The purpose of this study is to measure the amount of a protein in the brain known as tau using an imaging procedure called Positron Emission Tomography (PET/CT). 18F-AV-1451 is a specialized radioactive PET imaging agent that is able to detect the presence of tau in the brain. In this study, the investigator will use the 18F-AV-1451 to capture images of tau in the brain. Findings from this study will likely provide insight into the mechanisms and distinctions of age-related cognitive decline and that of very early Alzheimer's disease.