Pancreatic Islet beta-cells are responsible for synthesizing and secreting appropriate amounts of insulin to regulate blood glucose levels. One factor in the development of diabetes is the loss of beta-cells. Developing treatments to prevent or restore islet beta-cell mass (BCM) in diabetic patients is hampered by a lack of methods for the non-invasive imaging of these cells. This study is designed to evaluate a radiolabeled compound that binds to the pancreatic islet. The investigators will test the ability of one promising imaging compound, 18F-9-fluoropropyl-(+)-dihydrotetrabenazine (18F-FP-DTBZ), to measure the amount of pancreatic islet beta-cells in patients with long-standing type-1 diabetes and in age-weight-matched healthy control subjects.