymer coated IONPs wer e activated by treatment with ethyl 3 dimethyl amino propyl carbodiimide (EDAC) and sulfo N hydroxysuccinimide (sulfoNHS). Activated IONP were then incubated with the NIR 830 conjugated Z HER2:342 afiibody The resultant Z HER2:342 NIR830 IONP were purified using Nanosep 100Kgel filtration columns (Pall Corp, Ann Arbor, MI). 4 .3.2.3 Orthotopic human ovarian cancer xenograft model The ovarian tumor model was established by injecting 2 x 10 7 SKOV3 luciferase gene positive human ovarian cancer cells into the o varies of 6 8 week old female
74 athymic nude mice (Harlan). All mouse surgical and imaging procedures were approved by University Institutional Animal Care and Use Committee. Mice were imaged when orthotopically xenografted tumors reached around 5 mm in size, usually 4 6 weeks after tumor cell injection. Tumor growth was monitored and quantified weekly using a bioluminescence imaging system (Caliper Life Sciences, Hopkinton MA ) 4 .3.2 .4 Fluorescence molecular tomography imaging System. The photoacoustic syste m is the sa me as that used in Figure 3 1A The handheld optical fiber b ased FMT system is shown in Fig ure 4 12D In this system, a continuous wave (CW) 785nm laser (M5 785 0080, Thorlabs) was mounted in a 2D linear stage (Model 17AMA045, CVI MellesGriot). A convex lens was used to focus the laser into each source fiber in the array. For each source illumination, a 1024 1024 pixel CCD camera (Princeton Instruments, Trenton NJ) equipped with a high performance band pass filter (Thin Film Imaging Technologies, Greenfield MA) was used to collect the emission light from the detection interface. Optimal exposure time was determined for each experiment and binning of 4 4 pixels was used to improve the signal to noise ratio (SNR). A photograph of the imaging probe with 10 sources and 15 detectors is shown in Fig ure 4 12D Fluorescence images were reconstructed using an iterative finite element based algorith m, described by Zhao et al. 165 4 .3.2 .5 In vivo and ex vivo planar near infrared fluorescence imaging Planar N IR fluorescence imaging was performed using a Kodak i n vivo imaging system Fx (Carestream Health Inc., New Haven, CT) to demonstrate specific accumulation of the targeted IONPs in the ovarian tumors and anatomic localization of the tumors. All im ages were captured using an 800nm excitation and 850 nm emission
75 filter set with a 180s exposure time and a value of 0.2. For each optical image, a corresponding radiographic image was taken to establish the anatomical location of the tumor. 4 .3.2 .6 Histolog ical ana lysi s After imaging, mice were sacrificed and tumors were excised and fixed with 10% buffered formalin. Paraffin tissue sections were stained with Prussian blue or hematoxylin and eosin (H&E). Images were acquired at 20X magnification using a Zeiss Axioplan 2 upright microscope. 4 .3.2 .7 Statisti cal analysis All data obtained from the experiments were summarized using means standard error of the mean (SEM). 4 .3.3 Results To evaluate our latest PAT or FMT tomographic systems for imaging tumors, human ovarian tumor xenografts were generated orthotopically in the nude mice. Specifically, SKOV3 luc cells were injecte d into the mouse ovaries (Fig ure 4 13A ) and tumor growth was monitored by bioluminescence imaging (Fig ure 4 13B ). Planar NIR fluorescence images pai red with X ray were taken 24 and 48 hours after tail vein injection of 400 picomoles of NIR830 Z HER2:342 IONPs to show the optical signal in the luciferase positive tumor location. It seems that the fluorescence intensity reached to the strongest level in the ovarian cancer at 48 hrs fol lowing systemic delivery (Fig ure 4 13C ), indicating that the nanoprobes specifically accumulated in the ovarian tumor. Prussian blue staining of paraffin tumor tissue sections revealed the presence of tumor cells with blue i ron staining, indicating the internalization of IONPs in the tumor cells (Fig ure 4 13D ).
76 The absorption spectra of IONP for photoacous tic enhancement is shown in Fig ure 4 1 2B The IONP absorption decreases as the wavelength increases, with the highest abso rption occurring at 650nm. For clinical applications, the optimal operational wavelength lies within the tissue transparent window of 700 1000nm. The absorption of nanoprobes in photoacoustic phan tom studies (black dots, Fig ure 4 12B ) agrees well with the absorption spectra of IONP. Three groups of mice were imaged at 730nm, 800nm and 870nm and HER 2/neu targeted ovarian cancer nanoprobes (NIR830 Z HER2:342 IONP) had the highest photoacoust ic signal (left image in Fig ure 4 14A ), in contrast to surrounding no rmal tissues. In contrary, there was no significant photoacoustic signal enhancement with non target bovine serum albumin NIR IONP (NIR830 BS A IONP) (middle image in Fig ure 4 14A ) or in non injection controls (right image in Fig ure 4 14A ) Data from quant itative plots shown in Fig ure 4 14B revealed that the strongest photoacoustic signal of IONPs accumulated in the ovarian tumors was at 730nm, which was 500% higher than non injection controls and 300% greater than signal s from the mice injected with non ta rgeted BSA. In clinical applications, ovarian tumors are often located deeply inside the low peritoneal cavity. To evaluate the use of this technique in tumor imaging in humans, we examined the capability of detection of tumors in the deep tissue using PAM T by adding different thicknesses of chicken breast tissue placed on the top of mouse back. The top row in Fig ure 4 15A D shows the B scan images with the addition of different thicknesses of chic ken breast. The bottom row (Fig ure 4 15A D ) shows the MAP im ages with chicken breast additions. As shown, the ovarian tumors are clearly imageable even after the addition of 19mm thick of exogenous tissues. We also
77 investigated the imaging depth of FM T (Third and fourth rows in Fig ure 4 16A C ) and found an imaging depth of up to 10mm. We compared the spatial resolution of FMT and PAMT imaging at different depths (Fig ure 4 16 ). The 3D and 2D images of PAMT with different thicknesses of chicken breast are shown, as well as results wit h FMT. As shown, the axial and lat eral resolution s of FMT are relatively poor. Although the position of image center lies in the right depth, the axial and lateral size s are much larger than actual size s In contrast, the lateral and axial resolution s of PAMT are 820 m and 200m in our sys tem. Therefore, for PMAT, the reconstructed lateral and axial size s are very close to real size s The lateral and axial reconstructed size s of PAMT and FMT were plotted (Fig ure 4 17 ) and the average lateral and axial FWHM of the tumor in PAMT at three diff erent depths (3, 6 and 10mm) w ere 5mm and 3mm which are comparable to the size of the resected tumor. Using FMT, the lateral FWHM w as 5 to 13mm and axial FWHM was 3mm to 14 mm as the depth increased. 4 .3.4 Discussion In this report, we demonstrate d the f easibility of using PAMT and FMT to detect ovarian tumors in mice after systemic delivery of HER 2/neu targeted NIR830 Z HER2:342 IONP. Unlike gold nanoparticles, non targeted IONP has be e n approved for pilot studies on humans and is already used as a cont rast agent for magnetic resonance imaging ( MRI ) NIR830 Z HER2:342 IONP is a novel receptor targeted nanoprobe that specifically binds to ovarian tumors in vivo, as detected by planar NIR fluorescence imaging and histology. Planar NIR fluorescence imaging can monitor the delivery of nanoprobes and relay accurate tumor position and size information. However, it cannot map deeply positioned
78 ovarian tumors because of poor spatial resolution and a lack of depth information. In this study, our handheld FMT probe showed the potential to noninvasively map ovarian tumors in 3D at depths up to 10mm. The method was fast, taking less than 2 minutes to cover a 20 20mm imaging area. However, the poor axial and lateral resolutions affect the accuracy, which limits the fut ure applications in human patients. As a complementary method to FMT, PAMT was employed to detect ovarian tumors with contrast enhancement from IONP. PAMT had high axial and lateral resolutions of 200m and 820m, respectively, as well as imaging depth cap abilities up to 19mm. However, the system is not portable and image acquisition is slow (20 minutes to cover 18 18mm 2 area). For these reasons, PAMT is not yet suitable for imaging large areas. Here, we show a fast handheld FMT system can be used to mark a reas possibly containing ovarian tumors, and then use PAMT to image the marked area and accurately map the position of ovarian tumors. Our study has some limitations. The slow and bulky photoacoustic imaging system is not suitable for clinical application However, this is not a fundamental problem of PAMT and could be overcome using a commercial ultrasound array and/or a high frame rate of laser pulses with a high speed scanning system. Although our handheld FMT system is fast and portable, it offers limi ted imaging depth capabilities (10mm). For tumors positioned more than 10mm under the skin surface, our FMT system is not applicable. However we are currently investigating the use of high compact optical fiber bundles to make a small FMT probe containing more sources and detectors. From our simulation and phantom tumor detection experiments, we were easily able to image up
79 to 25mm away from the detector. A goal is to integrate these two systems into one handheld probe. In summary, noninvasive in vivo FMT a nd PAMT mapping of ovarian tumors after systemic delivery of NIR830 Z HER2:342 IONPs was successfully accomplished in a n ortho topic human ovarian tumor xenograft model in the mice. In this Chapter, we discussed about three applications of cancer research u sing ARPAM. All these three applications are cancer treatment related biological research. From our experience, it is difficult to translate ARPAM to clinical application such as image guided surgery that is very important for surgeons to improve the succe ss rate of surgeries. In next Chapter, I will introduce a MEMS based intraoperative photoacoustic probe for image guided surgery.
