Feasibility studies of simultaneous PET and SPECT dual-tracer imaging with a stationary multi-pinhole collimator inserted to animal PET detector

In this study, the feasibility for simultaneous PET and SPECT dual-tracer small animal imaging is investigated. A custom-made PET/SPECT imaging system was modeled in this simulation study. In this system, PET imaging was performed with a stationary L YSO detector ring, and SPECT imaging of Tc-99m was conducted with the same detectors and a ring-shape multi-pinhole collimator inserted into the detector. The PET projection data were acquired in 350-650 keV energy window and corrected for the collimator attenuation effect. The SPECT projection data were acquired in a 135-145 keV energy window by axial translation of object only to obtain sufficient angular sampling with the special design of our cylindrical multi-pinhole collimator. As we expect, the down-scatter effect from 511 keV photons of PET tracers is the main limiting factor on the quality of SPECT images in the dual-tracer system, we investigated two methods to compensate for this down-scatter effect. One is an auxiliary energy window (AEW) down-scatter correction method and the other is a triple energy window (TEW) down-scatter correction approach. The simulation results show that the artifacts caused by down-scatter events can be significantly reduced by applying both compensation methods. Compared to the SPECT image reconstructed from simulation data without down-scatter contamination, the reconstruction results after TEW correction can achieve comparable image quality. The TEW method demonstrated better performance of noise versus bias trade-off than the AEW method. Monte Carlo simulation results show that when the activity ratio is 1:10 and the acquisition time ratio is 1:30 for PET and SPECT imaging, simultaneous PET-SPECT dual-tracer imaging is achievable by using our stationary system and optimized scanning protocol.