PET
TRANSMISSION, SEPARATION, RESOLUTION
PET scanning was performed using a High Resolution Research Tomograph (HRRT; Siemens Healthcare) a dedicated brain PET scanner with a resolution of 2.5–3.0 mm and a 25 cm axial field of view. Transmission scanning was performed with a 137Cs rotating pin source to correct for attenuation. Two hours after a standard breakfast, a bolus of approximately 5 mCi of [18F]fallypride was infused i.v. using a Harvard pump. The specific activity was approximately 2,000 mCi/μmol at time of injection, and the radiochemical purity of the radiotracer was > 99%. PET emission data were collected starting at radiotracer injection over 3.5 hours, in 3 blocks separated by 2 10-minute breaks. Thirty-three volumes were acquired at times 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 90, 110, 130, 170, and 200 minutes. During each scan block, the room was quiet and dimly lit, and each subject was instructed to keep their head as still as possible, relax, and try to avoid falling asleep. The image reconstruction process corrected for head motion, which was tracked throughout each scan. Each scan consisted of 207 slices (slice separation = 1.22 mm). The fields of view were 31.2 cm and 25.2 cm for transverse and axial slices, respectively.The PET images were aligned within each scan block with 6-parameter rigid registration using seventh-order polynomial interpolation, and each block was aligned to the volume taken at 20 minutes of the first block. The final alignments were visually checked, with translations varying by < 5 mm and the rotations by < 5 degrees.