- We are investigating a high resolution, Angerlogic based PET block detector using large-area avalanche photodiodes (LAAPD). Such block detectors will be used in a simultaneous PET/MRI clinical scanner. Using a Monte-Carlo simulation tool, we systemically optimized the detector design taking into account the following factors: the dimensions of the scintillation crystal and the optical diffuser, the surface finish of crystals, the layout of APD arrays, and the signal-to-noise (SNR) of the system. Based on the simulation results, a block detector prototype was built with an 8×8 LYSO crystal array (crystal pitch size: 2.75 × 3.00 × 20 mm3) coupled to four APDs. The performance of the block detector with regard to the crystal resolving ability and the energy resolution is reported.