PET Imaging of Dopamine Synthesis Capacity

All participants were asked to fast (except water) 12 h before the scan. Imaging data were collected on a Siemens Biograph 6 HiRez PET scanner (Siemens, Erlangen, Germany) in 3D mode in line with our standard procedure (Egerton et al., 2010 (link); Howes et al., 2009 (link)). Subjects received 400 mg entacapone (a peripheral catechol-omethyl-transferase inhibitor) and 150 mg carbidopa (a peripheral aromatic acid decarboxylase inhibitor) orally 1 h before the scan to prevent the formation of radiolabelled metabolites that might cross the blood–brain barrier (Cumming et al., 1993 (link)). Participants were positioned with the orbitomeatal line parallel to the transaxial plane of the tomograph scanner. Head position was marked, monitored throughout the scan and movement minimized using a head strap. After acquiring a CT scan for attenuation correction, approximately 150 MBq of [¹⁸F]-FDOPA was administered by bolus intravenous injection. PET data were acquired in 32 frames of increasing duration over the 95 min scan (frame intervals: 8 × 15 s, 3 × 60 s, 5 × 120 s, 16 × 300 s).