Research On Methods And Technologies For Grating-based Imaging With Conventional X-ray Sources

Grating-based X-ray imaging with conventional X-ray tube is one of the cutting-edge and hot research fields in X-ray imaging. It can retrieve three different kinds of information of the sample: absorption, refraction and dark-field, from one single scanning. It maintains the advantages of conventional X-ray absorption imaging, and also has the advantages of phase contrast imaging and dark-field imaging, thus it has important research value and a promsing future for practical applications.Based on the classical-optical grating-based imaging system at the department of Engineering Physics, Tsinghua University, this dissertation investigates several key problems such as the system modeling,analysis and optimization, information retrieving algorithms and imaging reconstruction algorithms. The research in this dissertation makes solid theoretical foundation and technical accumulation for the clinical applications of garting-based imaging and takes our country to the forefront of this area. A series of research results were achieved:Firstly, the system modeling, performance analysis and design method of the classical-optical grating-based imaging method are investigated. A Monte Carlo simulation platform is built by implementing X-ray refraction process in Geant4. And from the three aspects of the error formula of information retrieving algorithm, the system sensitivity and the system geometric constraints, the optimized system design method is given.Secondly, a quantitative dark-field information retrieving algorithm and a multiple information retrieving algorithm based on three images are proposed. In the quantitative dark-field information retrieving algorithm, the relationship between the visibility degradation in grating-based dark-field imaging and the general scattering parameter of the sample is deduced. An important conclusion is that, the reconstruction problem of grating-based dark-field computed tomography (CT) can be solved by conventional reconstruction algorithm in absorption CT. The multiple information retrieving algorithm based on three images can reduce the imaging time consuming and the dose delivered to patients in grating-based imaging, while maintain the image quality.Thirdly, an iterative reconstruction algorithm (DART-CS) which benefits from the Compressed Sensing (CS) theory is proposed for differential phase contrast imaging (DPCI). By discretizing the projection process of DPCI into a linear partial derivative matrix, a differential algebraic reconstruction algorithm (DART) is proposed which can reconstruct the refractive index distribution of the sample from the refraction angle projections directly. Based on DART algorithm, the DART-CS algorithm is proposed by introducing the CS theory into the reconstruction of DPCI. It can accurately reconstruct the refractive index with incomplete projection data in low dose situation.