Ultra-High Resolution LYSO PQS-SSS Heptahedron Blocks for Low-Cost MuPET

We developed and built a solid detector ring for a new murine positron emission tomography (MuPET) system. We use cerium-doped lutetium yttrium orthosilicate (LYSO) crystals and regular round 19 mm photomultipliers (PMTs) arranged in a quadrant-sharing (PQS) configuration. The detector system comprised 180 PQS-SSS heptahedron-shaped blocks distributed in 6 subrings. Each block comprised a 13 × 13 crystal array with nominal dimensions of 19 × 19 × 10 mm³. To form a zero-gap solid ring, the rectangular blocks were ground into heptahedron-shaped blocks with a taper angle of 6^° on the edge crystals and optical surfaces. The two edge crystals were 1.76 mm wide, and the inner crystals were 1.32 mm wide. We explored the possibility of increasing the detector´s performance by implementing new design, materials, and production techniques; testing the detector´s performance; and measuring the detector´s timing resolution. List-mode data were acquired using a Ga-68 source, in-house high-yield pileup-event recovery electronics, and data-acquisition software. Four randomly selected blocks were used to evaluate the quality of the detector and our mass-production method. The four blocks´ performances were quite similar. A typical block had a packing fraction of 95%, a peak-to-valley ratio of 2.4, a light collection efficiency of 78%, and an energy resolution of 14% at 511 keV, and all 169 of the block´s crystal detectors were clearly decoded. Using a single crystal in coincidence with a block, the average coincidence timing resolution was found to be 430 ps (full width at half maximum). A block-to-block coincidence timing resolution of 530 ps is expected. Our PQS-SSS heptahedron block design indicates that it is feasible to construct a high resolution (~1.2 mm) MuPET detector ring using round 19 mm PMTs instead of the more expensive position-sensitive PMTs or solid-state detectors.