Experimental proof of record-short coincidence time resolution performance achieved in a 3 mm x 3 mm x 3 mm LYSO crystal

In this study, we re-evaluated the attainable coincidence time resolution (CTR) performance for 3×3×3 mm³ LYSO crystals coupled to matched 3×3 mm² SiPMs. This work was motivated by potential increased sensitivity in brain positron emission tomography (PET) detector blocks that would be enabled with ultrashort CTR (<100 ps). The recent progress in silicon photomultiplier (SiPM) technology, high-frequency read-out circuits, and optimized data processing is expected to lead directly to improved performance. The 3×3×3 mm³ LYSO crystals, with all sides polished to optical quality, were optically coupled to SiPMs designed and fabricated by Fondazione Bruno Kessler (FBK). An improved high frequency read-out circuit was designed and fabricated. CTR was measured using a 22Na positron source (<10 μCi) sandwiched between two identical LYSO/SiPM/read-out circuit stacks. Our studies show that a CTR of less than 80 ps, which, to the best of our knowledge, is the shortest reported CTR for 3×3×3 mm³ LYSO crystals. The results demonstrate, for the first time, that CTR performance in 3×3×3 mm³ LYSO crystals coupled to a 3mm×3mm² SiPMs is comparable to CTRs achieved for ultra-small LYSO crystals (2×2×3 mm³) coupled to large 4 × 4 mm² SiPMs. These results prove that an array of 3×3×3 mm³ LYSO/SiPM can be used to build a next generation high performance detector block with very high packing fraction, enabling ultimately very high gamma ray detection efficiency and very high system sensitivity.