Phase 1 Test-retest Evaluation of [18F]MNI-958 PET

The overall goal of this protocol is to evaluate \[18F\]MNI-958 also known as APN-0000455 or PM-PBB3, a tau targeted radiopharmaceutical. The specific objectives are: * To measure the dynamic uptake and washout of \[18F\]MNI- 958 in brain using positron emission tomography (PET) in patients with Alzheimer's disease, Progressive Supranuclear Palsy and healthy volunteers. * To measure blood metabolites of \[18F\]MNI-958 and perform kinetic modeling to assess its ability to measure tau protein in brain using the tracer plasma concentration or a reference region as indirect input. * To obtain test/retest reliability of the tracer binding parameters in patients with Alzheimer's disease, Progressive Supranuclear Palsy and healthy volunteers. * To acquire safety data following injection of \[18F\]MNI-958.