MIRD Pamphlet No. 32: A MIRD Recovery Coefficient Model for Resolution Characterization and Shape-Specific Partial-Volume Correction.

Accurate quantification in emission tomography is essential for internal radiopharmaceutical therapy dosimetry. Mean activity concentration measurements in objects with diameters less than 10 times the full width at half maximum of the imaging system's spatial resolution are significantly affected (>10%) by the partial-volume effect. This study develops a framework for PET and SPECT spatial resolution characterization and proposes 2 MIRD recovery coefficient models-a geometric mean approximation (RECOVER-GM) and an empirical model (RECOVER-EM)-that provide shape-specific partial-volume correction (PVC). The models were validated using simulations and phantom experiments, with a comparative PVC test on ellipsoidal phantoms demonstrating that the RECOVER models significantly reduced error in activity quantification by factors of approximately 1.3-5.7 compared with conventional sphere-based corrections. The proposed recovery coefficient models and PVC methodology provide a robust framework for improved region-based PVC, including corrections for nonspherical tumor volumes. This work is part of the ongoing MIRDsoft.org project that aims to enhance accessibility to advanced dosimetry tools for improved disease characterization, treatment planning, and radiopharmaceutical therapy dosimetry.