80 Fig ure 4 1 Comparative analysis of scAAV2 mediated transduction of MMECs Cells were infected wit h the WT or the triple quadruple or septuplete tyrosine mutant scAAV2 hGFP vectors at a of multiplicity infection (MOI) of 10,000 vgs/cell. Transgene expression was detected by fluorescence micro scopy, 72 hr post infection. A ) R epresentative images of hGFP expression in MMECs i nfected with differe nt tyrosine mutant scAAV2 hGFP. B) Quantitative analysis of AAV2 transduction efficiency in MMECs. hGFP expression is shown in arbitrary units calculated by multiplying the percentage of positive cells by the mean fluorescence intensity in each sample. Each value represents the average of three samples based on 10,000 counted cells.
81 Fig ure 4 2 scAAV2 encoding siRNAs against UPR proteins. A ) shows a scAAV2 construct with smCBA driving expression of siRNAs against mice UPR p roteins. B ) shows the sequences of the oligonucleotides encoding siRNAs agai 1 and ATF6. C ) Transduction efficiency of scAAV2 sept mut vectors in MMECs. Representative images of mCherry expression in MMECs infected with scAAV2 sept mut vectors at three days post infection (left column) and a comparison of mCherr y expression at different MOIs as indicated shown in arbitrary units calculated by multiplying the percentage of positive cells by the mean fluorescence intensity (right column). Each value represents the average of three samples (eight pooled wells of a 2 4 well plates/sample), based on 10,000 counted cells. UnST=unstimulated; scr=scrambed siRNA scr=scrambed siRNA
82 Fig ure 4 3 Pro angiogenic and survival role of the UPR proteins on endothelial cells. MMCEs were co cultu red with NeuT EMTCL2 for 48 hr. A ) M MECs were cultured between two layers of Matrigel for 48 hr. Morphometric analysis of the degree of tubule formation was then performed. B ) Representative photomicrographs of microscope fields showing tubule formation. C ) Annexin V propidium iodide posit ive cells were shown (top right, late stage apoptosis; bottom right, early apoptosis). D ) Cell apoptosis was expressed as a percentage of apoptotic cell s in the total cell population. E ) T he cell proliferation was assessed by crystal violet staining.
83 Fi g ure 4 4 Seria l photoacoustic imaging of the developing tumor vasculature and quantitative analysis for mice breast cancer xenograft models. Mice breast cancer xenografts were established by subcutaneous injection of 110 5 NeuT or NeuT EMTCL2 cells into t he mammary fat pads of the mice. Serial PA imaging was performed on the same tumor inoculation site on day 3, 5, 7 and 9 post tumor inoculations. A ) Representative serial PA images of NeuT model (top panel) and Ne uT EMTCL2 model (bottom panel). B ) Entropy extraction for change in the vessel density over different time points as indicated C ) Comparative analysis of normalized vessel volumes at different time point as indicated. D ) Tumor volume was calculated from daily caliper measurements of the large A an d smallest B diameters of each tumor using formula ab 2 0.4 Representative images of H&E staining for breast cancer tissue sections of NeuT (top) and NeuT EMTCL2 (bottom) xenograft models.
84 Fig ure 4 5 Knockdown of the siRNAs against the UPR protein r esulted in decreased tumor growth and tumor vasculature in mice breast cancer xenografts. Mice breast cancer xenografts received intratumoral scAAV2 sept mutant vector 1 or ATF6. PA imaging was performed on the same tu mor inoculation site on day 3, 5, 7 and 9 post tumor inoculations. A ) Representative PA images of different treatments: the scrambled 1 siRNA (third panel) and ATF6 siRNA (bottom panel). B ) Entropy extract ion for change in the vessel density over different time points as indicated C ) Comparative analysis of normalized vessel volumes at different time point as indicated. D ) Tumor volume was calculated from daily caliper measurements of the large A and smal lest B diameters of each tumor using formula ab 2 0.4
85 Figure 4 6 Illustration of NIR 830 dye and ATF conjugated IONP probe. A ) Schematic of NIR 830 dye. B ) NIR 830 dye is conjugated to mouse ATF peptide through a bond between maleimide esters and free thiol groups of cystidine residues of the the peptide. Dye labeled peptides were then conjugated to carboxyl group of the polymer coating on the IONPs. C ) Resulting optical probe has an Ex 800nm and Em 825nm. Fig ure 4 7 Schematic of NIR fluorescence imaging system.
86 Fig ure 4 8 In vivo and in vitro test of targeted nanoprobe using different wavelengths. A ) In vivo MAP photoacoustic images of tumor s u sing signals of 730nm, 800nm and 870nm 24 hours after injection of NIR830 ATF IO. B ) Absorption chara cteristics of NIR830 ATF IO, IO and blood. C ) PA signals using different wavelength inputs.
87 Fig ure 4 9 In vivo photoacoustic MAP and fluorescence images before and after injecti on. Micro graph s were merged with fluorescence images taken 24 hours post i njection with indicated agent ( A, E I ) Panels B thru L Photoacoustic MAP images were merged with images of blood vessels before injection ( B, F J ), and at 5 hours ( C, G, K ) and 24 hours post injection ( D, H, L ).
88 Fig ure 4 10 Quantitative plot and com parison of photoacoustic and fluorescent signals. A ) NIR fluorescence imaging of mice with resected tumor (top panel ), cross section of tumor along black dashed line (middle panel ) and section stained with Prussian blue (bottom panel). B ) Quantitative plot of fluorescence signals before and after injection of NIR830 ATF IO. C ) Quantitative plot of photoacoustic signals before and after injection of NIR830 ATF IO. D ) Comparison of fluorescence signals with different concentration s of injected NIR830 ATF IO. E ) Comparison of photoacoustic signals with different concentration s of injected NIR830 ATF IO.
89 Fig ure 4 11 In vivo photoacoustic images with adding chicken breast between detector and tumor A ) MAP image of tumor at 24 hours after injection without a dding chicken breast (left) and photograph of 31mm thick chicken breast (right). B ) SNR versus depth. C ) B scan images by adding different thickness of chicken breast
90 Fig ure 4 1 2 Schematic and spectrum of targeted nanoprobe and FMT system description. A ) Schematic representation of HER 2 targeted Z HER2:342 NIR 830 dye IONP: NIR 830 dye was conjugated to cysteine resid ues of HER 2 specific affibody The Z HER2:342 NIR 830 complex was conjugated to the C terminal end s of amphiphilic polymer ( 10nm in diameter). B ) Photoacoustic absorption spectra of IONP, NIR830 IONP and blood f rom 650nm to 900nm are shown. C ) Absorption and emission spectra of NIR830. The highest absorbing wavelength was 800nm and the strongest e mission wavelength was 825nm. D ) Schematic of the hand held FMT system.
