Optical Monte-Carlo Simulation to Evaluate Monolithic PET Detector Concepts

We present a framework based on Geant4 to evaluate the performance of different monolithic positron emission tomography (PET) detector stack setups based on a LYSO:Ce crystal with dimensions 32 mm x 32 mm x 12 mm. The optical properties of the scintillator were matched to measurements using an experimental calibration setup. We obtained a light yield of gLy = 33 000/MeV and an optical mean free path of l(p) = 18 cm. The readout characteristics of the digital sensor array, consisting of 4 x 4 digital photon counter (DPC) sensors from Philips Digital Photon Counting, were implemented to mimic the readout of the scintillation light captured by the DPC array in simulation and experiment. We generated calibration data using the simulation to train a positioning algorithm and determined the position of experimentally measured interactions. The method performed well with a spatial resolution of below 2 mm except for a systematical deviation in one edge of the crystal. Using experimental calibration data to position experimental interactions we obtained a resolution of 1.68 mm. Both in experiment and simulation the detector stack yielded the best performance when using retroreflectors as a wrapping. In the simulation the additional optical medium used to attach the reflector had a minor influence on the performance. Furthermore, the simulation was used to determine the influence of the optical coupling between scintillator and the glass plate protecting the DPC array. No benefit in the spatial resolution was obtained when changing the optical adhesive from a standard silicon glue to a high refractive-index coupling.