Edge resolved dual scintillator gamma ray detection system and method

#The present invention relates generally to an imaging system and method, and more particularly to a positron emission tomography (PET) detection system and method.

#Conventional PET detectors use an array of scintillator crystals (i.e., scintillator pixels) optically coupled to a position-sensitive photo multiplier tube (“PS-PMT”) to achieve high spatial resolution and compactness. However, the edge areas 3 of the detection surface (i.e., the front face) of a PS-PMT 1 are radiation insensitive, as shown by gray bands in FIG. 1. This prevents the use of the entire PS-PMT front to detect radiation emitted by the scintillator pixels. Thus, only the radiation sensitive central area 5 of the front face of the PS-PMT 1 is used to detect radiation, such as photons, emitted by the scintillator pixels when the pixels are irradiated by the gamma rays. This is termed an “edge problem.”

#The edge problem is particularly troublesome in PET systems having multiple detector rings, because a detection gap is formed at the ring edges. Since PS-PMTs are provided inside the rings, the ring edges contain the PS-PMT edge areas 3 which are radiation insensitive. When two rings are axially coupled, two PS-PMT radiation insensitive edge areas 3 are located adjacent to each other, exacerbating the edge problem. Thus, even if the detector rings contact each other, a detection gap (i.e., blank region) is formed in the image at the contact point of adjacent rings. In the prior art multiple detector ring PET system, in order to overcome the edge problem, an optical fiber (or optical waveguide) bundle 7 was used to couple radiation from the scintillator pixel array 9 to the PS-PMT 1, as shown in FIG. 2. However, the extra coupling, the absorption of the optical fiber, and most significantly the limited acceptance angle of optical fiber causes severe reduction of the number of photons (about 70-80% reduction) that can be detected by the PS-PMT. Alternatively, a single ring PET system may be used. However, such as system provides a limited axial coverage, which may be undesirable for some applications.

#FIG. 1 is a schematic of the detection surface of a prior art PS-PMT.

#FIG. 2 is schematic of a prior art way to overcome the edge problem using optical fiber coupling of the pixels to the PS-PMT.

#FIGS. 3 and 4 are plots illustrating the edge problem in the prior art PET detectors. FIGS. 3A and 4A are position maps, FIGS. 3B and 4B are row profile plots and FIG. 3C is a column profile plot.

#The present inventor has discovered that the edge problem can be solved by using a first scintillator having a first decay time adjacent to radiation sensitive area of the photodetector, and by using a second scintillator having a different, second decay time adjacent to radiation insensitive area of the photodetector, such that the first and the second pixels are optically coupled. Thus, a multiple detecting ring tomography system eliminates or substantially reduces image gaps between each detector ring. This significantly improves the detector performance since there is no need to use optical fibers to couple the scintillator pixels to the photo detector to solve the edge problem.

#When the inner and outer scintillator pixels are optically coupled to each other, the outer pixel, which is optically coupled to the radiation insensitive area at the edge of the detector can share the photons with an adjacent inner pixel, which is optically coupled to the radiation sensitive area of the detector. Therefore, the photons (i.e., radiation) emitted from the outer pixel can pass through adjacent inner pixels to the radiation sensitive area of the detector and get detected by the detector.

#In the case of a PS-PMT detector, the radiation from both outer edge and adjacent inner pixels is detected at the same photo cathode area, and sensed by the same anode near the PS-PMT edge (the last anode at the edge). The ratio for the signals from both pixels are about the same, which makes it difficult to distinguish the signals in a position sensitive manner. A similar effect occurs for other photodetectors with an edge problem.