Dynamic PET Image Processing
VOIs were manually drawn within descending aorta, duodenum, pancreas, and stomach using PMOD 4.3 software (PMOD Technologies Ltd.), where the descending aorta was selected to reflect the blood radiotracer concentration. To extract the TACs from the brain regions, all PET images were first processed by head motion correction and spatial normalization, both using Statistical Parametric Mapping software (SPM12, Wellcome Department of Imaging Neuroscience) in the MATLAB (2019b, The MathWorks Inc.) environment, followed by the co-registration of the PET images to the standard Montreal Neurologic Institute (MNI) space based on an MRI template25. Caudate, putamen, pallidum, and substantial nigra, which are the major brain regions associated with PD, were selected for the analysis. TACs of all organs were then extracted for kinetic analysis. The mean standardized uptake value (SUVmean) curves, which can be calculated as the average radioactivity within each VOI at each frame normalized by the ratio between the decay-corrected injected radiolabeled dose and body weight of the participant, were generated based on the TACs. In the SUVmean curve analysis of the brain nuclei, the separation time was calculated as the time when the difference in group means between the SUVmean of HC and PD reached to at least 10% of the mean SUVmean of HC group.