The aim of the study is to determine if regional cerebral blood flow, measured by dynamic arterial spin labeling (dASL), can be a biomarker for stage of Alzheimer's disease. The study is designed to be conducted in 2 parts in participants with mild to moderate Alzheimer's disease, and participants with normal cognition. Various imaging studies will be done using magnetic resonance imaging (MRI) and positron emission tomography (PET) along with neurocognitive assessments. Participants who meet the study-entry criteria will have up to 8 study visits. Repeat imaging studies may be required if the initial data are incomplete or un-interpretable. The maximum number of PET scans during the study will be limited to four.