Validation of the octamouse for simultaneous 18F-fallypride small-animal PET recordings from 8 mice.

UNLABELLED: Data collection in preclinical small-animal PET studies has been hindered by the small number of recordings typically obtained for a single radiosynthesis. Therefore, we tested procedures for obtaining 8 simultaneous small-animal PET recordings from the brains of 8 mice using an acrylic anesthesia distributor (the Octamouse), with the dopamine D(2/3) ligand (18)F-fallypride serving as a test substance for brain receptor imaging.
METHODS: The effect of scatter correction on the small-animal PET recordings was first evaluated in phantom studies in which sources of different radioactivity concentration were placed within the chambers of the Octamouse. Next, potential effects of mass on the (18)F-fallypride binding potential (BP(ND)) in the striatum were tested in groups of mice receiving (18)F-fallypride at 2 different specific activities (140 and 50 GBq/μmol), with and without scatter correction. Finally, the relationship between BP(ND) and injected dose of (18)F-fallypride (3.5-17 MBq/mouse) was tested.
RESULTS: Scatter correction improved the contrast between sources and air space within the Octamouse phantom. The magnitude of (18)F-fallypride BP(ND) in mouse striatum was invariant across the tested range of specific activities, and scatter correction increased BP(ND) by a mean of 6%; covariances of the inter- and intraoperator variability of BP(ND) were 10%. There was a positive correlation between radiochemical dose and BP(ND) with (R(2) = 0.53) and without (R(2) = 0.63) scatter correction, which was driven by increasing area under the percentage injected dose curve in the striatum.
CONCLUSION: The quantitation of emission sources placed within the Octamouse is linear over a wide range of source activities. In the striatum of living mice, the magnitude of (18)F-fallypride BP(ND) was highly reproducible between operators and was constant over a 3-fold range of specific activities, indicating a lack of significant occupancy. Scatter correction improved quantitation but did not entirely correct for the dependence of BP(ND) on injected dose, which was deemed to arise because of effects propagating from detector dead time when the total radiochemical dose in the field of view exceeded 50 MBq. Given this consideration, we were still able to quantify (18)F-fallypride BP(ND) in 16 mice from a single radiosynthesis, an economy that should be generalizable to brain studies of diverse radioligands.