OPTIMIZATION OF DEDICATED BREAST COMPUTED TOMOGRAPHY: BOWTIE FILTER DESIGN AND OPTIMAL SPECTRUM ANALYSIS
Jennings, Robert J
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Recently, researchers have been investigating the use of a new imaging modality called dedicated breast CT as a means of alleviating the problem of tissue superposition that comes from acquiring a two-dimensional image of a three-dimensional object in conventional mammography. Several groups have investigated the optimal spectrum for this new imaging modality using the dose efficiency as the FOM, but results are inconsistent. None of these groups have employed the use of bowtie filtration in their optimal spectrum studies. Given the right design, the inclusion of bowtie filtration will lead to improved dose efficiency as well as consistency in the metric independent of position in a given phantom. Bowtie filters can improve performance in several ways, including DR reduction, scatter reduction, patient dose reduction, and reduction of beam-hardening effects. In this dissertation, three different filter types with different choices for the tradeoffs between the performance improvements listed above are described. Examples of each type of bowtie filter are created for computational and Monte Carlo analyses, and two designs were fabricated for experimental analysis. Studies analyzing the material selection for each bowtie filter design and characterizing the scatter were also completed. Verification of the performance of the designs was done by calculating/measuring the HVL, intensity, and µeff behind the phantom as a function of fan-angle. The performance of the designs depended only weakly on incident spectrum and tissue composition. With various breast diameters, the calculated parameters varied the most, but the variation was substantially less than the no-bowtie filter case. For all designs, the DR requirement on the detector was reduced compared to the no-bowtie filter case. Simulation and experimental data showed that the use of our bowtie filters can reduce the peripheral dose to the breast by 61%, and provide uniform noise and CNR distributions. The best performing bowtie filter design was implemented in simulation studies analyzing the optimal spectrum through calculation of the dose efficiency metric. The results from this study show the improvement and consistency that can be obtained with the inclusion of the proper bowtie filter, and provide the research community with a methodology that will help lead to more consistent optimal spectrum results.