Exploring Anti-Coincidence Technique for Scattering Contamination Rejection in a Simultaneous PET/SPECT Imaging System.

Simultaneous PET/SPECT imaging instruments could enable comprehensive molecular characterization by combining the very high sensitivity and spatial resolution of PET with the ability to image a broad range of radionuclides, including therapeutic isotopes with SPECT. While PET and SPECT hardware can be integrated into a single hardware system, one of the major challenges in simultaneous PET/SPECT imaging is the down-scatter contamination from PET tracers, which can significantly degrade SPECT image quality and quantitative accuracy. In this study, we evaluated the efficacy of using anti-coincidence and active shielding techniques to reject the down-scatter contamination in SPECT data. Using GEANT4 simulations, we analyzed the performance tradeoffs resulting from different choices of active shielding component design, coincidence time windows, and SPECT detector materials (CZT versus GAGG). The anti-coincidence technique effectively rejected contamination events, achieving a signal-to-noise (SNR) enhancement from 0.29 to 1.48 and noise equivalent count rate (NECR) improvement from 41.3 to 89.1 cps with moderate active shielding. Even without active shielding, anti-coincidence alone significantly improved SPECT image quality (SNR: 1.38; NECR: 87.0 cps), making it practical for systems where shielding is constrained. In reconstructed images, anti-coincidence improved contrast from 0.78 (raw) to 0.96 and reduced noise from 1508 to 257 counts, outperforming traditional triple-energy-window (TEW) correction (contrast: 0.84; noise: 855). The anti-coincidence technique is also compatible with post-processing scatter corrections, offering a promising strategy for future hybrid PET/SPECT systems in both preclinical and clinical settings.