Mammography Tomosynthesis Using a Coupled Source And
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MAMMOGRAPHY TOMOSYNTHESIS USING A COUPLED SOURCE AND DETECTOR IN A C-ARM CONFIGURATION JOSEPH TOBIAS RAKOWSKI MEDICAL COLLEGE OF OHIO 2004 ACKNOWLEDGEMENTS I want to express my gratitude to my advisor, Michael J. Dennis, for his guidance; to Patti McCann and Richard Lane in the Department of Anatomy for providing the specimens; to Diane Ammons for editing my dissertation; to the members of my dissertation committee; and to the Medical College for providing this opportunity. I dedicate this to my family who taught me the value of education, and to my loving and patient wife, Linda, and my super son, Joseph Aaron. ii TABLE OF CONTENTS Acknowledgements ii Table of Contents iii Introduction 1 Literature 6 Materials 8 Methods 10 Results 32 Discussion 99 Summary 109 Conclusions 111 Bibliography 112 Appendix A 120 Appendix B 133 Abstract 145 iii INTRODUCTION Mammography is, by far, the best diagnostic tool for detecting early stage breast cancer (Baker, 1982). Tabar et al. (1987) demonstrated the importance of early detection in saving lives (Tabar, 1987). However, despite the technological and quality improvements in recent years, 10 - 30% of breast cancers are not detected, while other cancers are not detected early enough to allow effective treatment (Bassett et al., 1987; Baines et al., 1986; Haug et al., 1987). The primary reason for missed diagnosis is that the cancer is often obscured by fibroglandular breast tissue that is radiographically dense (Holland et al., 1982, 1983; Martin et al., 1979; Feig et al., 1977; Ma et al.,1992; Jackson et al., 1993; Bird et al., 1992; Mandelson et al., 2000; White, 2000; Boyd et al., 1998; Rosenberg et al., 1998; van Gils et al., 1998). A tool that could potentially prove valuable at improving the detection of early stage breast cancer, especially in radiographically dense breasts, is tomosynthesis. Tomosynthesis is the process of reconstructing planes of interest at any level in an object from limited angle projection data in a manner similar to conventional focal plane tomography. Like conventional tomography, tomosynthesis allows the radiologist to focus on a selected image plane rather than a conventional two-dimensional (2D) static projection of all overlying tissue. Unlike conventional tomography, digital tomosynthesis makes possible two further enhancements: 1) partial removal of overlying structures that lie outside the plane of interest, and; 2) selection of any plane through the breast using a single set of projections, with only a small increase in dose over a conventional film/screen mammogram. 1 Tomosynthesis reconstruction differs from the reconstruction techniques of the widespread modality of axial computed tomography (CT) in several ways. First, the primary CT image plane is defined in the source-detector plane, while the tomosynthesis image plane is perpendicular to the source-detector plane. Second, CT uses a much greater number of projections, typically from 800 to 1500 to produce an image, and typically samples through an arc of at least 180 degrees. The tomosynthesis arc is limited to about 40 degrees, with the greatest number of projections in the literature at 64 (Sone et al., 1996), thereby producing an incomplete data set from which to reconstruct the subject. The data set is incomplete in that there are not enough data points to solve for the attenuation coefficients of each subject voxel along the beam rays. The approximate number of projections needed to produce a complete data set is equal to the circumference of the scanned subject divided by the size of the smallest object one desires to image. For example, a 10 cm diameter breast at 0.2 mm resolution would require 1,571 projections. Third, the reconstruction method in modern commercial CT scanners is filtered backprojection typically using the Fast Fourier Transform (FFT), an approach that has not been applied to tomosynthesis. The methods better suited for tomosynthesis are Algebraic Reconstruction Technique (ART) and pixel shifting with iterative summation/subtraction techniques, both of which are used in this project. The algebraic/iterative techniques produce higher quality images than the filtered backprojection technique when working with incomplete sets of data, or when there is a large statistical uncertainty from a relatively small number of photons contributing to the projection images. Fourier based reconstruction methods improve contrast, i.e., soft tissue discrimination, but generate significant reconstruction artifacts when used with a limited 2 number of projections over a limited angular range. The ART approach was used in the early scanners; However, because of its computational complexity, it was replaced by the filtered