Quantitation of D2 receptor binding with dedicated small animal PET, small animal SPECT and HiSPECT - the impact of the reference region on the binding potential

In this study, we assess the impact of the reference tissue (frontal cortex, parietal cortex, cerebellum) on the magnitude of the binding potential obtained for the D2 receptor ligands [18F]N-methyl-benperidol and [123I]iodobenzamide in the rat striatum using dedicated small animal PET ("TierPET"), small animal SPECT ("TierSPECT") or a conventional dual-head SPECT camera upgraded with multipinhole-collimators ("HiSPECT"). With all 3 cameras employed in our investigation, striatum-to-cerebellum ratios were consistently higher compared to striatum-to-cortex ratios. Moreover, frontal-cortex-to-cerebellum ratios tended to exceeded parietal-cortex-tocerebellum. Findings are in line with previous in vitro results, and show that quantitation of both striatal and cortical D2 receptor binding is feasible using small animal imaging devices including the novel HiSPECT. With HiSPECT, both striatum-to-cortex and striatum-to-cerebellum ratios of IBZM were higher compared to TierSPECT, which underlines the greater suitability of the higher resolving camera. Findings, moreover, imply that the reference tissue should be carefully chosen with respect to the intended investigation.