Ability Of ([18F]-AV-45) PET Scan to Distinguish Alzheimer's Disease Subjects From Cognitively Normal Individuals

15 patients with a clinical diagnosis of probable Alzheimer's disease and 15 cognitively normal elderly control subjects will receive both \[18F\]-AV-45 and \[11C\]PIB to compare the diagnostic performance characteristics of each amyloid ligand to distinguish AD from normal subjects. In addition to clinical diagnostic category, ligand retention will be evaluated with respect to measures of symptom severity and cerebrospinal fluid levels of amyloid and tau.