91 Fig ure 4 13 Multi mo de in vivo imaging and histological validation. A ) SKOV3 luc cells were injected orthotopically into mouse ovar ies B ) Bioluminescence ima ging show ed the tumor l ocation. C ) Examination of tar get specificity of Z HER2:342 NIR 830 dye IONP s to ovarian tumors in vivo In these experiments, SKOV3 luc tumor bearing mice were injected with NIR830 Z HER2:342 IO conjugates via tail vein s and optical imaging was performed 24h and 48h post injection. The NIR signal scale was generated using the Kodak molecular imaging software. Anatomical X ray images were simultaneously captured to validate tissue position. D ) Pathological analysis of xenograft tumors All tumors were formalin fixed, paraffin embedded, se ctioned, and stained for analysis and photography. Representative images demonstrate the tumor tissue and presence of IO nanoparticles stained by P russian bl ue inside the tumor tissue (yellow arrows)
92 Fig ure 4 14 Quantitative analysis based on differe nt wavelengths A ) MAP photoacoustic images of ovarian tumor s injected with actively targeted NIR830 ZHER2:342 IONP (left), non targeted NIR830 BSA IO NP (middle) at 730nm 48 hours after injection and without injection/control case (right). B ) Quantitative c omparison of PA signals of actively targeted (HER 2 affibody), non targeted (BSA) and non injection cases at 730nm, 800nm and 850nm.
93 Fig ure 4 15 Image depth ability evaluation. B scan (top row) and MAP (bottom row) PA images of active ly targeted tumo r s with A ) 0mm B) 4mm, C) 10mm D) 19mm exogenous chicken breast tissues added between tumors and the detector. Scale bar 1mm, white arrow s indicate the chicken breast surface.
94 Fig ure 4 16 3D results for both PAMT and FMT. 3D PAMT images (top row), B s can PAMT images along white dashed lines (second row) and 2D slices along white dashed lines (third row) in 3D FMT images(bottom row) with activ ely target ed imaging agent and additional A) 3mm, B) 6mm C) 10mm chicken breast tissue
95 Fig ure 4 17 Quant itative comparison of spatial resolution between photoacoustic and fluorescence molecular tomography images with increased imaging depth. A ) Graph shows results for three 1D profiles taken from along the brown dashed lin es in the second row in Fig. 5 A C B ) Graph shows results for three 1D profiles taken from along the yellow dashed lines in the third row in Fig. 5a c. C ) 1D PA profiles along Z axis through tumor center at depths of 3mm, 6mm and 10mm D ) 1D FL profiles along Z axis through tumor center at depths of 3mm, 6mm and 10mm.
96 CHAPTER 5 INTRAOPERATIVE PHOTOACOUSTIC TOMOGRAPHY 5 .1 M otivation s In a lumpectomy, the surgeon removes the tumor along with some healthy, non cancerous surrounding breast tissue. The outside of the tumor, or margin, is typ ically examined using pathological methods to determine if the border of a tumor mass is more than 2mm away from the surrounding normal breast tissues; this is considered to be a negative margin. However, if the tumor cells come to the edge of the resected tissue, it is cons idered to be a positive margin. 166 167 Methods currently available for intraoperative margin assessment, such as gross examination, frozen section, ultrasound, and touch prep, have various limitations with false negative diagnoses in 20 to 50% of the patients. Therefore, there is an urgent need to develop sensitive and accurate intraoperative methods for detecting tumor margins in order to reduce tumor recurrence and to improve the survival rate of breast cancer patients. 168 169 Optical b ased technologies appear to be ideal for intraoperative imaging as they can be miniaturized, and are inexpensive, fast and sensitive. In recent years, an increasing number of optical studies using elastic scattering spectroscopy (ESS), Raman spectroscopy, Optical coherence tomography (OCT) or diffuse reflectance spectroscopy (DRS) have been carried out for tumor margin assessment. However, these methods are surface weighted which cannot provide depth information and their maximum sensing depth is limited fr induced photoacoustic imaging, where a single short pulsed light beam illuminates an object and the photoacoustic waves excited by thermoelastic expansion are measured using wideband
97 ultrasound transducer(s), retains the desired high optical contrast/sensitivity while frequency) than pure optical tomographic methods by detecting less scattering ultrasonic waves. The penetration depth for our iPAI system is 2 mm which is excellent for tumor margin assessment. The purpose of this study is to evaluate in vivo high resolution imaging of tumor margins and resection inspection by using iPAI system. We demonstrate this technique using a tumor bearing animal mode l. The results obtained show accurate correlation with the histology. 5 .2 Materials and M ethods 5.2.1 A nimal P rotocol Approval for animal studies was obtained from, and strict animal care procedures followed for all experiments that were set forth by, the Institutional Animal Care and Use Committee (IACUC), as based on the guidelines from the NIH guide for the Care and Use of Laboratory Animals. Human breast carcinoma cells (MDA MB 231 luc D3H2LN, Caliper Life Sciences) were implanted into the mammary fat p ad of 6 week old female nu/nu mice (Harlan Laboratories). Tumors were allowed to grow for 6 days prior to imaging experiments to a size of ~0.5cm. Animals were anesthetized via intraperitoneal injection for imaging and surgical procedures, and were subse quently sacrificed using IACUC approved techniques. 5 .2. 2 Intraoperative P hotoacoustic T omography S ystem In our microelectromechanical system (MEMS) based photoacoutc imaging system ( Figure 5 1 ), short laser pulses of 6 ns duration are generated from a Nd:YAG 532nm laser (NL 303HT from EKSPLA, Lithuania), and a reflection mirror is employed
98 to deliver light in to a miniaturized probe (Fig ure 5 2A ). Inside the probe, there are an aperture, a wedge shaped mirror ( Fig ure 5 2C ) and a MEMS mirror ( Fig ure 5 2B ). A custom designed ultrasound transducer (unfocused ring shape, 5.5MHz central frequency, Resource Center for Medical Ultrasonic Transducer Technology, University of South California) is attached to the end of the probe to detect photoacoustic waves. The structural design, dimensions and performance of the transducer are presented in Fig ure 5 3 Through the ultrasound transducer, the photoacoustic waves are converted to electrical signals which are amplified, filtered, and fed into a computer through a da ta acquisition card ( Figure 5 1 ). A MEMS mirror based on electrothermal bimorph actuation is utilized to realize two dimensional light scanning. The moveable mirror plate is as large as 0.8mm 0.8mm in a 2mm 2mm device footprint. The MEMS mirror is fixed on a ramp with an angle of 22.5 (Fig ure 5 4A ), allowing light to scan from position 3 to position 4 ( Figure 5 4B ), while the MEMS mirror moves from position 1 to position 2. The measured vertical resona nt frequency is 500Hz. In Video 1 online, four channels of voltage signals are u tilized to drive four actuators of the MEMS mirror. The probe is miniaturized through the integration of MEMS mirror and transducer. In the current design, the probe diameter is 12mm but further size reductions are achievable. Our miniaturized probe works effectively and stably in the front view and reflection mode, which is easier for a surgeon to operate during surgery than a probe in the side view or transmission mode. To d etermine the axial and transverse resolution versus target position, we imaged a 100m diameter tissue mimicking target (absorption coefficient =0.049mm
99 reduced scattering coefficient =0.5mm ) embedded in turbid media ( =0.012mm =0.35mm ). The target can be treated as an ideal line target because the central frequency of the transducer is much longer than the diameter of the target. Chicken breast with thickness of up to 2.3mm was added to investigate the resolution and signal noise ratio versus target position. 5 .2. 3 Imaging P rocedure The capabili ty of our iPAI system was qualitatively and quantitatively validated by mouse experiments involving tumor extirpation (n=5). The mouse was anesthetized during the experiments. The laser beam illuminated the tumor surface along Z axis at an energy density o f 8mJ/cm 2 that is lower than the American National Standards Institute safety limit of 20mJ/cm 2 Two 0~4V ramp signals with repetition frequency of 0.1Hz were used to drive two actuators moving along Y axis and the other two signals with the same magnitude but different repetition frequency of 0.002Hz were used to drive the actuators moving along X axis. The photoacoustic signals were recorded without averaging at each scanning point, amplified and converted to a one dimensional (1D) image assuming a consta nt ultrasound velocity in soft tissue (1.54 area (10mm 10mm) in XY plane was imaged to produce a 3D image of the tissue volume. 5 .2. 4 Histology Each tumor was cut into two pieces by a sharp blade along Z axis for H&E stain in XY plane and along X or Y axis for H&E stain in XZ or YZ plane. Tumors were harvested and preserved in 10% neutral buffered formalin for 10 hours at room temperature. Histological sections were stained with hematoxylin and eosin and
100 prepared by standard procedures. Aperio ImageScope V10 .2.2.2352 was used to microscopically image the slices and measure the actual size. 5 .3 Spatial R esolution of t he S ystem Fig ure 5 5 A shows the Hilbert transform of a typical A line acoustic signal and Fig ure 5 5 C function. The 6dB width of the signals shown in Fig ure 5 5 A and Fig ure 5 5 C present the axial and tra nsverse resolutions. In Fig ure 5 5 B (0.24mm~0.96mm) dashed lines. The signal to noise ratio (SNR) measured in turbid media is shown in Fig ure 5 5 D decreasing from 32dB to 18dB within 2.3mm. 5 .4 Phantom E xperiments As shown in Figure 5 6A the pencil lead was embedded 1.2mm below the surfac e of a 50mm diameter cylindrical solid phantom, consisting of Intralipid as the scatterer and India ink as the absorber. The phantom had (absorption coefficient) and (reduced scattering coefficient). A gar powder (2%) was used to solidify the Intra lipid/ink suspensions. Figure 5 6 B presents a typical A line signal collected (blue line), along with its amplitude after the Hilbert tr ansform (red line), while Fig ure 5 6C shows the 2D image of the object. Us ing the criteria defined by the FWHM of multiple A line signals, the size of the recovered object is estimated to be 0.72mm compared to the actual object size of 0.70mm. Figure 5 7A and B gives the photograph and recovered image of another phantom where m ultiple targets were embedded at different positions/depths. F rom the image shown in Figure 5 7B we see that the targets are all detected.
