Ultrasonic Computed Tomography of the Breast Using Linear Arrays

The performance of B-mode ultrasound in the clinical detection of breast cancer is often limited by poor contrast resolution. Contrast mechanisms based on alternative tissue characteristics, such as direct estimation of sound velocities and attenuation coefficients in different tissues, may offer additional diagnostic information. An approach based on limited-angle transmission tomography for reconstruction of the sound velocity distribution and attenuation coefficient distribution in the breast is proposed in this thesis. The imaging setup is similar to that of x-ray mammography. With this setup, the time-of-flight data and the attenuation data are acquired by a linear array positioned at the top of the compressed breast that both transmits and receives, and a metal plate is placed at the bottom as a reflector. Such a setup can be easily integrated with a B-mode system so that the acoustic data for all of the B-mode image, the sound velocity distribution, and the attenuation coefficient distribution can be simultaneously acquired. However, the acquired data are incomplete, and this results in inaccurate sound velocity estimation and attenuation coefficient estimation. In order to improve the estimation accuracy, a new reconstruction algorithm based on a convex programming formulation was developed. This improvement is mainly attributable to the proposed algorithm successfully incorporating information from the B-mode image of the same object. Furthermore, a technique based on the angular spectrum method was developed to compensate the effects of refraction on the attenuation data using the information on sound velocity distribution. Simulation results and experimental results demonstrate that the proposed approach to limited-angle transmission tomography using linear arrays is feasible. Both the sound velocity image and the attenuation coefficient image can be used to complement conventional B-mode image to enhance the detection of breast cancer.中文摘要 I ABSTRACT II LIST OF SYMBOLS III TABLE OF CONTENTS IV LIST OF FIGURES VI LIST OF TABLES XII CHAPTER 1 INTRODUCTION 1 1.1 SOUND VELOCITY, ATTENUATION COEFFICIENT, AND BREAST CANCER DETECTION 1 1.2 LIMITED ANGLE TRANSMISSION TOMOGRAPHY 2 1.3 THESIS ORGANIZATION 3 CHAPTER 2 RECONSTRUCTION OF THE SOUND VELOCITY DISTRIBUTION 5 2.1 BASIC EQUATIONS FOR THE IMAGING SETUP 5 2.2 A BRIEF INTRODUCTION TO THE CONVEX PROGRAMMING FORMULATION 6 2.3 RECONSTRUCTION ALGORITHMS 8 CHAPTER 3 NUMERICAL RESULTS—SOUND VELOCITY 13 3.1 B-MODE IMAGE GENERATION 13 3.2 GENERATION OF THE TIME-OF-FLIGHT DATA 16 3.3 RECONSTRUCTION WITHOUT THE B-MODE IMAGE: ALGORITHM I 19 3.4 RECONSTRUCTION WITH THE B-MODE IMAGE INFORMATION: ALGORITHM II 25 3.5 DISCUSSION 33 3.5.1 Accuracy of sound velocity reconstruction 33 3.5.2 Effect of pitch on sound velocity reconstruction 34 3.5.3 Miscellaneous issues 35 3.6 CONCLUDING REMARKS 36 CHAPTER 4 EXPERIMENTAL RESULTS—SOUND VELOCITY 37 4.1 EXPERIMENTAL SETUP AND METHODS 37 4.2 EXTRACTION OF TIME-OF-FLIGHT DATA 40 4.3 GENERATION OF CIMAGE 42 4.4 RECONSTRUCTED SOUND VELOCITY IMAGES 44 4.5 DISCUSSION 49 4.6 CONCLUDING REMARKS 51 CHAPTER 5 EXPERIMENTAL RESULTS—ATTENUATION COEFFICIENT 53 5.1 INTRODUCTION 53 5.2 RECONSTRUCTION METHOD 53 5.3 EXPERIMENTAL RESULTS 55 5.4 EXPERIMENTAL RESULTS WITH COMPENSATION FOR THE REFRACTION EFFECTS 58 5.5 CONCLUDING REMARKS 65 CHAPTER 6 DISCUSSION 67 6.1 ABERRATION CORRECTION 67 6.2 SNR ENHANCEMENT 69 CHAPTER 7 CONCLUSIONS AND FUTURE WORKS 71 APPENDIX A ARBITRARY WAVEFORM CODED EXCITATION USING BIPOLAR SQUARE WAVE PULSERS IN MEDICAL ULTRASOUND 72 A.1 INTRODUCTION 73 A.2 CODE CONVERSION 74 A.2.1 Sigma-delta modulation 74 A.2.2 Code translation 75 A.2.3 Preliminary evaluation 76 A.2.4 Code tuning 77 A.3 DESIGN OF THE COMPRESSION FILTER 80 A.3.1 Design criterion 80 A.3.2 Formulation for finding the optimal compression filter 82 A.4 PERFORMANCE OF THE PROPOSED METHOD 84 A.4.1 Simulation results 84 A.4.2 Experimental results 89 A.5 DISCUSSION 93 A.6 CONCLUDING REMARKS 94 REFERENCES 9