The Study of X-ray Phase-Contrast Imaging

The conventional X-ray radiography relies on the attenuation of X-ray energy to produce an image that describes the absorption property of the object. However, the small contrast between cancer tissues and normal tissues makes the conventional X-ray radiography ineffective in cancer diagnosis. Thanks to the emergence of X-ray phase-contrast imaging techniques, the low contrast of tissues can then be overcome. However, almost all the existing phase retrieval algorithms assume that incident beams are monochromatic, which usually are not available in modern clinical X-ray tubes. In order to implement the phase-contrast imaging in the clinical system, a quantitative polychromatic X-ray phase contrast imaging for tissue characterization was investigated in the first part of this thesis. It is a method that reconstructs the thickness distribution of the object that consists of a few homogeneous parts of known composition. By taking use of more incident spectra, we can apply the least-squares method to improve the reconstruction performance. The second part of this thesis is the iterative reconstruction of phase contrast tomography. It used the linear algebra operation to express the Radon transform. Since the phase retrieval process can result in a large noise amplification in the low frequency, the low frequency noise reduction became an important issue. In this study, the weighted least-squares conjugate-gradient method was utilized to reconstruct three-dimensional refractive index distribution. The statistical properties of the projected phase images were calculated as a priori information. This statistical properties could be taken into consideration to enhance the reconstruction quality and provided the reconstructed images with lower relative errors.