Integrated imaging now: PET, SPECT, and CT imaging in small animals

Abstract

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Objectives: Ideally, one would like to be able to take PET, SPECT, and CT images in an animal without changing the animal’s position, so that co-registration of images from the different modalities will be highly precise. Here we show that this is possible using the tri-modality FLEX system from Gamma Medica-Ideas, which combines SPECT, PET, and CT imaging systems in a single gantry. The principal challenge in conducting these experiments is the possibility of cross-talk between PET and SPECT. In particular, Compton-scattered gamma rays from F-18 have an energy peak at 170 keV, which is close to the imaging energy window for common SPECT isotopes such as Tc-99m, and can result in noise from F-18 being detected by SPECT cameras. Here we demonstrate that this issue has been overcome with the FLEX system.

Methods: We measured the down-scattered count rate from an F-18 point source into SPECT imaging windows set for Tc-99m. This allowed us to estimate the maximum amount of F-18 that could be injected in an animal without significantly interfering with a SPECT acquisition. We then measured the pile-up count rate into the PET system as a function of the amount of Tc-99m in the PET ring. This allowed us to estimate the maximum amount of Tc-99m that could be injected without interfering with a PET acquisition. Finally, we used these same amounts of activity to acquire SPECT and PET images of a mouse injected with F-18 FDG and Tc-99m MDP.

Results: With a 50 uCi point source of F-18, the down-scattered count rate detected by SPECT cameras was 0.4 kcps. With a 2 mCi point source of Tc-99m, the pile-up count rate detected by the PET ring was 3.2 kcps. PET and SPECT images of a mouse injected with 2 mCi of Tc-99m MDP and 50 uCi of F-18 FDG demonstrated that quality images could be acquired with these amounts of activity.

Conclusions: The high sensitivity of the FLEX PET system allows researchers to use a very small dose of F-18, thus minimizing the down-scattered count rate from F-18 into SPECT imaging energy windows. This, combined with the integration of SPECT, PET, and CT into a single gantry, makes it possible to produce precisely co-registered SPECT, PET, and CT images in small animals.

Cross-talk between PET and SPECT in the FLEX imaging system.