Timing performance evaluation of PMT-quadrant-sharing LYSO detectors for time-of-flight PET

Time-of-flight (TOF) data can improve the imaging quality of positron emission tomography (PET) cameras by estimating the location of the positron annihilation. In this study, we measured the TOF resolution of lutetium yttrium orthosilicate (LYSO) blocks based on the photomultiplier-quadrant-sharing (PQS) design, which has no external light guide and therefore has higher light collection efficiency than the conventional block design with a light guide. We investigated three digital timing methods to achieve better timing resolution: constant fraction discriminationleading edge discrimination, and linear fitting. A time-energy correction, estimated from a least mean square fit, was applied to minimize time walk caused by the variation of signal shapes. The average block-to-block coincidence timing resolution for two 13 ×13 blocks, each made of 4×4×20 mm³ LYSO crystals, with fast R9779 PMTs from Hamamatsu (51-mm diameter) was estimated to be 422 ps (full width at half maximum). Another 13×13 block of smaller 1.4×1.4×10 mm ³ LYSO crystals coupled to regular 19-mm-diameter XP1912 PMTs from Photonis was also tested and an average block-to-block timing resolution of 527 ps was achieved. This study shows that with PQS detector blocks, good time resolution suitable for TOF PET systems can be obtained, as well as high spatial resolution and lower PMT costs.