This research is designed to obtain efficacy and safety information for 123-I IMPY as an imaging biomarker for Alzheimer's disease (AD). The distribution of this agent will be measured by obtaining single photon emission computed tomography (SPECT) images of the brain serially over time to determine the relative localization of the radiopharmaceutical in regions of the cortex relative to background regions and develop an optimal technique of radiotracer administration (bolus or bolus with constant infusion). The researchers will then evaluate the utility of 123-I IMPY and SPECT in AD patients as an early diagnostic tool and subsequently serial evaluations of AD patients will be performed to determine if this technique may be useful as a tool for evaluation of progressive brain β-amyloid deposition in AD.