Impact of the Non-Negativity Constraint for In-Line Phase Contrast Computed Tomography

In-line X-ray phase-contrast computed tomography (IL-PCCT) can reconstruct high resolution images, and the core of IL-PCCT is the image reconstruction algorithm. As we all know, image reconstruction algorithm can suppress artifacts and produce high-quality reconstructed images using a priori constraint information. For conventional absorption X-ray CT, a common constraint is that linear attenuation coefficient has to be positive, as a negative value is physically not possible. Using the non-negative values priori information is believed to be beneficial in terms of image quality and convergence speed. Phase retrieval is a key step in achieving IL-PCCT. Due to considerations such as radiation dose, single-distance phase retrieval is typically employed. However, single-distance phase retrieval relies on limited data, and especially when the data is insufficient, it may lead to inaccurate reconstruction results. If they exceed the expected range or do not match the physical characteristics of the sample, they generally indicate errors in the retrieval process. In the image reconstruction process, when using projection data obtained from single-distance phase retrieval, whether to introduce a non-negative constraint as a regularization condition is a question worth exploring, but its validity has not yet been systematically studied. In this work, we present an investigation into this problem. The investigation is divided into various experimental studies that non-negativity constraint effect on the IL-PCCT.