Imaging Tau in Alzheimer's Disease and Normal Aging

Inclusion Criteria: 1. Age 50 and older. 2. Meet criteria for either 1. amnestic mild cognitive impairment (MCI) (single or mixed domain) or mild Alzheimer's disease (AD), or 2. have no cognitive impairment, based on history, exam, neuropsychological testing, and consensus diagnosis. MCI and mild AD patients must have Clinical Dementia Rating scale score of 0.5 or 1. Unimpaired subjects must have Clinical Dementia Rating scale score of 0. 3. Subjects unable to provide informed consent must have a surrogate decision maker. 4. Written and oral fluency in English or Spanish. 5. Able to participate in all scheduled evaluations and to complete all required tests and procedures. 6. In the opinion of the investigator, the subject must be considered likely to comply with the study protocol and to have a high probability of completing the study. Exclusion Criteria: 1. Past or present history of certain brain disorders other than MCI or AD. 2. Certain significant medical conditions, which make study procedures of the current study unsafe. Such serious medical conditions include uncontrolled epilepsy and multiple serious injuries. 3. Contraindication to magnetic resonance imaging (MRI) scanning. 4. Conditions precluding entry into the scanners (e.g. morbid obesity, claustrophobia, etc.). 5. History of kidney disease or presence of impaired kidney function based on laboratory tests at screening visit. 6. History of liver disease or presence of impaired liver function based on laboratory tests at screening visit. 7. Participation in the last year in a clinical trial for a disease modifying drug for AD. 8. Inability to have a catheter in subject's vein for the injection of radioligand. 9. Inability to have blood drawn from subject's veins.