Research On Scanning Trajectories Of Exact Cone-beam Reconstruction

In recent years, the technology of computed tomography has made a signification progress. Exact cone-beam reconstruction technique is taking the place of approximate reconstruction and becoming the mainstream in practical application fields. Meanwhile, the theoretical system of cone-beam exact reconstruction is in the process of constantly improving and developing.Although traditional standard helical scanning trajectory has some advantages such as continuous scanning, it shows limitations in the aspect of region of interest (ROI) reconstruction. Standard helical trajectory fits perfectly for high speed whole body scanning. However, practically ROI usually shifts away from original center, and also shapes anomalously. In order to reconstruct ROI, standard helical trajectory must scan an entire column region covering the ROI, which usually leads to longer scanning time, higher radiation dose, motion artifact, and unavoidable but unnecessary radiation harm to normal tissue.The main task for novel trajectory is ROI oriented scanning. On one hand, novel trajectory shall be designed to avoid some important organs to reduce local dose, and on the other hand, it shall be designed to speed up scanning to reduce integral dose. Practical needs have called for creations and applications of various non-standard helical trajectories for better ROI reconstruction. The core work in this thesis is to survey characteristics of scanning trajectories and to explore new trajectories based on exact cone-beam reconstruction. The main achievements include:1. Development steps of cone-beam reconstruction system from approximate algorithms to exact algorithms have been reviewed. It goes into details with respect to interrelationship and interactive effects between cone-beam reconstruction algorithms. It has been pointed out that research on ROI oriented exact cone-beam reconstruction would be an important direction in this field.2. A exact cone-beam reconstruction simulation system of standard helical trajectory has been established based on matlab platform. Effects on reconstruction results by integral discretization have been examined. Simulations show reconstruction results give star stripes under low sampling rate of scanning trajectories, and some details of reconstruction object will be lost in case of inadequate number of detector horizontal cells.3. Status quo of present trajectories research and design clues for new trajectories have been analyzed. Steps for new trajectory researching have been generalized as follows: reconstruction condition and reconstruction region; reconstruction algorithm; simulations and experiments. A method has been presented to determine reconstruction region for new trajectory and it has also been discussed that discretization error distributes non-uniformly in reconstruction region. A flow of Katsevich's exact reconstruction algorithm has been analyzed. It goes into the problem of detector efficiency according to filtering lines. A method of curve by optimizing filtering directions has been put forward and validated by simulation results.4. A novel kind of exp-helical dynamic pitch scanning trajectories has been proposed. The new kind of trajectories can be shaped flexibly and approach to standard helical trajectory and other kinds of trajectories with similar reconstruction results. Meanwhile, for ROI oriented reconstruction, integral and local dose can be reduced by this new kind of trajectories, in order to minimize scanning radiation harm to other important organs. This new kind of trajectories has some practical value, since they can not only be applied to present apparatus directly but can also achieve continuous scanning.5. Another novel kind of close circular scanning trajectory has been studied based on sinusoid circular scanning trajectory proposed by Yang et al. This new kind of trajectories has been defined mathematically and proved to satisfy reconstruction condition to realize exact reconstruction. Simulations show that discretization error in reconstruction can by reduced effectively by this kind of trajectories.