Quantitative dedicated cone beam breast CT imaging

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Shi, Linxi
Zhu, Lei
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In the United States, annual mammogram screening for early breast cancer detection is recommended. Such screening is known to have a significant impact on improving cancer mortality rates. However, the diagnostic function of mammogram is hampered due to their being two-dimensional projections, resulting in tissue superposition and compromised specificity and sensitivity. Dedicated cone beam breast computed tomography (CBBCT) is a recently approved diagnostic tool that produces high quality tissue-superposition-free volumetric images, demonstrating a potential to substantially improve breast cancer detection and diagnosis. Nevertheless, high scatter contamination stemming from large irradiation volume results in severe contrast lost and shading artifacts, impeding the quantitative uses of CBBCT in certain clinical tasks. Existing scatter correction methods demonstrate different drawbacks including low efficacy, dose or scan time increase, etc. In this thesis, we propose two scatter correction methods, library-based (LB) and forward-projection-based (FPB), to overcome the deficiencies while achieving high correction efficacy. In the LB method, a scatter library is precomputed via Monte Carlo simulation based on a simple breast model. Due to the relatively simple shape and composition, we find that a small library size with one input parameter of breast size is sufficient for effective scatter correction on general population. In the FPB method, we first estimate primary signals of CBBCT projections via forward projection of the segmented first-pass reconstruction. By subtracting the simulated primary projection from the raw projection, we obtain a raw scatter estimate containing both low-frequency scatter and errors. After discarding untrusted errors from the resultant raw scatter map, the final scatter is obtained via a novel Fourier-transform based local filtration algorithm. Both methods have demonstrated high correction efficacy on patient data, the LB method is superior in computational efficiency while the FPB method has better flexibility. By comparing these two proposed methods, we find that there is a large discrepancy between the scatter estimation of the two; and the FPB method tends to better preserve high spatial-resolution details than the LB method. We hypothesize that this is mainly due to the existence of off-focus radiation (OFR), which is a fundamental factor degrading the image spatial resolution. To quantitatively investigate the effect of OFR on spatial resolution, we designed an experiment to characterize the spatial resolution with and without OFR. The obtained results are consistent with the correction results using the two correction methods, therefore successfully validating our hypothesis.
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