An MLEM Reconstruction Method with Mixed Events Based on a Cascade Gamma Emission Imager System

Many medical radionuclides can emit cascade photons sequentially, yet commercial imaging equipment like PET or SPECT don't make use of this information. We have developed a cascade gamma emission imager system to detect the cascade photons in coincidence for those radionuclide imaging. In previous study, reasonable reconstruction images could be obtained using coincidence event based MLEM (CE-MLEM) but with a low signal-to-noise ratio (SNR), caused by the poor sensitivity of the coincidence event. In this work, we proposed an MLEM with mixed events (ME-MLEM) reconstruction method based on the coincidence events incorporated with the simultaneously detected single photons to improve the SNR while maintaining the spatial resolution. A myocardial phantom filled with 1 mCi 177Lu3+ solution was scanned for 1 hour on our prototype setup. 1.36xl04 coincidence events and 8.27×107 single photons were acquired under a 25.6 ns CTW and 10.8%/14.6% @ 208/113 KeV energy window. CE-MLEM, single photon based MLEM (SP-MLEM), and ME-MLEM with a 0.001 weighting factor between coincidence events and single photons were then used for the image reconstruction. The results showed the potential for ME-MLEM to improve the image SNR while keeping the same spatial resolution as CE-MLEM. Our results demonstrated that our system's image quality could be improved when combined coincidence events and single photons together.