The goal of this study is to assess [18F]MNI-777 PET imaging as a tool to detect tau pathology in the brain of individuals who carry a clinical diagnosis of a tauopathy, including: Alzheimer's Disease (AD),Parkinson's disease (PD) Progressive Supranuclear Palsy (PSP), chronic traumatic encephalopathy (CTE) and Frontal Temporal Dementia (FTD) and age- and gender-matched 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