Multimodal Imaging of Methylphenidate Effects

The participants underwent simultaneous PET/MRI imaging in a 3 T Biograph mMR scanner (Siemens; Medical Solutions, Erlangen, Germany). All studies were initiated at noon to minimize confounds from circadian variability. Venous catheters were placed in the left dorsal hand vein for radiotracer injection, and in the right dorsal hand vein for intravenous injection of medications. Heart rate (HR), systolic and diastolic blood pressures (BPs) were continuously monitored throughout the study with an Expression MR400 patient monitor (Philips, Netherlands). Thirty minutes before tracer injection, either 60 mg of MP or placebo was administered p.o. The participant was then positioned in the scanner. Earplugs were used to minimize scanner noise and padding was used to minimize head motion. A T1 weighted dual-echo image was collected for attenuation correction using an ultrashort-TE (UTE) sequence (192^{3 (link)} matrix, 1.56 mm isotropic resolution, TR = 11.94 ms, TE = 0.07 and 2.46 ms), and T1-weighted 3D magnetization-prepared gradient-echo (MPRAGE; TR/TI/TE = 2200/1000/4.25 ms; FA = 9°, 1 mm isotropic resolution) was used to map brain structure. List mode PET emission data were acquired continuously for 90 min and initiated immediately after a manual bolus injection of [¹¹C]raclopride (dose = 15.7 ± 1.9 mCi; duration 5–10 s). Thirty minutes after tracer injection, either 0.25 mg/kg MP or placebo were manually injected i.v. as a ~30-s bolus. The participants were instructed to remain as still as possible and to relax and keep their eyes open during scanning.