Alzheimer's disease (AD) and frontotemporal dementia (FTD) have overlapping features and can be difficult to distinguish based on symptoms, cognitive testing or even MRI. Beta-amyloid is a protein that aggregates in the brain in AD but usually not in FTD. Pittsburgh Compound-B (PIB) is an imaging tracer that binds to amyloid and can be detected by brain imaging with a PET scanner. The goal of this study is to determine whether brain imaging with PIB-PET can improve diagnostic accuracy in discriminating between AD and FTD, and thus govern appropriate specific treatment for each disease.