Experimental Design: A pulse labeling protocol with twenty participants was completed to simplify labeling. Pulse labeling experiments provided additional kinetics results to determine Aβ kinetic models. Of the next sixty participants most will be re-enrolled that have completed prior intravenous steady-state labeling Aβ SILK studies. All participants will have had a PET/PIB scan completed for fibrillar amyloid deposition measurements or CSF Aβ42 concentration measurements.