Longitudinal Imaging of Microglial Activation in Different Clinical Variants of Alzheimer's Disease

Inclusion criteria: 1. Age 50 and over at time of screening. 2. At screening, must have no cognitive impairment, or meet criteria for amnestic Alzheimer's disease, posterior cortical atrophy, or logopenic variant primary progressive aphasia. Diagnoses will be made based on history, exam, neuropsychological testing, brain MRI, and consensus diagnosis. 3. Patients must have Clinical Dementia Rating scale score of 0.5 or 1 at enrollment. Controls must have Clinical Dementia Rating scale score of 0 at enrollment. 4. Subjects unable to provide informed consent must have a surrogate decision maker. 5. Written and oral fluency in English or Spanish. 6. Able to participate in all scheduled evaluations and to complete all required tests and procedures. 7. In the opinion of the PI, 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 a brain disorder other than Alzheimer's disease (including presence of cortical infarct on MRI even in absence of clinical stroke). 2. Serious 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 MRI scanning. 4. Conditions precluding entry into the scanners (e.g. morbid obesity, claustrophobia, etc.). 5. Exposure to research related radiation in the past year that, when combined with this study, would place subjects above the allowable limits. 6. Exact medication exclusions are up to the discretion of the PI in consultation with the MD liaison.