Super-Cluster collimation for ultra-sensitive SPECT-PET: a simulation study

Objective.Clustered pinhole (CP) collimation currently supports sub-millimeter resolution imaging up to ∼1 MeV, enabling SPECT of alpha and beta emitters with gamma emissions, simultaneous multi-isotope PET and PET/SPECT, and positron range-free PET. Nonetheless, increasing sensitivity in the original CP designs by enlarging pinhole diameters is limited, as the resulting pinhole opening cones would overlap.Approach. To address this limitation, the use of Super-Cluster (SC) collimation was evaluated in a simulation study. Two SC designs were assessed: a standard configuration (SC-ST) offering a resolution-sensitivity trade-off similar to CP, and a high-sensitivity variant (SC-HS) with larger pinhole diameters to enhance sensitivity. Their performance was compared to CP collimation for¹⁸F at concentrations of 1.0, 0.1, 0.05 MBq ml^-1and ⁸⁹Zr at 2.0, 0.2, 0.1 MBq ml^-1, evaluating sensitivity, image resolution, recovery coefficients, and uniformity.Main results.CP and SC-ST showed comparable sensitivity and image resolution. Both resolved¹⁸F rods of 0.9, 1.4, and 1.8 mm at 1.0, 0.1, and 0.05 MBq ml^-1, respectively. For ⁸⁹Zr, rods down to 1.0 mm and 1.6 mm were resolved at 2.0 and 0.2 MBq ml^-1, but none at 0.1 MBq ml^-1. Compared to CP and SC-ST, SC-HS increased sensitivity threefold for¹⁸F and twofold for ⁸⁹Zr. At the highest activity, SC-HS showed slightly reduced resolution for¹⁸F (1.0 mm) and similar for ⁸⁹Zr (1.0 mm). However, it clearly outperformed both other collimators at lower activities, resolving¹⁸F rods of 1.2 and 1.4 mm at 0.1 and 0.05 MBq ml^-1, respectively, and ⁸⁹Zr rods of 1.4 and 1.6 mm at 0.2 and 0.1 MBq ml^-1. Additionally, SC-HS showed superior contrast recovery. Image uniformity remained consistent across all collimators, confirming effective angular sampling.Significance.The new SC geometry enables high-sensitivity collimation for high gamma energies, improving image quality at low activities. These results demonstrate SC collimation's strong potential for sensitivity-critical applications.