101 The results given in Figure 5 8 show the three dimensional imaging ability of our system. In this experiment, a penc il lead with a diameter of 0.7mm was embedd ed in one piece of chicken (Fig ure 5 8A ) at a depth of 1.5mm from the surface. The recovered 3 D images are presented in Fig ure 5 8B D (at coronal, sagittal and cr oss section views) and in Fig ure 5 8E (3D renderin g of the recovered image). Again, the object is accurately imaged in terms of its size, shape and location. 5 5 Human Blood V essels E xperiments Here we demonstrate the in vivo imaging ability of our MEMS based photoacoustic imaging system for visualizing blood vessels in a human hand. The photograph of the hand and the barely visible blood vessels u nder the skin are shown in Fig ure 5 9A while the recovered 3D image of the blood vessels is given in Fig ure 5 9B We can see that the blood vessels are clearl y imaged with correct shape and size for most part. We also note that some middle portions of the blood vessels (indicated by arrow in Fig ure 5 9B ) are notably distorted or missing. This is most likely attributed to the fact that these portions of the bloo d vessels are physically located deeper over other parts of the blood vessels of interest and that the sensitivity of the transducer responsible for this part of the imaging area is lower due to the limited directivity of the transducer used. It is noted t hat the shape of the vessels is not round likely due to the non linear effect of the MEMS mirror. We are seeking ways to resolve this issue by using a proper calibration method. 5 6 Preclinical Evaluation o f Intraoperative Photoacoustic Tomography The vol umetric shape, size and position of the tumor are shown in Fig ure 5 10B The surgeon removed the tumor marked wi ure 5 10B In these experiments, the line of surgical resection of tumor was intentionally carried out beyond the margin of the ima geable tumor, consistent with clinical practice. After the surgical procedure, the
102 surgical area shown in Fig ure 5 10C was inspected again by the same imaging method to ensure the complete resection of tumor. The image shown in Fig ure 5 10D confirmed the c ompleted resection of tumor. The signal (indicated by the arrow in Fig ure 5 10D ) was from the inner surface of the ring shaped transdu cer. This signal disappeared in Fig ure 5 10B due to strong signals from the tumor and normalized image procedure was used. Following tumor removal, the tumor size as determined by iPAT was compared with the actual tumor size as measured histologically. H&E stained sections along XZ plane were obtained after the photoacoustic experiments. A 2D PAT slice along the red dashed l ine in Fig ure 5 10B as selected and compared with the corresponding H&E stained section (the black dashed line in Fig ure 5 10B ) as show n in Fig ure 5 11A and B Fig ure 5 11C shows the photograph of the tumor in comparison with the H&E and PAT slices. Also, the red dashed line along the top margin of the tumor shown in Fig ure 5 11A matches well with the top margin (black dashed line in Fig ure 5 11B ) of the H&E section and photograph (red dashed line in Fig ure 5 11C ). The size along two typical directions (Lin es 1 and 2 in Fig ure 5 11A ) was estimated to be 2.1 and 1.2mm, respectively, in excellent agreement with the actual dimensions of 2.2 and 1.1mm Fig ure 5 11B ). Such comparisons were applied to other two H&E staining sections along XZ plane for this mouse, and we found that the measured error was less than 9.1% for all three slices. In another independent mouse tumor experiment, photoacoustic imaging demonstrated tumor nodules as shown in Fig ure 5 11D Th e tumor size imaged by PAT along lines 1, and 2 ( Fig ure 5 11D ) was 2.5 m m, 1.9mm, showing an accurate
103 measurement of the actual size of 2.5mm, 2.1mm, measured by histology along lines Fig ure 5 11E ). In this case, the largest observed error for the remaining two H&E sections was 9.5%. Photoacoustic slices with histological correlations from the third mouse are shown in Fig ure 5 12A C The structures indicated by the arrow in Fig ure 5 12B are consistent with each other. After quantitative analysis, the largest observed error was 6.5%. Fig ure 5 12D F show the PAT slices and histological correlations for the fourth mouse where we added 1.1mm thick chicken breast on top of the tu mor to simulate the real specimen resected from the human breast. The 3D top margins of the tumor were clearly seen in the PAT images and consistent with the H&E sections. The white dashed line shows the surface of the chicken breast. The error between th e PAT image and H&E sections was found to be less than 16.5%. A 1.8mm thick chicken breast was added to the top of the fifth mouse and the PAT images with the associated H&E section are shown in Fig ure 5 12G I The tumor shape imaged by PAT was close to th at from the H&E sections but the error increased to 21.5% in this case. 5 7 Discussion We have demonstrated that our iPAT technique with miniaturized MEMS probe can be effectively used for mapping tumors three dimensionally and for inspecting completenes s of tumor resection in small animals. The qualitative and quantitative results obtained are significant for several reasons. First, iPAT does not use any radioactive tracer or intravenous contrast. Second, the existing PAT techniques are not directly appl icable to intraoperative imaging due to their bulky size or inflexibility of the imaging probe. Our MEMS based probe breaks the limitation with its novel compact
104 design. Third, after tumor resection, a surgeon can inspect the tumor cavity wall to ensure co mpleteness of the procedure. Finally, iPAT has the potential to realize real time image guidance and allow acquisition of functional parameters of tumor when a multispectral laser with high repetition frequency is used. While our iPAT system has been demo nstrated to be practical for intraoperative tumor imaging, several challenges still remain. First, a handheld probe is necessary for a surgeon to operate during surgery. In the current study, our probe was fixed in position using a holder and an aperture w as used to generate a small light spot. This set up would not be applicable to real operative procedures. In the future, we will use an optic fiber coupled with a micro optical lens to replace the aperture and make the probe handheld. Second, imaging speed and area of our current iPAT system is limited by the 10Hz repetition frequency of the laser source currently available in our lab. Considerably faster lasers (up to 5kHz) are now commercially available. Since the resonant frequency of the MEMS mirror is up to 500Hz, the imaging speed will be reduced to 5 s when a laser having a repetition frequency of 500Hz is used. With this improvement, the probe can be moved easily to get a 70mm 70mm within 5 minutes. Third, the probe was sub merged in a tank filled with water which is not applicable to clinical procedures. We plan to make a MEMS probe with a water or ultrasound gel filled balloon attached to the surface of transducer which will then be used for clinical intraoperative tumor i maging. ( Fig ure 5 13 ). Finally, the bleeding problem during a clinical surgery should be considered because the absorption of blood on 532nm is very strong. During the mouse experiments, we used cautery to stop the bleeding and normal saline to wash out th e surgery area after tumor resection. For translating this
105 technique to clinical applications, near infrared pulsed laser should be used to avoid the strong affect from the blood and to increase the imaging depth. The ability of visualizing the tumor in th ree dimension permits the translation of this technology to real clinical image guided surgeries for breast cancer patients. Tumor margin and inspection of completed tumor resection allows the clinicians to diagnose the resected specimen and inspect the su rgical area timely in order to reduce tumor recurrence and improve the survival rate of breast cancer patients. Beside employing photoacoustic imaging for image guided surgery, it also necessary to develop a photoacoustic imaging system to detect tumor non invasively, especially for breast cancer that is the second most important public health problem In next Chapter, we proposed a new circular array based PAT system combined with DOT for noninvasive clinical breast cancer detection.
