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 the authors´ institution. The effectiveness of treatment can be evaluated through accurate time-dependent three-dimensional (3-D) dosimetry calculations based on SPECT images. The authors compared the absolute quantitative SPECT performance of a high-resolution rotating parallel-hole collimator (RPHC) designed to minimize septal penetration with that of a traditional medium-energy parallel-hole collimator (MEPHC). The authors scanned a phantom consisting of four spheres having inner diameters of 2.2 and 2.8 cm and whose activity concentration ratios with respect to the surrounding medium were between 5:1 and 17:1. Images were reconstructed using the OSEM algorithm (10 subsets, 20 iterations) with spatial resolution modeling. Quantitation of the MEPHC SPECT images yielded a large over-estimation of the surrounding activity concentration (50%) due to septal penetration. Quantitation of the RPHC SPECT image yielded a surrounding medium activity concentration within 8% of the expected value, and the activity within the spheres was estimated to within 20% of their absolute expected values for the RPHC. The authors also investigated the effect of out of field-of-view (FOV) uncollimated gamma rays on SPECT quantitation. It is concluded 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 clinical SPECT quantitation of I-131 RIT