Type 1 diabetes mellitus (T1DM) develops when there is impaired insulin production due to loss of insulin producing cells (beta cells). The amount of insulin that can be produced is imperfectly correlated with beta cell mass (BCM). The development of a reliable method to noninvasively quantify the total amount of insulin producing beta cells would be of great benefit by providing an important endpoint for the development of new treatments of diabetes. The investigators have previously identified a specific marker on islet cells called vesicular monoamine transporter 2 (VMAT2) that the investigators now propose to use in positron emission tomography (PET) scanning to determine islet beta cell mass. The PET radiopharmaceutical 18F-fluoropropyl(FP)-dihydrotetrabenazine(DTBZ) has been used previously in human subjects without adverse effects. It has shown promise in differentiating type 1 diabetes and non-diabetes. The investigators now hypothesize that repeat PET scans will be reproducible in the same subject. Subjects with normal BCM will be recruited from among normal weight non-diabetic people with plasma insulin levels within the normal range. Subjects with predicted reduced BCM will be recruited from among patients with T1DM who have low or non-measurable insulin levels. Two PET scan measurements will be taken in each subject and the amount of VMAT2 in the pancreas will be and compared for reproducible findings. Biochemical testing will also be performed and compared to PET scans as a potential indirect marker of beta cell mass.