Dosimetry of PET tracers in mice using microPET scans as an input function

Despite the growing use of microPET for preclinical imaging, little work has been done to evaluate the radiation dose received by mice from PET tracers. If the dose is too high, it can have physiological effects which can affect the results in longitudinal scanning of animals, particularly in tumour growth models. To calculate the dose received by a mouse during routine microPET scanning, four mice were imaged for 1 hour immediately following the injection of ¹¹C-raclopride. The data was summed into one static frame and used as the tracer distribution input function for an in-house computer package, VMDOSE, based on the MIRD formalism with S factors calculated at the voxel level. The dose calculation requires about 2.5 s, allowing rapid subject specific dosimetry. The output of VMDOSE is an image volume where the voxel values represent absorbed dose. Regions of interest (ROIs) were drawn in the PET image for various organs as well as the whole body. These ROIs were applied to the dose distribution data in order to calculate organ-specific doses. The doses ranged from an average of 3.8 mGy/MBq for the brain to 82.8 mGy/MBq gall bladder. The whole body dose average was 5.8 mGy/MBq. For a ¹¹C-raclopride injection of 7.4 MBq (200 μCi), this will result in doses of 42.9, 28.1 and 613 mGy to the whole body, brain and gall bladder, respectively.