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

The purpose of this study is to determine how inflammation is related to other changes in the brain that occur during the progression of Alzheimer's disease. The investigators are also studying how inflammation is related to the symptoms that first occur in patients with Alzheimer's disease (AD). For this reason, the investigators are asking people with different versions of Alzheimer's disease and/or other related dementias to participate. This includes patients with: * Mild Cognitive Impairment * Posterior cortical atrophy - a version of Alzheimer's disease with vision difficulties * Logopenic variant primary progressive aphasia - a version of Alzheimer's disease with language difficulties * Amnestic Alzheimer's disease - a "typical" version of Alzheimer's disease with memory difficulties * The investigators are also enrolling older adults with normal visual, language, and memory function.