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Simulation study of a DOI-based PET-Compton imaging system for positron emitters

PET is the most sensitive imaging technique in the field of nuclear medicine. However, the singles count rate is typically 10 times higher than the coincidence count rate even for positron emitters. This means that there are a lot of pairs of undetected annihilation photons. For single gamma imaging, Compton imaging is known as the only method to obtain directional information of an incoming gamma ray without using collimators. Compton kinematics collimation suffers from poor angular resolution due to Doppler broadening, energy blurring, and position blurring. Therefore, the spatial resolution of Compton imaging is worse than PET imaging in general. However, non-coincident singles photons have the potential to extend the field-of-view (FOV) beyond the scope of PET's FOV and to contribute to improved imaging performance. In this paper, therefore, we propose a system combining PET and Compton imaging by depth-of-interaction (DOI) detectors, and we show the feasibility of this system using a Monte Carlo simulator. The simulated system consisted of 48 DOI detectors arranged in five rings with a ring diameter of 80 cm. Detector materials of scatter layer were selected among Si, LaBr3 and GAGG. Absorber layer was only made of GAGG crystal for having both high stopping power of annihilation photon and high energy resolution. The thickness of the scatter layer was changed from 1 mm to 10 mm, and the thickness of the absorber layer was fixed to 2 cm. For the 70-cm NEMA phantom measurements, LaBr3 and GAGG had back scatter ratio of about 15% without an energy window, but had higher Compton sensitivity than PET sensitivity even after back scatter reduction based on the energy window. For all detector materials of the scatter layer, non-coincident singles events were up to 30 times higher than coincidence events, and ratios of Compton events in non-coincident singles were below 20%. Finally, Compton sensitivities were up to 3 times higher than PET sensitivities for all scatter layers over 5-mm thickness. The proposed system promises higher Compton sensitivity than PET sensitivity.