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). Tau has been shown to build up in the brains of patients with injury to brain cells, including Alzheimer's disease (AD), Parkinson's disease (PD), Lewy body disease (DLB) and Frontotemporal degeneration (FTD).