Design of an inexpensive high-sensitivity rodent-research PET camera (RRPET)

A small animal PET is being developed. The design goals are lower cost, higher sensitivity and same image resolution as commercial animal PET. It uses a new version of our PMT-quadrant-sharing (PQS) detector with a 98% crystal-packing fraction to maximize sensitivity and no light-guides to maximize light output (decoding resolution). It uses 168 low-cost 19-mm PMT and 9216 BGO for its high photoelectric fraction. Each block is 8×8 with an average pitch of 2×2 mm (10 mm deep) to provide image resolution similar to commercial systems using LSO, position-sensitive PMT and optical fibers. The number-of-crystals-per-PMT decoding ratio achieved was 55, similar to an animal PET using GSO that has 3 times more light output than BGO. To maximize image resolution, we "circularize" the PQS detector design, where each block is one side of a 24-sided polygon (the detector ring). Circularizing PQS detector requires the block to be ground slightly into a pentagon (166° apex). The edge rows of crystals in the block are also tapered, so that blocks can be glued together to form a solid BGO ring with nearly 100% packing. The ring diameter is 13 cm. 6 rings provide a 12-cm axial FOV. The large axial FOV increases coincidence sensitivity. An automatic PMT-equalization system can tune the PET in 1 minute without radiation, for PMT tuning before each study to minimize the effects of prior radiation loading and temperature drift. Detector-pileup-recovery electronics were used to prevent imaging artifacts and count-loss in BGO detectors. Experiments showed that 2×2×10 mm BGO can be decoded (energy resolution 23%). Monte Carlo simulation showed a detection efficiency of 5.9% (350-700 KeV) for a central point source. The 3.7× increase in coincidence sensitivity comes from (a) larger axial-FOV (1.8×), (b) BGO higher photoelectric fraction (1.5×) and (c) high packing fraction (1.3×).