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

Abstract

18F-FDG Positron Emission Tomography (PET) is used to monitor tumor response to 131I-therapy, but is confounded by prompt emissions (284, 364, 637, and 723 keV) from 131I, particularly in animal PET imaging. We propose a method for correcting this emission in 18F-FDG PET. The 131I 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 131I prompt emission fraction was 12% when 300 µCi of 131I and 100 µCi of FDG were administered, and increased exponentially with escalating 131I activity for all energy windows. The difference in spill-over ratio was reduced to <5% after 131I 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 131I prompt emission-correction, whereas it was overestimated by 38% before correction. Contrast was improved by 18% after 131I prompt emission correction. We first found that count contamination on 18F-FDG follow-up scans due to 131I spilled-over count after 131I rituximab tumor targeted therapy. Our developed 131I prompt emission-correction method increased accuracy during measurement of standard uptake values on 18F-FDG PET.