A substantial portion of people covered by Medicare will develop Alzheimer's disease and other forms of dementia that together devastate the lives of millions of people in the United States, and cost us a total of over $200 billion every year. Getting a brain scan with a PET scanner to look for abnormal brain metabolism patterns is recognized as "reasonable and necessary" for some patients with "a recently established diagnosis of dementia" (Centers for Medicare and Medicaid Services (CMS), Decision Memo CAG-00088R), but the evidence is considered less clear for patients having less severe cognitive problems, and/or for patients getting a brain scan with a PET scanner to look for abnormal proteins in the brain (CMS Decision Memo CAG-00431N). This project employs a scientifically rigorous design (prospective, multi-centered, randomized controlled trial) to determine whether such PET scanning can help distinguish more accurately than is being done in current clinical practice those patients with early molecular changes in their brains who will benefit from Alzheimer related treatments from those patients who will not, as proven by measuring to what extent the PET scans actually lead to earlier appropriate therapy, and in fact result in improved outcomes for Medicare beneficiaries and for the health care system in which they obtain care.