backprojection technique. Fourth, CT reconstruction creates a cross sectional map of the attenuation coefficients of the voxels in the subject tissue, which are translated proportionately to Hounsfield numbers for display purposes, scaled relative to the attenuation of water. This characteristic creates tremendous contrast relative to radiographic imaging, allowing discrimination of soft tissue. Conversely, radiography displays only the amount of radiation transmitted through the subject along each ray from the source to detector. Tomosynthesis, however, can improve contrast by focusing on individual planes in a subject and partially removing interfering out-of-plane structures, commonly referred to as blur in conventional film tomography. Finally, the quality and independence of tomosynthesis planes are compromised by blurring artifacts produced from unregistered details located outside the plane of reconstruction. Digital tomosynthesis methods will be implemented for use with isocentric stereotactic breast biopsy units with digital imaging capabilities. The methods used in this project provide for planar reconstruction orthogonal to the source-receptor axis, as well as tilted relative to that axis. The images are collected on a Lorad stereotactic prone breast biopsy unit. The imaging system comprises an x-ray source and image receptor coupled in a c-arm configuration. The image receptor comprises a scintillation screen that is lens-coupled to a Charged-Coupled Device (CCD) array with a 56 mm x 56 mm effective field of view. The CCD array size selections are 512 x 512 or 1024 x 1024. The general objective of this project was to quantify the imaging performance of the tomosynthesis process performed on an isocentric digital stereotactic digital breast 3 imaging system (Lorad Stereoguide with DSM digital image receptor). The image quality will be quantified in terms of the line spread function, the Modulation Transfer Function (MTF), the appearance of the ACR digital mammography phantom, and the Signal to Noise Ratio (SNR), Contrast to Noise Ratio (CNR) and Signal to Background Ratio (SBR) of a low contrast detectability tool developed for this project. Specific objectives were to: 1. Design C++ code to implement the tomosynthesis reconstruction methodology for the C-arm configuration. 2. Design C++ code to implement the tomosynthesis plane subtraction methodology for the c-arm configuration. The goal of plane subtraction is to remove the blur artifacts produced by structures positioned outside the reconstructed plane. a. Demonstrate a linear technique. b. Demonstrate a logarithmic technique. c. Demonstrate an iterative multiplane technique. d. Evaluate the effectiveness of each technique using the test phantoms and cadaver breast specimen. 3. Design C++ code to implement the iterative reconstruction methodology for the C-arm configuration, similar to ART. a. Evaluate the effectiveness of the ART using the test phantoms and cadaver breast specimen. 4. Evaluate the effect of reconstruction on system resolution through the Line Spread Function (LSF) and MTF using an edge phantom. 4 a. Compare the results from objective 4 with performance of the zero angle single projection image. 5. Develop a low contrast detectability phantom. a. Evaluate the effect of iterative and non-iterative reconstruction on Contrast, CNR, SNR, and SBR without structured noise. b. Modify the low contrast phantom with in-plane and out-of- plane structures designed to mimic fibroglandular breast tissue, i. e., structured noise. c. Evaluate the effect of iterative and non-iterative reconstruction with structured noise subtraction on Contrast, CNR, SNR, and SBR. d. Compare the results from objectives 5a. and c. with the performance of the zero angle single projection image. 6. Image a cadaver breast specimen. Perform both iterative and non- iterative reconstruction. Compare results with a single projection image. 7. Develop and image a step phantom to measure the effective slice thickness of a tomosynthesis plane. 5 LITERATURE Niklason et al. (1997), at Massachusetts General Hospital, using a full field digital General Electric (GE) mammographic system with a stationary detector geometry, demonstrated that tomosynthesis may improve the specificity of mammography with improved lesion margin visibility and may improve early breast cancer detection, especially in women with radiographically dense breasts. Webber, et al. (2000), at Wake Forest University in Winston-Salem, North Carolina, have developed an extension of tomosynthesis in their method of Three Dimensional Mammography Using Tuned Aperture Computed Tomography (TACT™). Webber et al. (1997) first presented their method in an earlier publication on dento-alveolar imaging. The TACT™ method produces a three-dimensional image of a limited slab by