Performance evaluation of S-PET, a compact, LYSO/BGO phoswich detector small animal PET prototype

Objective.This work aims to evaluate the performance characteristics of S-positron emission tomography (PET), a compact, phoswich detector small animal PET prototype developed at Shenzhen Bay Laboratory (SZBL), in accordance with the National Electrical Manufacturers Association (NEMA) NU4-2008 protocol.Approach.The S-PET system comprises twenty flat-panel detectors arranged in two rings, with an inner diameter of 80 mm and an axial field of view (FOV) of 104.5 mm. Each detector adopts a phoswich depth of interaction design, consisting of two layers of scintillator crystal arrays, an oblique edge-cut glass lightguide, and two multi-pixel photon counter arrays. The front layer (annihilation photon entrance) is a 30 × 60 pixelated cerium-doped lutetium yttrium orthosilicate (LYSO) array (0.79 × 0.79 × 5 mm³per crystal), while the back layer (towards multi-pixel photon counters) is a 20 × 40 pixelated bismuth germanate (BGO) array (1.22 × 1.22 × 7.5 mm³per crystal). This phoswich design enables identification of most cross-layer crystal scatter (CLCS) events. Performance evaluation was conducted following NEMA NU4-2008 standards, including sensitivity, energy resolution, spatial resolution, noise equivalent count rate (NECR), scatter fraction (SF), image uniformity, spillover ratio, and recovery coefficient (RC).Main results.Within the 350-650 keV energy window, the peak absolute sensitivity at the FOV center was 14.9% (with CLCS events) and 11.9% (without CLCS events). The average system energy resolution was 10.7% ± 2.0% for LYSO and 28.1% ± 4.5% for BGO, derived from averaging individual crystal spectra. For three-dimensional ordered-subsets expectation maximization reconstructed images with spatially variant point spread function modeling, the spatial resolution of a point source in air ranged from 0.65 mm to 1.08 mm, with an average of 0.78 ± 0.12 mm across all measured locations. For the mouse-sized phantom, the peak NECR was 237.4 kcps at 11.9 MBq, with a SF of 13.0%; for the rat-sized phantom, these values were 110.3 kcps at 11.9 MBq and 23.3%, respectively. For the NEMA image quality phantom, the uniformity was 6.3%, spillover ratios in water-filled and air-filled cold region chambers were 11.1% and 7.9%, respectively, and RCs ranged from 0.36 to 0.89.Significance.The comprehensive performance results demonstrate that the S-PET prototype developed at SZBL possesses high spatial resolution and high sensitivity. These characteristics meet the requirements of preclinical research, particularly forin vivoimaging of small animals such as mice and rats.