A Study to Assess Regional Cerebral Blood Flow as an Alzheimer's Disease Biomarker Compared to Positron Emission Tomography in Patients With Mild-to-Moderate Alzheimer's Disease and Cognitively Normal Elderly Subjects (Study MK-0000-068)(COMPLETED)

Inclusion Criteria: The prospective participant must meet, at least, all of the criteria below (among others determined by the study staff) to be eligible for study participation. The participant: * Has mild-to-moderate Alzheimer's Disease (AD), OR is considered cognitively normal; * Has been on stable doses of any regularly used medications for 4 weeks prior to study start; * Must have been on a stable dose for 12 weeks prior to study start, of any medications taken for AD. Exclusion Criteria: If the prospective participant meets any of the criteria below (among others determined by the study staff) they will NOT be eligible for study participation. The participant: * Is living in a nursing home or skilled nursing facility; * Has severe AD; * Cannot undergo MRI; * Cannot undergo PET scans; * Has a history of neurological or neurodegenerative disorders, other than AD within 2 years prior to study start; * Has taken Tacrine or anti-parkinsonian medications within 3 months of study start; * Has taken corticosteroids, blood thinners, narcotic analgesics, benzodiazepines, or certain antihistamines within 1 month of study start; * Initiates, discontinues, or changes the dose of any AD treatment during the study.