Development of a SiPM based preclinical PET SPECT imaging system imaging system

Objectives: SiPM based detector technology has been well established in PET imaging systems. In this work we aim at developing a hybrid PET/SPECT preclinical imaging system with one set of LYSO+SiPM detectors and an add-on multi-pinhole collimator.

Methods: The detector block is made of a staggered dual-layer LYSO crystal array (15x15 crystals in inner-ring layer, 16x16 crystals in outer-ring layer, and 2x2x7 mm³ crystal size in both layers), coupled to an 8x8 SiPM array (SensL FJ 30035 series). The output signals of SiPM array are first readout by a self-developed 64-channel ASIC, converted to analog E, X and Y signals with built-in Anger logic circuits and digital T signal with leading edge discrimination in the ASIC, and then digitized by an 80 MHz 12-bit ADC in AD board. Crystal index identification and energy window discrimination are performed inside FPGA on a DAQ board. Timing synchronization and coincidence sorting are performed with an open source Ethernet-based White Rabbit switch. The imaging system has 8 detector modules, forming a 175 mm bore size. Each detector module consists of 2 x 3 detector blocks. The designed PET FOV is 70mm(Φ) x 101mm(L). In SPECT imaging, a concentric multi-pinhole collimator is inserted inside the detector ring. The collimator is a tungsten tube with 80 mm inner diameter. 36 0.3-mm-diameter knife-edge pinholes are arranged in 3 axial rows. With helical scanning orbit by translating the animal bed and rotating the collimator, the designed SPECT FOV is 40mm(Φ) x 80mm(L). Rat imaging pinhole sets were also constructed with 3 x 8 0.6-mm-diameter pinholes, aiming at a 70mm(Φ) x 250mm(L) desired FOV. The amplification gains inside ASIC and bias voltage for SiPM were optimized for both PET and SPECT photons detection. Energy window and crystal lookup table was calibrated at each energy level. A list-mode OS-EM reconstruction algorithm and a multi-pinhole SPECT OS-EM reconstruction algorithm were developed for PET and SPECT image reconstruction respectively. For performance evaluation, positioning, energy and initial timing accuracy measurement were conducted. Initial PET and SPECT imaging phantom experiments were performed.

Results: Both 15x15 and 16x16 crystals in the two detector layers were clearly separable in the crystal identification map. The average energy resolution was ~13% at 511 keV and ~25% at 140 keV. Preliminary coincidence timing test showed the coincidence resolving timing (CRT) was 288 ps with a pair of crystals. In phantom studies, 1.2 mm hot rods were separable in PET image. In SPECT image with a 5-rod phantom and the mouse imaging pinhole sets, all the rods from 1 to 5 mm diameter were clearly visible, while the 1 mm rod intensity recovery was limited by resolution.

Conclusion: The developed SiPM based detector is feasible for both PET and SPECT photon detection. A hybrid PET/SPECT imaging system has been successfully constructed. Initial phantom studies show satisfactory imaging performance for both PET and SPECT modes. Imaging System optimization and full performance tests are under way. Research Support: The research was supported by the National Natural Science Foundation of China (No. 11375096, No.11575096, No. 11605008) and National Key Research and Development (R&D) Plan of China (Grant ID. 2016YFC0105405). $$graphic_285C92EF-C7CB-4F3E-9B9F-659DC03E2D20$$