High-resolution absolute SPECT quantitation for I-131 distributions used in the treatment of lymphoma: A phantom study

Clinical lymphoma I-131 radioimmunotherapy (RIT) studies are underway at ou r institution. The effectiveness of treatment can be evaluated through accu rate time-dependent three-dimensional (3-D) dosimetry calculations based on SPECT images. We compared the absolute quantitative SPECT performance of a high-resolution rotating parallel-hole collimator (RPHC) designed to minim ize septal penetration with that of a traditional medium-energy parallel-ho le collimator (MEPHC). We scanned a phantom consisting of four spheres havi ng inner diameters of 2.2 and 2.8 cm and whose activity concentration ratio s with respect to the surrounding medium were between 5:1 and 17:1. Images were reconstructed using the OSEM algorithm (10 subsets, 20 iterations) wit h spatial resolution modeling. Quantitation of the MEPHC SPECT images yield ed a large overestimation of the surrounding activity concentration (50%) d ue to septal penetration. Quantitation of the RPHC SPECT image yielded a su rrounding medium activity concentration within 8% of the expected value, an d the activity within the spheres was estimated to within 20% of their abso lute expected values for the RPHC. We also investigated the effect of out o f field-of-view (FOV) uncollimated gamma rays on SPECT quantitation. We con clude that for the present quantitation application, the benefits of septal penetration suppression by the RPHC outweigh the increased image noise due to low count rate. These results point to the feasibility of using a large -FOV collimator based on the imaging principles of the RPHC for accurate cl inical SPECT quantitation of I-131 RIT.