MAPT and Alzheimer disease: Accurate interpretation of quantitative positron emission tomography (PET)outcomes hinges on understanding the test–retest variability (T-RT).Previous studies of the tau-PET ligand [18F]MK-6240 reported adequateT-RT performance of tau burden estimates over a short-term 21-day and over alonger-term 6-month T-RT period, primarily involving Alzheimer’s disease(AD) and cognitively normal (CN) subjects, respectively.