106 Figure 5 1 System description of the experimental platform. The mouse was fixed into position on the holder with the head exposed to air. An aquatic heater was placed in the tank to maintain a constant body temperature for mouse. The probe was immersed in the water. The ou tput trigger signal of the laser was used to synchronize the data acquisition and function generators (AFG3022B, Tektronics). The magnitude of ultrasonic signals were amplified by a pre amplifier with a gain of 17dB and further amplified by an amplifier wi th a changeable gain from 5 dB to 20dB. The data acquisition sampling rate was 50MHz and a high pass filter (Panametrics, Waltham, Massachusetts) with 1 MHz cutoff frequency was used to cut off the low frequency noises.
107 Figure 5 2 Schematic representat ion of MEMS based photoacoustic imaging system. A ) The illuminating beam from a Nd:YAG laser passes through a mirror mounted in a 45 plane, a miniaturized probe mounted on a three dimensional (3D) linear stage and irradiates the sample s urface. B ) 3D rend ering of the probe in a; an aperture is used to form a small light spot of 0.2mm in diameter and plastic film is used to protect the MEMS mirror from water or ultras ound gel. C ) One surface of wedge shaped silicon base plated with aluminum redirects the l i ght beam to the MEMS mirror. D ) Photograph of the MEMS mirror with four actuators and five pads bonded with gold wire.
108 Figure 5 3 Schematic and performance of a ring sh aped ultrasound transducer. A ) non focused ring shape ultrasonic transducer was fabr icated. To obtain high detection sensitivity, high performance PZT (DL 53HD, DeL Piezo Specialties, LLC, Wellington, FL) ceramic was used to fabricate the ring ultrasonic transducer. The inner and outer diameters of the transducer are 9mm and 11mm respect ively. B ) The central frequency of the transducer was measured as 5.5MHz and the 6dB bandwidth of the transducer was larger than 50%.
109 Figure 5 4 Schematic of the miniaturized probe A wedge shaped mirror plated with aluminum is fixed in position to r edirect the light beam to the MEMS mirror. The MEMS mirror is fixed on a ramp with an angle of 22.5 allowing light to scan from position 3 to position 4 while the MEMS mirror moves from position 1 to position 2.
110 Figure 5 5 Performance evaluation of t h e system. A ) Hilbert transform of a typical A line photoaco ustic signal from the target. B ) Axial and transverse res olutions versus target depth. C ) Transverse point s pread function of the target. D ) Signal to noise ratio (SNR) versus target depth.
111 Fi gure 5 6 Phantom experiment with single target. A ) Photograph of phantom. B ) Raw signal (blue) and signal a fter Hilbert transform (red). C ) Image result of the phantom Figure 5 7 Diagram of the phantom and imaging result with multiple targets
112 Figu re 5 8 Ex vivo experiment with single target. A ) Diagram of the sample. B ) 2D and 3D result of the sample Figure 5 9 In vivo experimental evaluation using human hand. A ) Photograph of the image area. B ) 3D image result of the blood vessels
113 Figure 5 10 In vivo three dimensional (3D) tu mor mapping in a mouse model. A ) Photograph of the mouse with tumor implant ed in abdomen before surgery. B ) In vivo 3D photoacoustic image. The distance shown along Z axis includes the tumor depth and the distance be tween the surface of transducer and mouse skin. C ) Photograph of the mouse after tumor resection. D ) 3D photoacoustic image of the tumor cavity after surgery to examine the complet eness of the tumor resection. E ) Photograph of the tumor resection guided by 3 D photoacoustic image shown in B
114 Figure 5 11 Quantitative analysis of the photoacoustic sli ces and H&E stained sections. A ) Photoacoustic slice. Lines 1 and line 2 indicate the imaged dimensions al ong two different directions. B ) H&E stained sectio n in the same position. s in the H&E stained section. C ) Photograph of the tumor. All lin es indicated by arrows in Fig. 5 11A Fig. 5 11B and Fig. 5 11C show the top margins of the tumor. D ). Transverse pho toacoustic slice from another mouse. Lines 1 and line 2 indicate the imaged dimensions al ong two different directions. E ) H&E stained dimension s in the H&E stained section. F ) Photograph of the tumor. All lin es indicated by arrows in Fig. 5 11D Fig. 5 11E and Fig. 5 11F present the margins of the tumor in transverse plane. Scale bar, 500m.
115 Figure 5 12 Quantitative analysis of photoacoustic images in correlation with H&E sections with increasing image depth. ( A B C ) PAT slices and H&E sections for a tumor beneath the skin of mouse (0.3mm depth) ( D E F ) PAT slices and H&E sections for a tumor beneath the mouse skin and a 1mm thick chicken breast. ( G H I ) PAT slices and H&E sections fo r a tumor beneath the mouse skin and a 1.8mm thick chicken breast. White dashed lines show the surface of chicken breast. Scale bar, 500um.
116 Figure 5 13 Design of future imaging probe. A transparent and deformable balloon filled with water or ultrasound gel is attached to the ultrasound transducer. The balloon serves as the ultrasound coupling medium while protecting the transducer from direct contact with blood.
117 CHAPTER 6 CIRCULAR ARRAY BASED PHOTOACOUSTIC TOMOGRAPHY AND ITS APPLICATION TO BREAST CANC ER DETECTION 6.1 M otivation s To date several PAT systems for breast cancer detection have been reported. Oraevsky et al. developed a laser based optoacoustic imaging system (LOIS) with an arc shaped 64 element acoustic array capable of providing a spatial resolution of 1mm. Haisch et al. reported a combined photoacoustic and ultrasound imaging system. Manohar et al. built a three dimensional PAT system that could cover a 90mm field of view. In their system, a planar array of 590 PVDF transducers was used, offering a spatial resolution of 2.3 3.9mm given an imaging depth of up to 32mm. Pramanik et al. described a hybrid PAT and thermoacoustic tomography system. Kruger et al. recently reported an interesting PAT study of breast angiography using a 646450mm field of view given a 40mm imaging depth. While the results from these studies are promising, PAT has limitations. For example, PAT can provide high quality images only for certain size of targets due to the limited detection band of an ultrasound transd ucer. It is difficult to accurately detect a the limited directivity of a transducer may lead to imbalanced image quality for the entire imaging area/volume. Yet, it still remains a major challenge for PAT to recover tissue scattering coefficient, an important parameter for breast cancer detection. DOT is capable of overcoming the above mentioned limitations associated PAT, although it has relatively low spatial reso lution. Thus, a combination of PAT and DOT appears to be an ideal approach for breast imaging. Towards this, we recently reported for the first time a hybrid system that integrated PAT and DOT in a single platform. 13
118 The goal of the current work is to deve lop an improved G2 PAT/DOT system using a ring shaped 64 element array with improved sensitivity and directivity over the previous 32 element array used in our first prototype. The G2 PAT/DOT is validated using phantom and ex vivo experiments. 6 .2 Material s and M ethods 6.2.1 System D escription Fig ure 6 1 A presents the schematic of our PAT/DOT hybrid system. Detailed description of the DOT part has been reported previously. Briefly, light generated from a diode laser is delivered sequentially by an optical s witch and optical fiber bundles to 16 source positions. For each source position, diffused light is detected by 16 detection fiber bundles coupled with detection units and data acquisition board and used for DOT image reconstruction. For the PAT part, a pu lsed light from a Nd:YAG or Ti:Sapphire laser with a 6ns pulse duration and 10Hz repetition rate is sent to the object through a light delivery system. The laser generated ultrasound signals are collected by 64 transducers which are connected via a mechani c switching system to a 16 channel pre amplifier and 16 channel data acquisition (DAQ) board. The sampling rate of the DAQ board triggered by the laser is 50MHz. Fig ure 6 1 B the finished optical fibers/transducers/object interface contains a home made 6 4 elements acoustic transducer array. We used commercial PVDF film (110m) with silver electrode in both sides. The film was shaped into 32 rectangle units with a size of 5mm30mm. For each unit, a hole in the center was drilled for the placement of a fibe r bundle of DOT. The positive electrode was divided into two parts with an equal size of 2.3mm30mm and the PVDF film was attached to backing material
119 with the same size as the film by transparent epoxy resin. The positive and negative electrodes of a coax ial cable were connected to the film using silver epoxy. A copper housing was used to shield electromagnetic noise for each unit to improve the signal to noise ratio. We initially tested a fiber bundle based light delivery approach and found it was not fea sible since the damage threshold for a typical fiber bundle was 20mJ which was much lower than the at least 60mJ needed for us to illuminate an object area of 3cm 2 (calculated based on the maximum permissible exposure to light for human tissue surface). Th erefore, we developed a light delivery system by mounting three prisms to a two dimensional step motor (Fig ure 6 2 A ). And a concave lens and ground glass indicated by arrow 3 in Fig ure 6 2 A were attached to the front of output end of the light delivery sys tem to extend the light beam to be 3 cm 2 The output laser beam was delivered from bottom of the exam table to the phantom. During an imaging experiment, the light beam is scanned in two dimension (arrows 1 and 2 in Fig ure 6 2 A ) via two step motors to reali ze the light illumination to cover a large area. Ultrasound signal generated by photoacoustic affect on a sphere absorber is distributed in bipolar shape and the Fourier spectrum of this signal reveals the major frequency components the acoustic energy con tains. Therefore, the frequency bandwidth of the transducer determines the detectable range of target size. We used an impulsive wave method described by Manohar et al. 8 to measure the frequency response of each element. Fig ure 6 2 B shows that the 6dB ba ndwidth of the transducer is from 380kHz to 1.48MHz and maximum frequency response is up to 2MHz.
