This is a study to evaluate biodistribution, pharmacokinetics and safety of [18F]THK-5351 positron emission computed tomography. Ten cognitively healthy subjects and 10 patients with Alzheimer's Disease will be enrolled. The primary outcome measures are to evaluate pharmacokinetics of [18F]THK-5351 Positron Emission Tomography imaging . Tracer biodistribution will be evaluated by global and regional standardized uptake value ratio of [18F]THK-5351 in the brain. Safety. For safety assessment, a physical examination, Electrocardiogram and vital signs will be performed at baseline and at the completion of all imaging to assess for interval change.