The project will apply positron emission tomography (PET) of acetylcholinesterase to study non-dopaminergic (i.e. cholinergic) brain changes over time in subjects with Parkinson disease. Acetylcholinesterase PET imaging was used to assess cholinergic changes over time in this study. Acetylcholinesterase PET imaging is a diagnostic test as the investigator does not assign specific interventions to the subjects of the study based on the acetylcholinesterase PET. Therefore, this is an observational study as defined as following: studies in human beings in which biomedical and/or health outcomes are assessed in pre-defined groups of individuals. Subjects in the study may receive diagnostic, therapeutic, or other interventions, but the investigator does not assign specific interventions to the subjects of the study.