120 Fig ure 6 2 C presents the directivity of a transducer measured in the plane parallel to the short side of the transducer. It was estimated to be 30 based on the 6dB level. The methodology we used for the estimation was described by Ermilov et al. 7 The normalized sensitivity for the 64 elements is shown in Fig ure 6 2 D where we see that the sensitivity lied in the range of 0.7 1.0. We used this set of data to calibrate the measurements from each element for image reconstruction. 6 .2 .2 Quantitative Reconstruction M ethods of PAT and DOT Various quantitative reconstruction methods have been explored for PAT and DO T. The reconstruction methods for both PAT and DOT in our PAT/DOT system are finite element (FE) based. For PAT, the FE based dual meshing reconstruction algorithm described by Yao and Jiang 14 was used to recover absorption coefficient from the photo acoustic measurements. The algorithm includes two steps. The first is to obtain the map of the absorbed optical energy density. The second step is to recover the distribution of the absorption coefficient from the absorbed energy density. The core procedur e of our PAT reconstruction algorithm can be described by the following two equations: ( 6 1) ( 6 2) where is the pre ssure wave; is the wave number described by the angular frequency, and the speed of the acoustic wave in the medium, ; is the thermal expansion coefficient; is the specific heat; is the absorbed energy density
121 that is the product of the absorption coefficient, and optical fluence, (i.e., ); and and are the observed and computed complex acoustic field data for boundary locations; is the update vector for the absorbed o ptical energy density; is the Jacobian matrix formed by at the boundary measurement sites; is the regularization parameter determined by combined Marquardt and Tikhonov regularizati on schemes; and is the identity matrix. Thus here the image formation task is to update the absorbed optical energy density distribution vi a the iterative solution of equations ( 6. 1) and ( 6. 2) so that an object function composed of a weighted sum of the squared difference between the computed and measured acoustic data can be minimized. The second step is based on the iterative solution to the following radiation transport equation (RTE): ( 6. 3) wher e is the scattering coefficient; is the radiance; is the source term; denotes unit vector in the direction of interest. The kernel is the scattering phase function describing the probability density that a photon with an initial direction will have a direction after a scattering event. If the incident laser source strength and the absorbed energy density are estimated in advance, absorption coefficient distribution can be determined by iterative solution procedure using the finite element method. In the PAT reconstruction, are assumed constan t in this study. A fine mesh of 6285 nodes and a coarse mesh of 1604 nodes were applied in the reconstruction, and the images were converged within 50 iterations using a parallel computer.
122 For DOT, both the absorption and scattering coefficient images wer e recovered from the algorithms described in detail by Li and Jiang and Jiang et al.. In short, the distribution iteratively in order to minimize an object function composed of a weighted sum of the squared difference between computed and measured optical data at the medium surface. The computed optical data (i.e., photon density) is obtained by solving the photon diffusion equation with a finite element method. The core proc edure in our reconstruction algorithms is to iteratively solve the following regularized matrix equation: ( 6 4) where is the photon density, I is the identity matrix, and can be a scalar or a diagonal matrix. is the update vector for the optical property profiles, where is the total number of nodes in the finite element mesh used and is the diffusion coefficient. and where and respectively, are measured and calculated data for boundary locations. is the Jacobian matrix that is formed by and at the boundary measurement sites. In DOT, the goal is to update the and or distributions throug h the iterative solution of equation ( 6 4) so that a weighted sum of the squared difference between computed and measured data can be minimized. A single mesh of 700 nodes was used, and the images were converged within 15 iterations in a 3GHz PC with 1GB memory. 6 .3 Performance Evaluation and Phantom E xperiments Since detailed system performance for DOT has been evaluated previously, 5 here we focus on evaluating the PAT part of the PAT/DOT system on the imaging depth,
123 active imaging area and multi target imaging ability using the light delivery system, and conduct a comparison between PAT and DOT in these areas. We also test our PAT/DOT system using ex vivo tumor tissue embedded in a tissue mimicking phantom. 6 .3.1 Performance of PAT Spatial resolution of a PAT system is determined by several factors including the number of transducers, inter element spacing, and sensitivity and frequency response of the transducer as well as the reconstruction methods used. Two experiments were perfo rmed to estimate the best spatial resolution of our system. In the first experiment, we put three thin metal wires (0.15mm in diameter each) in the center of a phantom background. The distance was 0.5mm and 0.7mm, respectively, between wires 1 and 2 and be tween wires 2 and 3. In the second experiment, we placed two metal wires into the phantom background and the distance between the two wires was 0.3mm. The reconstructed PAT images from the tw o experiments are shown in Fig ure 6 3 A and B respectively, where we note that the three targets ca n be clearly distinguished (Fig ure 6 3 A ), while the two targets are not resolvable (Fig ure 6 3 B ). Hence, we determine that the best spatial resolution of our system is 0.5mm. The detail parameters of the tissue mimicking p hantom (Intralipid + India ink) and 5 targets used for evaluating the PAT p erformance are listed in Table 6 1 These experiments were performed using the 532nm Nd:YAG pulsed laser. The light beam was guided and extended by our light delivery system to be a n area source of 3 cm 2 and the PAT images were reconstructed by a delay and sum method. The light energy density at the surface of the phantom was 20 mJ/cm 2 Fig ure 6 4 A and B show the PAT images of target 1 located at 0 and 22mm below the phantom surfa ce, respectively. While the target is clearly better imaged with
124 0mm depth, the target with 22mm depth is still detectable. To detect a 2mm radius centrally located target, we found that the maximum depth was 7mm and 22mm for the G1 and G2 systems, respect ively. In addition, due to the use of electromagnetic shielding cases for the transducers in the G2 system, the signal needed not to be averaged while it needed to be averaged 100 times for the G1 system. The active imaging area for PAT is valid ated by the result shown in Fig ure 6 4 C where target 3 (30mm off center positioned) is detected. Due to the limited directivity of transducers, we note that the shape of reconstructed target was distorted and stronger artifacts were seen around the target. We also te sted and found that when a target was placed at a >30mm off center position, the image quality became unacceptable. For the single target experiments, the light beam was directly delivered to the target area without s canning. The image shown in Fig ure 6 4 D demonstrated the ability of imaging multiple targets using the developed light delivery system. Three targets (2, 4, and 5) separated with a large distance were imaged by scanning the laser beam 9 (33) times to cover the whole phantom surface. The 9 imag es obtained from the 9 laser beam positions were then fused int o a single image shown in Fig ure 6 4D 6.3.2 P hantom E xperiments Fig ure 6 5 presents the reconstructed quantitative absorption coefficient images by PAT and absorption and scattering coefficien t images by DOT for some of targets 1 5 and for a relatively large target, target 6 positioned 20mm off center (radius=5mm, absorption coefficient=0.028mm 1 and scattering coefficient=4.0mm 1 ). These quantitative images were recovered using our finite elem ent based PAT and DOT. Fig ure 6 5 also gives the reconstructed optical property profiles depicted along one cut
125 line crossing through the center of the target (red dashed line) for each case in comparison with the exact property values (black solid line). F rom Fig ure 6 5 A we see that target 5 (radius=1mm) is accurately recovered by PAT in terms of its size, position and absorption coefficient value, while it is not detectable by DOT in both the absorption and scattering images. For target 2 (radius=2mm), a gain PAT can accurately reconstruct its size, position and ab sorption coefficient value (Fig ure 6 5 B ). In this case, DOT can detect the target from both the absorption and scattering images; however, the values of both absorption and scattering coefficient s are sig nificantly underestimated. Fig ure 6 5 C shows that target 3 (radius=3mm) is quantitatively recovered by both PAT and DOT while we note an overestimated target size by DOT. Interestingly, the largest target 6 (radius=5mm) is poorly recovered by PAT (Fig ure 6 5 D ), due to the loss of low frequency signals given the limited frequency response of the transducers. Here we see that only the edge of the target is recovered by PAT and the reconstructed absorption coefficient value is considerably underestim ated. DOT in this case provides accurate recovery of the target in terms of its size, position, and absorption and scattering coefficient values. Overal l from the results shown in Fig ure 6 5 and the summary in Table 6 2 PAT can provide both qualitatively and quantitatively better images than DOT when the target size is equal to or small than 3mm in radius, while DOT can offer better image quality when the target size is larger than 3mm in radius. We also notice that the smallest detectable target size is 2 mm in radius for DOT given the experimental conditions used in this study.
