System design and development of a lower-cost animal PET-CT (MuPET) with large axial solid PET ring of 1.25-mm LYSO detectors

This paper presents the system design and engineering of a lower-cost, large axial field-of-view micro PET-CT (MuPET) for small animal imaging. The system has been constructed and being tested. 30,420 LYSO detectors (1.25 transaxial × 1.4 axial × 10 mm³) form a 16.5-cm detector solid ring and a large 11.6-cm axial field for high detection sensitivity. The 30240 crystals are configured into 180 pentagon-shaped position-sensitive blocks and are precisely glued together into a solid crystal ring consisting 78 detector rings and 390 detectors per ring. Each block decodes 13 × 13 crystals using our PMT-Quadrant-Sharing (PQS) scheme and the entire solid detector ring with 30,420 crystals are decoded by only 210 round PMT (19-mm), with an average decoding ratio of 145 detectors/PMT. The 180 PQS detector blocks were produced by an efficient slab-sandwich-slice method with a 94% volumetric packing fraction and internal mirror film decoding light-guides for high light-yield. The high-resolution PQS block has an average energy-resolution of 14.1% ranged from 12.9% to 15.8%. The front-end FPGA based digital HYPER (High-Yield-Pileup-Event-Recovery) detector-decoding electronics can process 25 million random "singles"/sec with a 10% loss. The coincidence system has a capability of online programming of both prompt and delayed-coincidence windows between 0 to 16-ns with 95-ps resolution. The system coincidence-timing window is 1.9-ns. The preliminary timing resolution is 650-ps, despite using non-time-of-flight PMT and very narrow crystals. The electronics has an online baseline-correction and automatic PMT-gain control using LED for stabilizing detector-decoding and a PID detector temperature controller using peltier cooler for minimizing the effect of room temperature variation. The CT section is a cone-beam CT using a flat panel detector with 43-micron pitch and 80KVp (0.5 mA) x-ray generator. The reconstructed Na-22 point source image resolut- - ion using 2-D filtered back-projection is 1.35 mm and 1.9 mm at the center and 4.0 cm off center, respectively. With resolution-recovery reconstruction, resolution less than 1.2 mm can be achieved at the center and 1.35 mm at 4.0 cm off center. The preliminary result of absolute sensitivity was 5.0% for a point source at the center of the PET with a 300-750 keV window.