Heng Zhang

Positron emission tomography (PET) is widely used in the diagnosis of tumors, cardiovascular system diseases, and neurological diseases. Scintillation crystals are an important part of PET scanners; they can convert γ photons into fluorescent photons to obtain their energy, time, and position information. Currently, an important research goal in PET is to find scintillation crystals with better performance. In this work, the principle of scintillation crystals is introduced, and the properties and requirements of scintillation crystals in different PET scanners are analyzed. At present, Lu_2(1−x)Y_2xSiO₅ (LYSO) is the scintillation crystal with the best comprehensive properties. LaBr₃ performs even better regarding the timing characteristics and light output; however, LaBr₃ has not been used in any PET scanner because of its deliquescence. Detectors made of Gd₃(Ga, Al)₅O₁₂ (GAGG) exhibit a high depth of interaction (DOI) resolution and have considerable application potential. The application fields of PET are constantly expanding, and its future development aims to achieve high spatial resolution and high sensitivity, which require scintillation crystals with better performance.

Dual-layer-offset or multi-layer-offset design of a PET detector can improve spatial resolution while maintaining high sensitivity. In this study, three dual-layer-offset LYSO detectors with three different reflectors (ESR, Toray, and BaSO₄) were developed. The top layer consisted of a 17 × 17 array of crystals 1 × 1 × 6.5 mm³ in size and the bottom layer consisted of an 18 × 18 array of crystals 1 × 1 × 9.5 mm³ in size. Neither light guides nor optical glue were used between the two layers of crystals. A custom-designed electronics system, composed of a 6 × 6 SiPM array, two FPC cables, and a custom-designed data processing module, was used to read out signals. An optimized interaction-decoding algorithm using the center of gravity to determine the position and threshold of analog signals for timing methods was applied to generate decoding flood histograms. The detector performances, in terms of peak to valley ratio of the flood histograms and energy resolutions, were calculated and compared. The dual-layer-offset PET detector constructed with BaSO₄ reflectors performed much better than the other two reflectors in both crystal identification and energy resolution. The average peak-to-valley ratio and the energy resolution were approximately 7 and 11%, respectively. In addition, the crystals in the bottom layer showed better performance at crystal identification than those in the top layer. This study can act as a reference providing guidance in choosing scintillator reflectors for multi-layer dedicated DOI detectors designed for small-animal PET imaging.