This study is being done to learn about tau tangles in Alzheimer's disease. A type of positron emission tomography (PET) scan is used to measure the abnormal accumulation of protein called tau in the brain. These are thought to be involved in Alzheimer's disease. The investigators will also perform brain MRI and to tests to measure the participant's memory and thinking.

This study is being done to determine the relationship between tau tangles and cognitive impairment in elderly subjects with and without Alzheimer's disease (AD). Subjects will undergo screen that includes neuropsychological testing and brain MRI. This study uses a special type of scan called a PET scan to take pictures of the brain. During the PET scan, a special radioactive dye called 18F-MK-6240 is injected into the body. 18F-MK-6240 sticks to abnormal tangles made of the protein tau. Subjects will have the option to have lumbar puncture performed to measure CSF concentrations of biomarkers.

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.

During the scanning procedure (1 day), and at 1 year follow-up