Vestibulopathy as a Cause of Imbalance in Parkinson

Inclusion Criteria: * PD based on the United Kingdom Parkinson's Disease Society Brain Bank Diagnostic Research Criteria (n=64, gross recruitment) * M/F * age 45 years or older * duration of disease \> 5 years and/or Hoehn \& Yahr stages 1.5-4 able to ambulate independently and no evidence of dementia. Exclusion Criteria: * History of Meniere disease or recent onset of acute vestibular dysfunction, such as otolith disorders (BBPV etc). * Other disorders which may resemble PD, such as progressive supranuclear palsy (PSP), vascular dementia, normal pressure hydrocephalus, multiple system atrophy (MSA), corticobasal ganglionic degeneration, or toxic causes of parkinsonism. Prototypical cases have distinctive clinical profiles, like early and severe dysautonomia (MSA) or appendicular apraxia, which may differentiate them from idiopathic PD and PSP. The use of the UKPDSBRC clinical diagnostic criteria for PD will mitigate the inclusion of subjects with atypical parkinsonism. * Evidence of a stroke or mass lesion on structural brain imaging (MRI). * Participants in whom MRI is contraindicated including, but not limited to, those with a pacemaker, presence of metallic fragments near the eyes or spinal cord, or cochlear implant. * Severe claustrophobia precluding MR or PET imaging. * Subjects limited by participation in research procedures involving ionizing radiation. * Pregnancy (test within 48 hours of each PET session) or breastfeeding.