Optimization of Crystal Arrangement on 8-Layer DOI PET Detector

Previously, we proposed an 8-layer depth-of-interaction (DOI) encoding method for a PET detector and proved its validity. The layer of interaction is identified by hybrid method: scintillation light control by the original reflector arrangement for 4-layer DOI encoding and pulse shape discrimination for 2-layer DOI encoding. In the 8-layer DOI detector, four layers then consist of the scintillator of different pulse shape from another scintillator for the other four layers. The two kind crystal layers can be arranged in two ways: stacked alternately (LSLS) or set in the upper and lower four layers (LLSS). Since the two crystal arrangements are expected to show different detector performance, we investigated the difference to understand the characteristics of the DOI detector for its optimization. Gd₂SiO₅ (GSO) crystals of 0.5 mol% Ce dopant and 1.5 mol% Ce dopant were used for the measurement. The former is in dimensions of 2.9 mm times 2.9 mm times 3.75 mm and the latter is 2.9 mm times 2.9 mm times 3.6 mm, respectively. The experimental results show better performance of the LLSS arrangement in pulse shape discrimination, while inferior in the 4-layer DOI encoding. There was no particular difference between the two crystal arrangements in light output and energy resolution of each layer.