Improved Quantification of (18)F-FDG PET during (131)I-Rituximab Therapy on Mouse Lymphoma Models after (131)I Prompt Emission Correction

(18)F-FDG Positron Emission Tomography (PET) is used to monitor tumor response to (131)I-therapy, but is confounded by prompt emissions (284, 364, 637, and 723 keV) from (131)I, particularly in animal PET imaging. We propose a method for correcting this emission in (18)F-FDG PET. The (131)I prompt emission effect was assessed within various energy windows and various activities. We applied a single gamma correction method to a phantom and in vivo mouse model. The (131)I prompt emission fraction was 12% when 300 µCi of (131)I and 100 µCi of FDG were administered, and increased exponentially with escalating (131)I activity for all energy windows. The difference in spill-over ratio was reduced to <5% after (131)I prompt emission correction. In the mouse model, the standard uptake value (SUV) did not differ significantly between FDG PET only (gold standard) and FDG PET after (131)I prompt emission-correction, whereas it was overestimated by 38% before correction. Contrast was improved by 18% after (131)I prompt emission correction. We first found that count contamination on (18)F-FDG follow-up scans due to (131)I spilled-over count after (131)I rituximab tumor targeted therapy. Our developed (131)I prompt emission-correction method increased accuracy during measurement of standard uptake values on (18)F-FDG PET.