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 LYSO 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.