Clearance of brain Aβ occurs by enzymatic digestion (e.g. Insulin Degrading Enzyme, Neprilysin, etc.), cellular uptake and breakdown, transport across the blood-brain-barrier, and transport from the brain to cerebrospinal fluid (CSF) and then to blood. However, the relationship between CNS Aβ and blood Aβ is not known in humans and only partly understood in other animals. The goal is to determine the kinetics of Aβ in the CNS and blood to test the hypothesis that altered Aβ kinetics in the CNS in AD is associated with altered blood Aβ labeling kinetics. Understanding blood and CSF Aβ kinetics will contribute to a better understanding of Aβ production, transport, and breakdown within and between the brain, CSF and blood compartments. These fundamental measurements of Aβ kinetics in AD will help determine the effects of peripheral Aβ metabolism on pathophysiologic changes in AD. This information will provide key insights into whole body Aβ metabolism and will be useful for understanding the causes of AD. Further, these results may lead to a specific blood biomarker for AD.