To date cerebral tau pathology in vivo was only estimated by cerebrospinal fluid (CSF) tau or CSF phospho-tau which precludes a study of topical distribution and interplay with Abeta pathology. 18F-AV-1451 offers the chance to visualize tau pathology and to study effects of tau on brain structure, brain physiology and cognitive function. Ideally, these effects are studied in well characterized individuals in whom other important pathological factors are already known.