1. ¹Hammersmith Imanet, Hammersmith Hospital, London, UK
2. ²MRC Clinical Sciences Centre, Hammersmith Hospital, London, UK

Correspondence: Dr M-C Asselin, Wolfson Molecular Imaging Centre (University of Manchester), 27 Palatine Road, Manchester, M20 3LJ UK. E-mail: marie-claude.asselin@manchester.ac.uk

Received 19 August 2005; Revised 10 March 2006; Accepted 16 April 2006; Published online 24 May 2006.

Abstract

The very high-affinity position emission tomography (PET) radioligand [¹¹C]FLB 457 was developed in order to study extrastriatal tissues, where the density of dopamine D₂/D₃ receptors is one to two orders of magnitude lower than in the striatum. The present study investigated the validity of using the cerebellum as a reference region. Ten healthy volunteers underwent a 90-min dynamic PET study after the bolus injection of [¹¹C]FLB 457. The total volume of distribution (VD_t) was estimated for the thalamus, hippocampus, frontal cortex, and cerebellum using a two-tissue compartmental model with a metabolite-corrected arterial plasma input function. VD_t was sensitive to co-injected stable FLB 457 in all regions, including the cerebellum. Ex vivo saturation studies were also conducted in 17 rats where the dose of stable ligand was varied over five orders of magnitude. Specific binding was estimated to account for more than half of the rat cerebellar uptake of [¹¹C]FLB 457, questioning the latter as an estimate of nonspecific binding in human PET studies. To check whether the cerebellum is a reference region, the binding potential (BP) was calculated either from the VD_t ratio or using the simplified reference tissue model (SRTM). A non-negligible density of D₂/D₃ receptors in the cerebellum was shown to lead to underestimation of BP as well as erroneous estimation of differential occupancies. Binging potential estimates from the SRTM were found to be sensitive to changes in cerebral blood flow, providing further evidence for caution in the use of the cerebellum as a reference region in measures of [¹¹C]FLB 457 binding.