Electronics for a prototype variable field of view PET camera using the PMT-quadrant-sharing detector array

The electronics for a prototype high resolution PET camera with 8 modules of position sensitive detectors has been developed. Each module has 16 BGO blocks (each block is composed of 49 crystals). The design goals are component and space reduction. The electronics is composed of 5 parts: front-end analog processing, digital position decoding, fast timing, coincidence processing and master data acquisition. The front-end analog circuit is a zone based structure (each zone has 3×3 PMTs) with 8 trigger clusters (each cluster is composed of 6 PMTs) generating a trigger and identifying an active zone to be digitized by 9 ADCs. The trigger is sent to a fast timing board to get a time mark, and the 9 digitized signals are passed to the position decoding board, where a real block (4 PMTs) can be picked out from the zone for position decoding. Lookup tables are used for energy discrimination and to identify the gamma hit crystal location. The coincidence board opens a 70 ns initial timing window, followed by two 20 ns digital coincidence time mark look up table windows, to identify the true and accidental events. The data output from the coincidence board can be acquired either in sinogram mode or in list-mode with a Motorola/IRONICS VME-based system