Dual-photopeak joint image reconstruction for pinhole SPECT, PET and PET-SPECT

Objective.Many SPECT and PET radionuclides, along with radionuclides used in targeted alpha or beta therapy and their imaging surrogates have multiple gamma and/or positron emissions. Images of these radionuclides are usually obtained from the photopeak with the most convenient energy and/or highest intensity or by adding counts from different photopeaks. Smart utilization of multiple energy peaks may improve reconstructed images, especially in low-count scans.Approach.We investigate and compare various dual-photopeak joint reconstruction (JR) approaches, namely (i) Single-Band (SB-JR)-projections from two energy windows are summed and reconstructed with a system matrix at a single average energy, (ii) mixed Multi-Band (mMB-JR)-like SB-JR but the system matrix incorporates the element-wise contributions from the photopeak energies, (iii) Multi-Band (MB-JR)-separate projections for each window and separate system matrices at relevant gamma energies are utilized. We evaluate these methods for a multi-pinhole PET-SPECT system (VECTor, MILabs, the Netherlands) using Monte Carlo generated Derenzo phantom projections of²²⁵Ac (218 keV and 440 keV gammas),²²⁶Ac (158 keV and 230 keV gammas) and⁸⁹Zr (511 keV annihilation gammas and 909 keV prompt gammas) at three different activity concentrations. A contrast-to-noise ratio (CNR) based quantitative performance analysis was done.Main results.The MB-JR scheme of JR showed superior visual image quality and highest CNRs in almost all cases, across all radionuclides and activity concentrations. The CNR improvement over images acquired from the single best-performing photopeak ranged from 30%-65% for²²⁵Ac, 20%-54% for²²⁶Ac, and 25%-47% for⁸⁹Zr, respectively, for the smallest visible rods in the Derenzo phantom. CNR improvements/degradations for the other two methods, mMB-JR and SB-JR, were: for²²⁵Ac, -16%-51% and -21%-51%; for²²⁶Ac, 9%-61% and 0.2%-38%; and for⁸⁹Zr, 19%-52% and -3%-16%, respectively.Significance.We believe the proposed image reconstruction methods can enhance SPECT, PET, and PET-SPECT imaging of a wide range of radionuclides that emit gamma's with multiple energies.