Ultrahigh-Resolution L(Y)SO Detectors Using PMT-Quadrant-Sharing for Human & Animal PET Cameras

The goal of this study is to develop lower-cost ultrahigh resolution detectors for PET systems, using the PMT-quadrant-sharing (PQS) decoding technology on L(Y)SO scintillation crystals. For this work, L(Y)SO PQS block detectors for both animal and human PET cameras were developed and studied. Both simulation and experimental detector design studies were carried out to achieve efficient light distribution and crystal decoding. The effects of crystal finishes and reflector patterns on light distribution, light output and energy resolution were investigated and used to derive the highest resolution PQS-L(YSO) block-detector. The PQS-L(Y)SO detector performance was measured on the best performing blocks. For performance evaluation, list-mode data from the detectors were acquired and analyzed to extract light-collection efficiency, energy-resolution distribution, and pulse height distribution for individual crystals. The potential PET imaging resolution performance was investigated using Monte Carlo simulation studies with the GEANT4/GATE software for both detectors developed for small animal PET and human PET applications. From these studies, we have the following findings: 1) For light distribution studies on the crystal surface finish, 4 mum lapping was found to be the preferred finish for achieving the best position decoding together with good overall light output for all the crystals in both, the human and animal detector arrays; 2) Intricate reflector patterns between crystals can be made from the ESR mirror film (3M Inc.) for optimally controlling the light sharing between crystals and to the four decoding PMT´s, with high packing fractions on the PQS-blocks; 3) For the PQS-LYSO detector block for animal PET systems, using 19-mm circular photomultipliers (PMT), we achieved decoding a 14 times 14 arrays with a crystal pitch of 1.27 times 1.27 mm². This animal detector has a packing fraction of 95.6%, an energy resolution ranging between 12.9%~15.8% for- individual crystals (average energy resolution of 14%), the pulse height for the least favorable crystal is 63.5% of the most favorable crystal; 4) For PQS-LSO detector block for human PET systems using very large circular 51-mm PMT, we achieved decoding a 15 times 15 array with a crystal pitch of 3.25 times 3.25 mm². The human PET detector has packing fraction of 98.2%, an energy resolution range 12.9%~15.8% (average energy resolution 14%). The pulse height of least favorable crystals is 80% of the most favorable crystal. 5) From Monte Carlo simulations for LSO small animal PET, a spatial resolution of 1.1-1.2 mm may potentially be achieved using low cost 19-mm circular PMT. For human PET systems, 3-mm spatial resolution may potentially be achieved using very large 51-mm circular PMT for cost reduction.