Time-of-flight (TOF) is an effective method for improving signal-to-noise ratio of the images for PET. Basic performance of TOF-PET such as sensitivity, TOF-gain, and energy resolution depends on the scintillator thickness. Hence, determining the appropriate scintillator thickness is crucial when designing TOF-PET systems. However, the optimal thickness for the Lu-based scintillators is often not clear-cut. In this paper, we introduce a new concept (figure of merit,FOM) to evaluate the optimum scintillator thickness with regards to the above factors for a TOF-PET system employing Lubased scintillators coupled to silicon photomultipliers (Si-PMs).We optically coupled Lu1.9Gd0.1SiO5 (LGSO) scintillators of different thickness (length) to Si-PMs and measured the timing resolutions with a high bandwidth digital oscilloscope. Then we calculated the TOF-gain for each crystal.Wealso calculated the coincidence detection efficiencies. Finally, for each LGSO length, we evaluated the relative sensitivity with FOMs that take into account the TOF-gain (TOF-sensitivity). From this study, we found that LGSO with 20-25mmthickness was optimum based on the calculated TOF-sensitivity FOM.Weconclude that the evaluation methods and results can provide an objective method in which one may use to determine the optimum thickness of the scintillators for TOF-PET systems.
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