Research On Image Reconstruction Algorithms In 3D Cone-Beam X-ray Computed Tomography

X-ray CT has been widely used in the world, since the birth of the first CT (Computed Tomography) in 1972。The broad using of CT also generates great impetus for CT research. Several big changes have been taken place during the past 30 years. These changes mainly involve in two aspects--speed and image quality.X-ray CT image is calculated from projection data by computer. So there are two steps to get image. Firstly, use X-ray to collect projection data. Secondly, calculate projection data to get image. There are three kinds of scan mode: parallel-beam, fan-beam and cone-beam. Parallel-beam projection data can't be realized in the project directly. Compared with the fan-beam projection data, the cone-beam projection data get more information once. So it is in favor of improving scan speed and image quality. But the relationship between cone-beam projection data and the cross section image is very complex, and it increases the complexity of reconstruction algorithm and scan frame. In recent years, many research in the cone-beam scan technology appeared .But many of the algorithms are not very perfect, so the commercial cone-beam CT is not appeared at present. Three main questions have been researched in this thesis in order to solve the problem existed in CT.1. In the process of research in the accuracy of model computation and the feasibility of three-dimension reconstruction algorithm, the design of model and computer simulation of projection data is a necessary part. 3D Shepp-Logan head phantom is used as an example. The thesis introduces its design and the projection simulation method in detail. The projection data simulation is performed based on 3D Shepp-Logan head phantom. The results of image reconstruction using FDK algorithm indicate the accuracy of model design and the feasibility of computing projection data.2. Cone-beam helical CT is complex and time-consuming in reconstruction algorithm for the moment. The helical Cone-Beam approximate reconstruction algorithm is introduced in the thesis. And the thesis researches on reconstruction speed from the structure of algorithm,the technique of implementation and the optimization of code. Computer simulation experiment express that the methods can efficiently improve the speed of image reconstruction.3. In the image reconstruction basing on optimization without constraint,in order to improve iterative efficiency and reconstructed quality, the thesis firstly applies the variable metric method to reconstruct image, and describes variable metric method,steepest descent Method,conjugate gradient method respectively. The thesis compared three algorithms above at iterative efficiency and reconstruction quality respectively by simulating experiment. The result shows that variable metric method is better than other algorithms in the aspects of iterative efficiency and reconstruction quality.In this thesis, some familiar cone-beam scan reconstruction algorithms are also analyzed. These algorithms can be sorted by scan mode, reconstruction precision. The cone-beam scan reconstruction algorithms are introduced in this paper include FDK,P-FDK,T-FDK,HT-FDK and S-FDK, etc.