F 18 T807 Tau PET Imaging in Dominantly Inherited Alzheimer's Network (DIAN Project)

The investigators hypothesize that in vivo tau imaging will ultimately: * Demonstrate the presence of tau fibrils in the brain during the pre-symptomatic stages of cognitive decline, prior to cerebral atrophy, hypometabolism (as measured by 18F-FDG PET imaging), and dementia. * Demonstrate that the phenoconversion from cognitively normal (CN) status to early stages of cognitive impairment will be closely correlated with neocortical F 18 T807 uptake and that amyloid positive CN individuals who are positive for F 18 T807 will demonstrate conversion to early dementia. * Correlate closely with the appearance of CSF markers of tau, including tau, p-tau, and VILIP-1. * Co-localize with specific cognitive deficits (i.e. patients with tau deposition in the left lateral temporal lobe will have primarily language deficits). * Predict the onset of dementia more accurately than existing biomarkers.