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

<i>Objective.</i>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.<i>Approach</i>. 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.<i>Main results.</i>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.<i>Significance.</i>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.