Feasibility study of phase-contrast cone beam CT imaging systems

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2010.Attenuation-based x-ray imaging techniques have been developed for many decades. One of the state-of-the-art imaging modalities is the cone beam computed tomography (CBCT) that efficiently scans an object and reproduces high-resolution and isotropic three-dimensional images of it. However, attenuation-based imaging shows a limitation in soft tissue imaging where the absorption contrast is low. Recently several phase-contrast techniques have been developed that are expected to improve low-contrast details by using the phase information of the object. The idea of this thesis is to incorporate the phase-contrast techniques into the current cone beam CT systems to combine the advantages of both phase-contrast imaging and CBCT. From a practical view of medical imaging, two phase-contrast cone beam CT systems are proposed by using the in-line phase-contrast technique and the differential phase-contrast technique, respectively. An in-line phase-contrast image is a Fresnel diffraction pattern in the near field. The image is edge-enhanced, and for soft tissues it is possible to retrieve the phase projection from a single in-line image. Therefore, this technique can be utilized in either of two methods. The first method is to produce edge-enhanced reconstruction images of the attenuation coefficient, and the second is to reconstruct the phase coefficient using the retrieved phase projections. In order to investigate this modality, computer simulations were performed for both working modes. The results using the in-line phase-contrast technique demonstrate superior image quality than that of the attenuation-based technique. A bench-top in-line PC-CBCT system was designed and constructed on top of an optical table, and a simple phantom was imaged and reconstructed using both modes to validate the principle of the proposed imaging scheme. The grating-based differential phase-contrast technique is able to produce the first derivative of phase projections using the principle of shearing interferometer. The proposed differential phase-contrast cone beam CT (DPC-CBCT) system was firstly evaluated using computer simulations as well to investigate its application in volumeof- interest breast imaging. The results demonstrate that compared with attenuationbased CBCT, the DPC-CBCT is able to reveal higher spatial resolution at a similar dose level, and the transverse truncation associated with volume-of-interest (VOI) imaging is not an issue any more. A bench-top DPC-CBCT system was designed and constructed. The initial results from the system encourage the principle of DPCCBCT imaging