Positron Emission Tomography

All the patients except one who took a long-acting injectable risperidone were taking their antipsychotic drugs at night once a day. The patients were instructed to take their antipsychotic medication at 21:00 h a day before the scan. The date of the positron emission tomography (PET) scan was randomly assigned to participants to avoid a possible bias by consecutive scans from the same group. The PET scan was performed at 14:00 h and participants were required to fast and abstain from smoking and drinking from midnight on the day of the scan and received 150 mg carbidopa and 400 mg entacapone orally 1 h before scanning to reduce the formation of radiolabeled metabolites (Turkheimer et al, 1999).

Participants underwent a short computed tomography (CT) for attenuation correction and PET imaging on a Biograph 40 Truepoint PET/CT scanner (Siemens, Knoxville, TN, USA) for 95 min after an intravenous bolus injection of ∼370 MBq (10 mCi) of [¹⁸F]DOPA with minimum specific activity of 1.30 × 10⁷ Ci/mol. Head movement was monitored with a mark and minimized using a light head strap. After routine corrections for uniformity, decay corrections, and attenuation (using the CT), the PET imaging data acquired in a list mode were reconstructed with a filtered back-projection using a Gaussian filter. Images were collected in a three-dimensional mode with 148 axial slices, an image size of 256 × 256, a pixel size of 1.3364 × 1.3364 mm², and a slice thickness of 3 mm. The dynamic volumetric images were sequenced using the following framing: 2 × 30, 4 × 60, 3 × 120, 3 × 180, and 15 × 300 s.

For the analysis of volume effects in the striatum, high-resolution T1-weighted magnetic resonance images (MRIs) were also acquired after the PET scan (TE=1.89 ms, TR=1670 ms, flip angle=9°, 208 slices, matrix=256 × 256, FOV=250 mm).