Evaluation of [18F]MNI-777 PET as a Marker of Tau Pathology in Subjects With Tauopathies Compared to Healthy Subjects

Inclusion Criteria: For all subjects: * Written informed consent or assent is obtained. * Willing and able to cooperate with study procedures. * For females, non-child bearing potential or negative urine pregnancy test on day of \[18F\]MNI-777 injection. Alzheimer Disease subjects: * The participant is 50 years or older. * Participants have a clinical diagnosis of Alzheimer's disease based on National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) criteria (McKann, 1984) * Modified Hachinski Ischemia Scale score of ≤ 4. Parkinson's Disease subjects: * The participant is 30 years or older. * Participants have a clinical diagnosis of PD based on the UK Brain Bank Criteria (Hughes, et al., 1982). * The duration of diagnosis of PD is \<20 years prior to the imaging visit * PD subjects must be on stable doses of medications for a period of at least 30 days prior to the imaging visit. * Treatment with dopamine replacement therapies or other symptomatic therapies for PD is permitted; however, subjects must be on a stable dose of medications 30 days prior to the imaging visit. Progressive Supranuclear Palsy subjects: * The participant is 30 years or older. * Participants have a clinical diagnosis of PSP based on National Institute of Neurological Disorders and Stroke/ (NINDS) and the Society for PSP (SPSP) criteria (Litvan, et al. 1996). Chronic Traumatic Encephalopathy subjects: * The participant is 18 years or older. * Subjects with a diagnosis of probable CTE based on a prior history of repetitive brain trauma and at least one concussion, and a current mood disorder (depression, apathy, irritability, suicidal ideation), cognitive symptoms (memory loss, impaired executive function) or behavioral symptoms (disinhibition, aggression and increased violence) (Jordan, 2013). Frontal Temporal Dementia/Pick's disease subjects: * The participant is 50 years or older. * Participants have a clinical diagnosis of FTD based on consensus for clinical diagnosis of frontotemporal dementia (Neary, et al., 1998) Healthy Control subjects: * The participant is 18 - 85 years old. * Negative history of neurological or psychiatric illness based on evaluation by a research physician. * MMSE score must be 29 or above. Exclusion Criteria: All subjects will be excluded from participation for the following reasons: * The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness. * The subject has any disorder that may interfere with drug absorption distribution, metabolism, or excretion (including gastrointestinal surgery). * The subject has evidence of a structural lesion on MRI that may interfere with interpretation of PET imaging. * The subject has evidence of clinically significant gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease. * The subject has participated in another clinical study within the previous 30 days. * Pregnancy or women who are nursing or breastfeeding