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 mm
³
. 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.