A Novel Low Dose and Accurate Phase Sensitive Breast Tomosynthesis (PBT) System based on Photon Counting Detector Technology for Cancer Imaging Project Summary: This project will focus to build a clinically driven phase sensitive breast tomosynthesis (PBT) imaging system using the photon counting x-ray detector technology to reduce the radiation dose and improve the accuracy of phase retrieval methods. Digital breast tomosynthesis (DBT) and PBT prototypes use the energy integrating detectors which introduce considerable electronic noise at low dose levels that limit the ability to reduce the total dose. With the spectral averaging of polychromatic x-ray beams, the quantitative information such as the projected electron densities of tissues is not accurately retrieved in phase sensitive imaging as the phase retrieval methods hold rigorously for monochromatic x-ray beams. One can employ energy resolving spectrometer to suppress the electronic noise and take the full advantage of the phase retrieval process. However, such an approach will be time-consuming and cannot be realized in the clinical world. We propose the utilization of photon counting detector technology with phase sensitive imaging technique to address the stated limitations. Photon counting detectors with their ability to eliminate the electronic noise potentially allow the phase sensitive imaging of breast at reduced radiation dose levels while preserving excellent imaging quality, enhanced tissue contrast, and tissue type identification capabilities. With narrow energy bin imaging, photon counting detectors limit the contributions of various photon energies in the polychromatic x-ray beam, and this will potentially help in accurately retrieving the quantitative information of the tissue. Such a technology for breast cancer imaging is neither available nor optimized. These are the primary aims proposed in this proposal: (1) Developing an inline PBT prototype that incorporates a high-resolution two-dimensional photon counting detector. We will fully characterize the spectral PBT; determining the detective quantum efficiency, measuring the spatial resolution, evaluating the noise properties, count rate performance, determining the width and location of energy bin; (2) Optimizing the phase retrieval methods for accurate quantification of the phase maps and projected electron densities of the breast tissues; (3) Conducting the image quality and dose saving comparisons with the existing PBT prototypes and DBT systems that utilize the energy integrating detectors. The proposed project will further facilitate the translation of the phase sensitive imaging of breast to the clinical world and enhance the sensitivity and specificity of breast cancer detection while reducing the radiation dose.