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New PMT-Quadrant-Sharing shallow block detector development for high performance TOF PET applications

The time-of-flight (TOF) technology is being adopted by more and more human PET systems in recent years. The most important specification for a TOF PET system is the time resolution. Among many factors that affect the time resolution, one of the most important factors is the block detector design. The PMT-Quadrant-Sharing (PQS) block design has several advantages over non-PQS detector block for TOF purposes. First, the cancellation of external light guide in PQS design enable more optical photons to travel to the PMT cathodes resulted in higher photoelectron number which is critical for time resolution. Second, optical photons' travel distance from the scintillator to PMT cathode is shorter for PQS block compared to non-PQS block, which could reduce the time distribution of the transition time for light photons. Those factors will improve the time resolution for PQS block in TOF applications. In this study, we developed two high performance, lower-cost and high-resolution PQS block detectors for TOF PET purposes. The blocks have an area of 39×39 mm2 and two dimensions in length: 12 mm and 15 mm. The matrix of both blocks is 16×16 using 2.4×2.4 mm2 LYSO crystals, which can achieve a sub 3-mm intrinsic resolution in a typical human PET geometry that is significantly higher than any other similar systems, together with a 400-500 ps time resolution possibility. Two sets of masks are developed and good decoding map are achieved. Peak-tovalley ratio is 2.6 for the 12-mm block and 2.1 for the 15mm block. The energy resolution is 12.3% and 14.8% for the 12-mm and 15-mm block respectively. Two PET modules consisting of 7 blocks each are constructed. PET tomographic results are obtained by the coincidence data from these two modules and the rotation of the object placed between the modules. The performance measurements demonstrate the successful development of two large area shallow block detectors with high spatial resolution and time resolution which will be the ideal candidates for the construction of high-performance low-cost TOF PET systems.