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

<i>Objective.</i>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.<i>Approach.</i>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).<i>Main results.</i>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.<i>Significance.</i>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 for<i>in vivo</i>imaging of small animals such as mice and rats.