The investigators propose using DaTscan in patients with REM sleep behavior disorder (RBD), mild cognitive impairment (MCI), Parkinson's disease (PD), dementia with Lewy bodies (DLB), Alzheimer's disease (AD), and other neurodegenerative syndromes and disorders, to test several hypotheses - some confirmatory, and some novel. Such use will provide new data on the potential clinical and research utility of DaTscan in neurodegenerative diseases. The findings on DaTscan 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.