Study On Iterative Reconstruction Algorithms For Half-Covered CT

Computed Tomography(CT)utilizes computer technology to process projections obtained by scanning a test object at multiple angles to generate a cross-sectional(tomographic)image of a specific area of the test object,allowing the user to view the inside of the test object without cutting.Due to the limitations of the production process,production costs and the physical factors of the radiation,the size of the detector is limited.In the case of limited detector size,the scanning scheme must be designed in order to expand the Field of View(FOV).In practical application,the FOV is usually expanded by two methods of translating turntable or offsetting detector.In order to ensure that each point in the FOV is passed by at least a ray,the radiation must cover at least half of the FOV,so CT that utilize this scanning method is called half-covered CT.Since translating turntable and offsetting detector are equivalent theoretically,for ease of description,this thesis describes half-covered scanning via offsetting detector.For half-covered CT reconstruction,a new weighted method,called WSART(Weighted SART)algorithm,is developed based on the redundancy of rays and SART(Simultaneous Algebraic Reconstruction Technique)algorithm.This thesis compares the reconstruction performances of Weighted Filtered Back Projection(WFBP)and WSART algorithm.Simulated and real projection data experiment results show the effectiveness of the WSART algorithm.Sparse View CT not only reduces the radiation dose but also reduces data storage for quick calculation,but reconstruction by using traditional FBP and SART algorithms will cause severe streak artifacts.The ASD-POCS(Adaptive Steepest DescentProjection on Convex Sets)iterative reconstruction algorithm based on Total Variation(TV)minimization has achieved good reconstruction results in sparse view CT reconstruction.However,when the sampling view is too sparse,the algorithm can not effectively suppress artifacts.Prior Image Constrained(PIC)and sparsity of Gradient Amplitude Image is introduced as prior information for the half-covered sparse CT reconstruction.Based on ASD-POCS(Adaptive Steepest Descent-Projection on Convex Sets)and the WSART algorithms,a PIC-ASD-WSART reconstruction algorithm has been developed.Simulation and real projection data experiment results show the effectiveness of the algorithm.