The purpose of this study is to evaluate radiolabeled [18F]MK-3328 as a PET tracer for estimating the regional distribution and extent of amyloid plaques in participants suffering from amnestic mild cognitive impairment (aMCI) and Alzheimer's Disease (AD) versus healthy young and elderly participants. The study hypotheses will test whether [18F]MK-3328 can discriminate between AD participants and cognitively normal elderly control participants as measured by brain regional tracer uptake.