Until recently, islet cell mass visualization has not been clinically feasible. We have previously identified a specific marker on pancreatic beta cells called vesicular monoamine transporter type 2 (VMAT2). In preclinical studies, we have shown that imaging beta cell mass with PET scanning using this radioligand is feasible in rats and in dosimetry studies conducted in baboons. This radioligand, [11C]DTBZ, has been used previously in human subjects in clinical trials evaluating P.E.T scanning of the brain in patients with bipolar illness and schizophrenia compared to healthy control subjects. In the current protocol, we propose to use a radioligand, [11C]DTBZ, that binds to VMAT2 in positron emission tomography (PET) scanning to assess whether PET can measure beta cell mass in human subjects.