RecruitingPhase 3Drug trialPET Imaging Tau Accumulation in FTLD and Atypical Alzheimer's Using [18F]-PI-2620The investigators will compare \[18F\]-PI-2620 tau PET scans from patients with frontotemporal lobar degeneration (FTLD), patients with non-amnestic presentations of Alzheimer's disease (naAD), and demographically matched cognitively normal subjects.