Approximately 6 subjects with Parkinson disease, 6 subjects with Alzheimer disease and 20 healthy control subjects will be recruited to participate in this study. All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a baseline physical and neurological evaluation and baseline cognitive evaluations. Subjects will be asked to undergo a bolus injection of [18F] CFPyPB followed by serial PET imaging scans and plasma sampling for measurement of [18F] CFPyPB in plasma (both protein bound and free) over a period of up to 4 hours. The imaging analyses will be performed by an image-processing specialist who will remain masked to the procedures employed with each imaging acquisition. The primary imaging outcome measure will be the brain regional distribution volumes expressed as a brain tissue to plasma ratio of the radioligand, [18F] CFPyPB. Time to the peak uptake and amplitude of the peak uptake will be evaluated for all brain regions and the results for the PD and AD subjects will be compared with the HC subjects.