126 6 .4 Ex V ivo E xperiment For the ex vivo experiment, a tumor was removed from a rat bearing a 4T1 tumor of approximately 3.5mm in radius and embedded in the phantom background. In t his case, 730 nm pulsed light from a Ti:Sapphire laser was employed and provided an energy density of 20 mJ/cm 2 at the phantom surface. Fig ure 6 6 shows the recovered absorption and/or scattering images where we see that the tumor is detected by both PAT an d DOT. The absorption coefficient of the tumor recovered by PAT is consistent with that by DOT. Through quantitative analysis of ex vivo PAT and DOT images, the size of tumor (3mm in radius) estimated by the absorption image of PAT and by the scattering im age of DOT is consistent with the actual tumor size, while it is overestimated by the absorption image of DOT to be 5mm in radius. 6 5 Multilayer Ultrasound Transducer w ith Improved Sensitivity a nd Bandwidth In our PAT&DOT system, the circular array were made of PVDF because PVDF owns some immediate advantages compared with other materials (PZT, PMN PT). PVDF has a high mechanical damping ability and a unique permittivity, resulting in a high bandwidth. In addition, the acoustic impedance (Z 0 ) of PVDF is 2 .7 MRays, which is much lower than that of PZT (>30MRays). Hence it provides a good coupling efficiency to human tissue or commonly used ultrasound gels. PVDF is flexible and can be easily fabricated in convex shapes replacing the acoustic lens commonly us ed in PZT or PMN PT. We note, however, that the sensitivity of the PVDF transducers currently used for PAI is not comparable to the P ZT and PMN PT based transducers leading to limited SNR especially when targets are deeply located within tissues. In order to improve the sensitivity of the PVDF transducers, several transducer designs based on the same thickness PVDF layers including folded transducer (FT), barker
127 coded transducer (BCT) and switchable Barker coded transducer (SBCT) have been suggested in the field of ultrasound imaging 170 172 While the sensitivity is improved, there is no significant improvement in bandwidth from these designs. Here we propose a design of PVDF transducer with variable thickness layers to provide improvement in both the sensi tivity and bandwidth for photoacoustic imaging. The design of our multilay ered transducer is shown in Fig ure 6 7 A where the PVDF films (Mettalized Film Sheets, Measurement Specialties) having three different thicknesses (28m, 52m and 110m) are sandwic hed by electrodes. The fabrication procedure of this transducer is as follows: (1) shaping the lateral dimension of the three PVDF films to 10mm3mm; (2) removing the unneeded silver coating from both sides of each film to ensure electric insulation betwee n the positive and negative electrodes; (3) stacking/gluing the films/backing material together using ultrasound transparent epoxy (EPO TEK 301 2, Epoxy Technology); and (4) connecting coaxial cables to the electrodes. The multilayered transducer fabricat ed is tested using a circular scanning photoacoustic imaging system ( Fig ure 6 7B ). A laser beam generated from a pulsed Nd:YAG laser (532nm) was redirected through the center of the rotator and expanded by a lens to illuminate an imaging area of 6.25cm 2 A two dimensional linear stage (NLS4 Series, Newmark Systems) was used to optimize the position of transducer. A water tank with a through hole in the bottom sealed with transparent membrane was used to provide coupling for ultrasound transmission. The sign al received by the PVDF transducer was amplified by two low noise amplifiers and digitalized/stored by a data acquisition card at 50MHz sampling rate. A pulse/echo test system ( Fig ure 6 7C ) was
128 used to evaluate the bandwidth of the multilayered transducer. The transducer was driven by a pulser/receiver (5073PR, Panametrics NDT) and a piece of metal with a thickness of 5mm was placed in front of the transducer to act as a reflector. The reflected pulse was received by the transducer, amplified by the interna l amplifier of the pulser/receiver and recorded using an oscilloscope. Before the performance evaluation and in vivo experiments, the transducer was calibrated through an experiment with a hair embedded solid phantom. The solid phantom was a mixture of Int ralipid (scatter), India ink (absorber), and 2% Agar powder (solidifier). The absorption and reduced scattering coefficients of the phantom were 0.007mm 1 and 1.0mm 1 A human hair was inserted in the solid phantom, which was ill uminated by the light beam. Fig ure 6 8 A B and C show the acoustic signals from different layers of the transducer where we note a time shift between these signals, caused by the different distance of these three layers from the target. Hence, calibration of this time shift is neede d before further evaluating the performance of the transducer. Assuming that the arrival times of the signals are T1 T2 and T3 for the layers of 28, 52 and 110m, respectively, and setting the arrival time for the 28m thick layer as the baseline, we ca n then calculate the time shift of the other two layers relative to T1 by the following relationships: ( 6.5 ) ( 6.6 ) Thus, the photoacoustic (PA) signal from t he multilayered transducer can be obtained as ( 6.7 )
129 where P1, P2, and P3 are the phase of the PA signals which are 1, 1, 1 in our case for the 28m 53m and 110m t hick layer, respectively. Fig ure 6 8 D shows the calibrate d signal (red dashed line) and non calibrated signal (blue dashed line). We see that the SNR of the calibrated signal is improved significantly (by 2.5 times) compared with that without the calibration. Fig ure 6 9 B C and D show the frequency characteristi cs by fast Fourier transforming the reflected acoustic waves detected by each of the three layers using the pulse/echo system. For the 110m 52m and 28m thick single layer transducer, the central frequency and the bandwidth ( 6dB level) are, respectiv ely, 5MHz/85%, 10MHz/78% and 15MHz/59%, while the central frequency of the multilayered transducer is 10MHz and the 6 dB level bandwidth is improved to be 140%, as shown i n Fig ure 6 9 A In vivo animal experiments were performed to evaluate the merits of the improved bandwidth and sensitivity for photoacoustic imaging. The brain of adult BALB/C mice (~25g) was imaged. Before imaging, the hair was removed with a hair remover lotion. The mice were anesthetized by a mixture of Ketamine (85mg/kg) and Xylazine during the imaging, and were sacrificed afterwards using the University of Florida Institutional Animal Care and Use Committee (IACUC) approved techniques. The laser beam provided an incident energy density of 9 mJ/cm 2 at the skin of the mice, which is muc h lower than the safety standard provided by ANSI (20mJ/cm 2 at 532nm). For each imaging experiment, we collected PA signals at 120 positions by scanning the transducer around the mouse brain with a scanning step of 3. The PA image was reconstruct ed by a d elay and sum algorithm
130 Fig ure 6 10 A shows a typical signal from in vivo mouse brain measured by the multilayered transducer without (top panel) and with (bottom panel) calibration. Without the calibration, the SNR is low and the image formed with such si gnals has relatively poo r quality. As we see from Fig ure 6 10B (left), in such case, some blood vessels inside the brain are not visualized and almost every blood vessel imaged is not continuous, as compared to the photograph of the brain after removal of the scalp (Fig ure 6 10B (right)). With the calibration, the SNR is significantly improved as sho wn in the bottom panel of Fig ure 6 10 A From the image f ormed with the calibration (Fig ure 6 10B (middle) ) we see that the eyes, blood vessels and other tissue s are identified clearly in comparison with the photograph. In a separated in vivo experiment, we compared the sensitivity of the multilayered transducer with the single layer transducers and the PA image s obtained are presented in Fig ure 6 11 We immedia tely note the clearly higher SNR shown by the image using the multilayered transducer (Fig ure 6 11A ) compared to that using the single layer transducer (Fig ure 6 11 B C and D ). In particular, we can identify the tissues, organs and blood vessels inside the brain from Fig ure 6 11A much more clearly than that from Fig ure 6 11B C and D as indicated by red arrows. 6 .6 D iscussion and F ut ure D irections. We have developed a second generation prototype PAT/DOT system and evaluated its validity using phantom and e x vivo tumor experiments. This system takes full advantages of both PAT and DOT to provide high resolution absorption image and scattering image. While we plan to test this system in breast cancer patients in the near future, we are aware that there are st ill several improvements that need to be undertaken before this testing. First, multi spectral PAT imaging will be considered to
