The purpose of this research is to find out whether changes in the brain in people with Alzheimer disease can be detected using a brain imaging test. Most people with Alzheimer disease have changes in the brain that result in deposits of a protein called beta-amyloid. In this study, the investigators will be using a radioactive drug, [18F]MNI-558 that binds to beta-amyloid. This drug is experimental and has not been approved by the FDA. Brain imaging using PET (positron emission tomography) will be done to see if the investigators can evaluate the areas of beta-amyloid in the subjects with Alzheimer disease. The investigators will compare these scans with those done in healthy normal volunteers. PET is a brain-scanning test used in medicine and scientific research to see how the brain is working. The PET imaging test used in this study is not being done for diagnostic purposes.