This study will use neuroimaging to understand how the connections in the brain change in Alzheimer's Disease. Changes will also be examined in individuals with mild cognitive impairment and healthy aging. The goal is to discover which brain changes are present in healthy aging and MCI so that future studies can assess the risk for developing Alzheimer's Disease. The study involves blood draw, cognitive testing, MRI, PET scanning, and a 1-year follow-up visit to repeat cognitive testing and MRI scanning.