Local High Spatial Resolution Pet Imaging Based On All-digital PET Detectors

Positron emission tomography (PET) is a nuclear medicine instrument, which has shown great value in the early detection and diagnosis of tumor. As the wide application, PET is expected to provide better imaging quality. To improve the imaging quality of PET, acquiring high spatial resolution is one of the key directions in the development of PET instrument. Higher spatial resolution always means better lesion detectability to detect tiny lesions earlier.The research and development of traditional PET focus on the technology of common design mode for general purpose. After the implementation of PET instrument, the performances are almost fixed and can’t be adjusted or modified to fit the variation of the application’s requirements. We noticed that different applications always have different requirements and the focus point is always the region of interest (ROI). Usually, only the ROI requires high imaging quality. The all-digital PET detector adopts the design of single event and modularization. Digital technology is the core of the detector. The detector can be utilized to easily realize PET imaging with variable structures. Therefore, based on the characteristics of all-digital PET detectors, we proposed a new local high spatial resolution PET system in this thesis. We investigated the relation between the layout of the detectors and the spatial resolution requirement of ROI with different positions and sizes. Then the imaging characteristics of the application of the liver lesion detection were analyzed. At last, we initially investigated a new fast and simple normalization correction method based on thin planar source, which aimed at the real implementation of the proposed system. The overall summaries are outlined as following:The proposed system was composed of mixed detectors having different intrinsic spatial resolutions. To meet the requirement of application, the proposed system can provide high spatial resolution in the ROI and normal spatial resolution in the region outside of the ROI, by changing the layout of the detectors and without sacrificing the field of view (FOV). Firstly, the design concept and imaging method of the new PET was proposed. Then, we designed one kind prototype of the proposed system with two types of detectors, including normal resolution detectors (NRDs) and high resolution detectors (HRDs). The NRDs and HRDs were located on the detector ring, with all the HRDs (NRDs) distributed as one arc. The effects of the location and the size of the HRD arc on the spatial resolution of every sub-region in the FOV were analyzed. And we proposed an uniform distribution of the HRDs, which was a complementation to the one arc distribution. These two distributions led to local high spatial resolution for any ROI in the FOV. These work established the relation between the spatial resolution requirement of ROI with different position and size and the layout of the HRDs. Customized PET system with mixed detectors can be designed for high spatial resolution of ROI.Based on Monte Carlo (MC) simulation, we analyzed the imaging characteristics of the proposed system with one arc distribution in the application of liver lesion detection. The convergence properties, the imaging performances and the effect of the random events on the imaging quality were analyzed. The results shown that the convergence rate and the imaging performances of ROI directly related to the HRD layout, which was consistent with the conclusion of the effects of the HRD layout on the ROI’s spatial resolution. And the random fraction had little effect on the relative imaging performances of the proposed system with respect to the traditional system.To meet the high time requirement of normalization correction due to the off-line and on-line changement of system structure, we designed a fast normalization method based on a thin planar source. For the off-line, a single complete correction is needed to obtain all correction factors, and for the on-line, only part factors are needed to update. Because the method is also fit to the system composed with homogeneous detectors, the constructed Trans-PET(?) system was adopted to verify the method. The results has shown that this method can meet the basic requirement of short-time normalization for the off-line structure changement of the proposed system. Combination with the analysis of this method, it indicated that the method has the potential to provide fast normalization correction for the proposed system with on-line structure changement.