Development of DOI Detector for PMT Blind Region

To avoid loss of data sampling and sensitivity between detectors in a PET system, it is necessary to fill up scintillation crystals in the region between two PMTs, PMT blind region, and recognize gamma-ray detected location by a special crystal identification technique for the region.We have developed a 2-layer depth of interaction (DOI) detector for the PMT blind. The detector is composed of two layers of scintillation crystal array standing over two PMTs without a light guide plate. Crystal identification, that is 2-layer DOI encoding, is performed by Anger-type calculation of the PMT signals. Because all upper layer crystals and lower crystals in the PMT blind region are not coupled directory to the PMTs, materials between crystals must be chosen to guide scintillation light appropriately from the crystal of detection to the PMT useful area. Not only reflectors, air gap and the translucent film that is expected to pass scintillation light much more than reflector but less than air gap can be the candidates of the material.The basic study to determine proper reflector arrangement was carried out on one position sensitive PMT (PS-PMT) with using distant anodes instead of preparing two PMTs. Lu2(1-x)Y2xSiO5 (LYSO) in dimensions of 1.46 mm x 1.46 mm x 4.5 mm was used as the crystal element and a 256ch flat panel PS-PMT (FP-PMT) was done as the PS-PMT. The FP-PMT has 16 x 16 multi-anodes with 3.04 mm interval. It has 52 mm x 52 mm opening area and 89% of useful area. Since the reflector arrangement was optimized through the basic study, we measured the 2-layer DOI detector performance in the more realistic system with the existence of the PMT blind region made by two FP-PMTs.World Congress on Medical Physics and Biomedical Engineering 200