To investigate the sensitivity, specificity, and validity of the tau PET tracer [18F] PI-2620 for measuring toxic tau accumulation in the brain based on how the intensity and anatomical distribution of its uptake differs between cognitively normal adults, amyloid-positive individuals with suspected non-amnestic Alzheimers disease (naAD), and amyloid-negative individuals with suspected frontotemporal lobar degeneration (FTLD). To investigate differences between normal cognition, naAD, likely FTLD-tau, and likely FTLD-TDP patients in the anatomical distribution of tau uptake, as measured by [18F]PI-2620, demonstrating that a) [18F]PI-2620 uptake is higher in AD and FTLD tauopathies than in TDP-43 disease and cognitively normal seniors. To test associations between [18F]PI-2620 tau PET imaging measures and patients cognitive, motor, and functional impairment at baseline and approximately one year after the PET study visit.