Inclusion Criteria:  Age 50-80 Diagnosis of Parkinson's disease Ability to provide informed consent Ability to speak English Normal cognition or mild cognitive impairment Willingness to go off parkinsonian medication for 12 hours prior to two of the study visits  Exclusion Criteria:  Contraindication for MRI Abnormal clinical brain MRI, specifically with evidence of large-vessel stroke or mass lesion History of stereotactic or ablative brain surgery Pregnancy Recent participation in other research studies involving radiation such that the annual research radiation dose would exceed FDA Limit if participating in this study Prior brain injury (eg., TBI) Baseline cognitive impairment due to genetic or developmental disorder Active illicit drug use or alcohol abuse Incapable of staying still for a 2-hour PET or MRI study Use of CNS-penetrating medications affecting the cholinergic system, including cholinesterase inhibitors and anticholinergics, up to 60 days prior to study participation