Primary progressive aphasia (PPA) is an umbrella term that encompasses a group of neurodegenerative syndromes characterized by varying combinations of progressive speech and language problems. Three clinical variants of PPA have been described and are well recognized: the agrammatic variant characterized by grammatical errors in speech and writing and typically associated with phonetic errors in speech; the semantic variant characterized by poor naming from loss of knowledge about the meaning of words; and the logopenic variant characterized by word retrieval problems and poor sentence repetition from impairment of working memory and phonemic errors. Pathological studies of PPA patients that died with postmortem examination of their brains have demonstrated that PPA is associated with a number of different abnormal cellular proteins that do not have perfect associations with the three PPA variants. One such protein is the microtubule associated protein, tau, which is the most common abnormal protein found in the brains of patients with PPA. Tau is an important protein that has been linked to the neurodegenerative process in many diseases. No neuroimaging studies have investigated tau deposition in PPA and hence the binding characteristics of AV-1451 (the Tau binding drug used in this study) in PPA are unknown. Understanding the binding characteristics of AV-1451 is crucial to help determine whether it can serve as a biomarker for tau deposition in the brains of patients with PPA.