Animal-specific positioning molds for registration of repeat imaging studies: comparative microPET imaging of F18-labeled fluoro-deoxyglucose and fluoro-misonidazole in rodent tumors

IntroductionComparative imaging of multiple radiotracers in the same animal can be invaluable in elucidating and validating their respective mechanisms of localization. Comparative imaging of PET tracers, particularly in small animals, is problematic, however: such tracers must be administered and imaged separately because simultaneously imaged positron emitters cannot be separated based on energy discrimination.ObjectiveAs part of our ongoing development of hypoxia imaging radiotracers, the intratumoral distributions of sequentially administered F18-fluoro-deoxyglucose (FDG) and the hypoxia tracer F18-fluoromisonidazole (FMiso) were compared in rats by registered microPET imaging with positioning of each animal in a custom-fabricated whole-body mold.MethodsNude rats with a hindlimb R3327-AT anaplastic rat prostate tumor xenograft and a hindlimb FaDu human squamous cell carcinoma (each up to 20×20×30 mm in size) were studied. Rapid-Foam (Soule Medical, Lutz, FL) was used to fabricate animal-specific molds for immobilization and reproducible positioning. Each rat was injected via the tail vein with ~33 MBq (900 μCi) of FDG and imaged in its mold at 1 h postinjection (pi) on the microPET. The next day, each rat was injected with ~22 MBq (600 μCi) of FMiso and positioned and imaged in its mold at ~2 h pi. Custom-manufactured germanium-68 rods (10 μCi each, 1×10 mm) were reproducibly positioned in the mold as fiduciary markers.ResultsThe registered microPET images unambiguously demonstrated grossly similar though not identical distributions of FDG and FMiso in the tumors — a high-activity rim surrounding a lower-activity core. There were subtle but possibly significant differences in the intratumoral distributions of FDG and FMiso, however. These may not have been discerned without careful image registration.ConclusionAnimal-specific molds are inexpensive and straightforward to fabricate and use for registration (±1 to 2 mm) of sequential PET images and may aid image interpretation.