All study procedures will be conducted at the Institute for Neurodegenerative Disorders (IND) and Molecular NeuroImanging (MNI) in New Haven, CT. Approximately 25 patients with mild to moderate Alzheimer's disease (AD) and 20 healthy controls will be recruited to participate in this study. AD patients will be eligible to participate if they have a diagnosis of AD of less than 3 years duration. Healthy controls will be examined to ensure that there is no evidence of neurodegenerative changes including cognitive decline. All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a baseline physical and neurological evaluation and baseline cognitive evaluations. Subjects will be asked to undergo either a bolus injection or bolus injection followed by continuous infusion of 123-I IMPY. Following injection, subjects will undergo serial SPECT imaging scans and serial venous plasma sampling for measurement of 123-I IMPY in plasma (both protein bound and free) over a 3.5 - 8 hour period. The imaging analyses will be performed by an image-processing specialist who will remain masked to the procedures employed with each imaging acquisition. The primary imaging outcome measure will be the brain regional distribution volumes expressed as a brain tissue to plasma ratio of the radioligand, 123-I IMPY.