The FDA-approved indication is to assist in the evaluation of adult patients with suspected Parkinsonian syndromes (PS). In these patients, DaTscan may be used to help differentiate essential tremor from tremor due to Parkinsonian syndromes (such as idiopathic Parkinson's disease, multiple system atrophy and progressive supranuclear palsy). DaTscan will be used as an adjunct to other diagnostic evaluations. Identifying dopaminergic dysfunction is also important in other settings such as those with cognitive impairment with or without parkinsonism, and in subjects with REM sleep behavior disorder. The findings on DaTscan in subjects with these various disorders will be correlated with clinical diagnoses and other multimodal imaging studies (e.g., MRI, MRS, FDG-PET, and amyloid-PET) to enhance our understanding of neurodegenerative diseases. A subset of subjects will undergo a 2nd DaTscan at least 1 year after the initial scan was performed to determine if changes over time provide any prognostic information.