Simulation study to optimize the number of photo-detection faces and inter-crystal materials for the X´tal cube PET detector

We are developing a novel PET detector with 3D isotropic resolution, the X´tal cube. Our original idea was to cover all 6 surfaces of a segmented crystal block with multi-pixel photon counters (MPPCs) for effective scintillation photon acquisition. On the other hand, modification toward practical use is important in terms of assembling efficiency and production costs. In addition, when the X´tal cubes are arranged to form a PET scanner, insensitive inter-detector gaps made by the MPPC arrays should not be too wide or better yet, they should be removed. Therefore, reducing the number of faces to be connected to the MPPCs has become our top priority. In this paper, we evaluated the effect of reducing the number of MPPCs on positioning accuracy through numerical simulations. We optimized the X´tal cube in terms of the gap material of crystal segments and the number of MPPC connection faces. We simulated the X´tal cube with (3.0 mm)³ crystal segments; the crystal block was composed of 6 × 6 × 6 array of (3.0 mm)³ LGSO crystals. Each surface of the crystal block was covered with a 4 × 4 array of MPPCs, each of which had a 3.0 × 3.0 mm² active area. Outer surfaces of the crystal block, except for the partitions that the MPPCs were arranged on, were covered with reflectors. For material between crystal elements, we compared optical glue and air gap. The air gap showed better crystal identification performance than the optical glue, although good crystal identification performance was obtained even with optical glue for the 6-face MPPC connection. We showed that the number of MPPC connection faces could be reduced to two when the gap material was air.