Optimization of the SiPM Pixel Size for a Monolithic PET Detector

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Abstract Molecular imaging of small animals with PET requires high detection efficiency (DE). PET detectors consisting of monolithic scintillator coupled to position sensitive photo-detectors can yield high DE by reducing detection-inactive space. Silicon photo-multipliers (SiPMs) are compact photo-detectors with high gain, high photon detection efficiency (PDE) and fast response. There is a substantial interest in employing SiPMs for developing monolithic scintillator-based PET detectors. In this work, we investigate the optimization of the pixel size of an SiPM array to read out a monolithic scintillator. The pixel size of the SiPM affects the spatial resolution of the resulting detector in two ways. First, in general smaller pixels can measure more accurately the distribution of the scintillation lights at the exit surface of the scintillator to attain higher spatial resolution. On the other hand, a smaller pixel detects fewer light photons and yields higher pixel noise. As a result, the spatial resolution and energy resolution may be degraded. Consequently, the optimal pixel size of the SiPM array to use for achieving the best spatial resolution must be determined by considering the trade–off between these two factors. We conducted Monte-Carlo simulation to analyze the relationship between the SiPM pixel size in a monolithic-scintillator PET detector and the resulting spatial resolution. In our simulation, the scintillator was 10 mm thick, the gamma rays were assumed to interact at the center of the scintillator and the PDE of the SiPM was set to 20%. A range of scintillator light output was considered, including 30,000 photons per MeV that is typical of LYSO. The 3-d position of the interaction point of the gamma ray was estimated from the light distribution pattern measured by the SiPM array by employing the Nonlinear Least Squares Position Estimation method developed by Li et al. The results show that, for achieving the best resolution at the center of the detector the optimal pixel size of the SiPM array is between 5.4 × 5.4 mm2 and 6.3 × 6.3 mm2 Also, for achieving the best overall sptial resolution for the detector, the optimal pixel size was between 5 × 5 mm2 and 8 × 8 mm2.