Estimation of optimum scintillator thickness of Si-PM detectors for time-of-flight (TOF)-PET

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 Lu-based 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. We also 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 20-25 mm thickness was optimum based on the calculated TOF-sensitivity FOM. We conclude 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.