Radionuclide Dependence of Distance-Dependent Spatial Resolution and Resolution Recovery in SPECT: A SIMIND Monte Carlo Simulation Study

The distance dependence of spatial resolution is an important characteristic of SPECT imaging and a key factor in resolution-recovery reconstruction. This study evaluated radionuclide differences in distance-dependent spatial-resolution degradation and examined whether similarity in fitted degradation functions was reflected in comparable resolution-recovery performance between Tc-99m and I-123. Monte Carlo simulations were performed using SIMIND version 8.0 with four parallel-hole collimators. Planar line-source simulations were performed at source-to-collimator distances of 50–400 mm using a 45° acceptance angle to include photons contributing to septal penetration and collimator scatter. Spatial resolution was evaluated using the full width at half maximum (FWHM) of the line spread function, and radionuclide- and collimator-specific linear degradation functions were derived. Septal penetration and collimator-scatter fractions were also evaluated. The degradation functions were incorporated into resolution-recovery reconstruction, and Tc-99m- and I-123-derived functions were cross-applied to evaluate their resolution-recovery performance. For both radionuclides, FWHM increased approximately linearly with source-to-collimator distance for all collimators (R² = 0.96–0.99). The fitted slopes showed broadly similar trends between Tc-99m and I-123 but varied according to collimator type. Septal penetration and collimator scatter were greater for I-123 than for Tc-99m and showed radionuclide- and collimator-dependent distance dependence. Resolution recovery using radionuclide-specific degradation functions improved FWHM by approximately 50% under most conditions. Cross-application generally produced FWHM and FWTM values close to those obtained using radionuclide-specific functions; however, condition-dependent differences of up to approximately 5–6% remained. These findings demonstrate that Tc-99m and I-123 exhibit broadly similar linear distance-dependent spatial-resolution characteristics, but similarity in fitted degradation functions does not necessarily establish equivalent resolution-recovery performance. Direct evaluation of cross-application performance is therefore important when considering the use of degradation functions across radionuclides.