Optical Monte-Carlo Simulation to Evaluate Monolithic PET Detector Concepts

We present a framework based on Geant4 to evaluate the performance ofdifferent monolithic positron emission tomography (PET) detector stacksetups based on a LYSO:Ce crystal with dimensions 32mm x 32mm x 12mm. Theoptical properties of the scintillator were matched to measurements using anexperimental calibration setup. We obtained a light yield of $g_{LY=}$ 33000/MeV and an optical mean free path of $l_{p=}$ 18cm. The readout characteristics of the digital sensorarray, consisting of 4 x 4 digital photon counter (DPC) sensors from PhilipsDigital Photon Counting, were implemented to mimic the readout of thescintillation light captured by the DPC array in simulation and experiment.We generated calibration data using the simulation to train a positioningalgorithm and determined the position of experimentally measuredinteractions. The method performed well with a spatial resolution of below2mm except for a systematical deviation in one edge of the crystal. Usingexperimental calibration data to position experimental interactions weobtained aresolution of 1.68mm. Both in experiment and simulation the detector stackyielded the best performance when using retroreflectors as a wrapping. Inthe simulation the additional optical medium used to attach the reflectorhad a minor influence on the performance. Furthermore, the simulation wasused to determine the influence of the optical coupling between scintillatorand the glass plate protecting the DPC array. No benefit in the spatialresolution was obtained when changing the optical adhesive from a standardsilicon glue to a highrefractive-index coupling.