131 provide tissue functional information. Second, we will use the absorption coefficient recovered by PAT as a priori knowledge to improve the scattering image reconstruction by DOT, or use the scattering coefficient provided by DOT to enhance the absorption recovery of PAT. Third, the data acquisition for PAT will be improved by using a fast electronic switch with in board pre ampli fiers that connects between the 64 transducers and the 16 channel data acquisition board. Finally, we note that when the light beam was delivered to the region near the boundary where the transducers are located, the acoustic signal generated by light deli vered to the surface of PVDF film resulted in notable artifacts in the reconstructed photoacoustic images. We are currently investigating some ways to minimize this effect including attaching a thin layer to the transducer to absorb the light and preproces sing the photoacoustic signals using wavelet transform before reconstruction. We have shown that the novel design of variable thickness multilayered PVDF transducer provide s significant improvement in both the sensitivity and bandwidth compared to the con ventional single layer transducer. However, we note several limitations associated with the current design, which should be overcome in future. First, the sensitivity of the current multilayered transducer is still not comparable with that of a commercial PZT unit. T his however, can be overcome by stacking more layers (>3) together to make a more sensitive transducer Second, the PVDF films used in the current work are commercial units with electrodes on both si d e s of each film After stacking them togethe r, there are two layers of electrodes between two adjacent films. As a result the improvement of the sensitivity is not linearly proportional to the number of the layers stacked due to the attenuation from the electrodes. This problem can be
132 solved by usi ng raw film liquid which will allow us to use only one much thinner electrode (just a few m) between two adjacent films during depositing the films Third, no matching layer and electromagnetic shield (EMS) housing were used in the current design Hence we had to average 50 times on the signals collected to reduce the electromagnetic noise resulting in a relatively long data acquisition time. Figure 6 1 System description. A ) Sch ematic of the PAT/DOT system. B ) Photograph of the interface and schema tic of a single transducer.
133 Figure 6 2 Performance evaluation of the hybrid system. A ) 3D schematic of the light delivery system. B ) Frequency response of a transducer. Red dash ed line shows the 6dB level. C ) Directivity of a transducer. Red dash ed l ine shows the 6dB level. D ) Normalized sensitivity for all the elements in the transducer array.
134 Figure 6 3 System resolution evaluation. A ) PAT image of three separated metal wires (0.15mm in diameter each) embedd ed in the phantom background. B ) PAT image of two metal wires (0.15mm in diameter each) placed in the phantom background.
135 Figure 6 4 System performance evaluation by phantom experiments. A ) PAT image o f target 1 without any depth. B ) PAT image of target 1 positioned at 22mm beneath the su rface. C ) PAT image of target 3 (30mm off center positioned). D ) Fused image of multiple targets.
136 Figure 6 5 Quantitative PAT and DOT images of ta rget s. A ) Target 5. B ) target 2 C ) target 3 D ) target 6
137 Figure 6 5 Continued
138 Figure 6 5 Conti nued
139 Figure 6 5 Continued
140 Fi gure 6 6 Reconstructed optical properties of ex vivo tumor. A) A bsorption image by PAT B ) Absorption image by DOT. C ) S cattering image by DOT.
141 Figure 6 7 Schematic of the multi layered transducer and performance ev aluation systems. A ) Configuration of the multilayered PVDF transducer. B ) Circular scanning based photoacoutic imaging system. C ) Description of pulse/echo evaluation system. F igure 6 8 Calibration of multi layered transducer. Signals of 28 m ( A ), 52 m ( B ) and 110 m ( C ) layer transducer from hair. D ) Signals with (red) and without (blue) calibration.
142 F igure 6 9 Frequency response of mult ilayer ed and single layered PVDF transducer A ) Frequency response of multilayered PVDF transducer. B) 110 m PVDF transduce r C ) 52 m PVDF transducer D ) 28 m PVDF transducer
143 F igure 6 10 In vivo experimental evaluation of multi layered transducer. A ) In vivo phtoacoustic signals of multilayer transducer with (bottom panel) and wit hout (top panel) calibration. B ) Reconstructed brain images using signals without (left panel) and with (middle panel)calibration. Photograph of the brain after removing the scalp (right panel).
144 F igure 6 11 In vivo photoacoutic imaging of mouse hea d using multilayer ed and single layere d transducer A ) Multilayered transducer. B ) 110 m single layered transducer. C ) 52 m single layered transducer. D ) 28 m single layered transducer. Table 6 1 Exact size (diameter), location (off center), depth, and absorption and reduced scattering coefficients of the five targets Size (mm) Location (mm) Depth (mm) (mm 1 ) (mm 1 ) Target 1 4 0 0 (22) 0.028 2 Target 2 4 20 5 (15) 0.028 2 Target 3 6 30 5 (10) 0.028 2 Target 4 6 20 5 (15) 0.028 2 Target 5 2 20 5 (15) 0.0 28 2
14 5 Table 6 2 Exact and reconstructed target size (diameter), location (off center) and absorption and reduced scattering coefficients for the phantom experiments Size (mm) (mm 1 ) (mm 1 ) Location (mm) Case 1 Exact 2 0.028 2 20 PAT 2 0.026 NA 20 DOT NA NA NA 20 Case 2 Exact 4 0.028 2 20 PAT 4 0.025 NA 20 DOT 10 0.017 1.1 20 Case 3 Exact 6 0.028 2 20 PAT 6 0.024 NA 20 DOT 10 0.024 1.8 20 Case 4 Exact 10 0.028 4 20 PAT 8 0.015 NA 20 DOT 12 0.028 4 20
146 CHAPTER 7 CONCLU SION AND FUTURE WORK In this dissertation, four modalities of photoacoustic imaging covering from macro scale to micro scale are described For each modality, we show the potential applications for different cancers studies. However, there are still severa l aspects need to be improved for each modality. Firstly, for ORPAM, more applications should be excavated. Secondly, for ARPAM, one improvement should be focused on employing quantitative photoacoustic microscopy to exhume functional parameters such as he moglobin concentration, blood flow, oxygen consumption et al with multi wavelength s Quantitative photoacoustic microscopy is also very helpful to explore the binding rate The other aspect is to investigate a real time handheld photoacoustic probe which is a basic requirement for clinical applications. For intraoperative photoacoustic tomography, and integrated PAT and DOT system we ar e focusing on making it suitable for clinical tr ies
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163 BIOGRAPHI CAL SKETCH Lei Xi received his B.S. in the College of Optoelectronic Science and Engineering from Huazhong University of Science and Technology, China, in 2007. In Fall 2008, he sta rted to pur sue his Ph.D. in the Department of Biomedical Engineering at the University of Florida. His research focused on photoacoustic tomography for clinical breast detection, intraoperative photoacoustic imaging and photoacoustic microscopy for biological cancer research.