Research On Extraction Methods Of Phase Information And CT Reconstruction Algorithms In Diffraction Enhanced Imaging

Diffraction enhanced imaging (DEI) is one of up-to-date subjects in the X-ray imaging field. It can inspect weakly absorbing low-Z samples and its spacial resolution is on the order of microns or even sub-microns. So DEI has its particular advantages over traditional X-ray absorption imaging technology. The primary experiments on DEI have been done sucessfully in medical, biological and material fields. Thus, the research on DEI has significant value and perspective.This dissertation focuses on the research of extraction methods of phase information and reconstruction algorithms of DEI-CT based on the solution of the DEI's computer simulation experiment model. The research results are all proved by the real experimental data from the DEI setup at the Beijing synchrotron radiation facility. The main work and the originality of the dissertation are summarized as follows:Firstly, the equivalent rectilinear propagation model of the computer simulation experiment is presented according to the geometric-optics approximation theory of X-ray diffraction by a perfect crystal with symmetrical Bragg geometry. The new model partly simplifies the geometric structure of the beam path and is applied to analyze quantificationally the position offsets of the DEI images in the different positons of the rocking curve. The correction method is proposed to correct the images of the computer simulation experiment and ensure the validity of the simulated results computed by the extraction methods of phase information and CT reconstruction algorithms.Secondly, in the research on extraction methods of phase information, the limitation of the current methods, such as the geometric-optics approximation method and the multiple-images statistical method, are analyzed detailedly, and three more effective methods are presented here, including the statisticalgeometric-optics-approximation method, the maximum refraction-angle method and the Gaussian curve-fitting method. A strategy how to select appropriate extraction methods in the real experiment is drawn up. More accurate results of refraction angle information can be obtained by these new methods than current methods in the simulation and real experiments.Finally, in the research on reconstruction algorithms of DEI-CT, a direct filtered backprojection algorithm based on refraction angle information for the scan type of the CT rotation axis parallel to the lattice plane of the analyzer crystal is presented here, which can reconstruct the distribution of the sample's diffractive index decrement in one step. The results of the simulation and real experiments indicate that the new algorithm based on the new extraction methods can reconstruct the distribution of the sample's diffractive index decrement quite accurately, whose calculation precision and image quality are better than those obtained by the CT reconstruction algorithms based on the current extraction methods.