Department of Internal Medicine IV, Division of Endocrinology, Diabetology and Nephrology, University Hospital of Tübingen, Tübingen, Germany; Institute of Diabetes Research and Metabolic Diseases of the Helmholtz Centre Munich at the University of Tübingen (IDM), Tübingen, Germany; German Center for Diabetes Research (DZD), Neuherberg, Germany. Institute of Diabetes Research and Metabolic Diseases of the Helmholtz Centre Munich at the University of Tübingen (IDM), Tübingen, Germany; German Center for Diabetes Research (DZD), Neuherberg, Germany; Section on Experimental Radiology, Department of Diagnostic and Interventional Radiology, Eberhard Karls University Tübingen, Tübingen, Germany. Institute of Diabetes Research and Metabolic Diseases of the Helmholtz Centre Munich at the University of Tübingen (IDM), Tübingen, Germany; German Center for Diabetes Research (DZD), Neuherberg, Germany. Department of Information Engineering, University of Padua, Padua, Italy. Institute of Diabetes Research and Metabolic Diseases of the Helmholtz Centre Munich at the University of Tübingen (IDM), Tübingen, Germany; German Center for Diabetes Research (DZD), Neuherberg, Germany; Department for Diagnostic Laboratory Medicine, Institute for Clinical Chemistry and Pathobiochemistry, University Hospital of Tübingen, Tübingen, Germany. Department of Internal Medicine IV, Division of Endocrinology, Diabetology and Nephrology, University Hospital of Tübingen, Tübingen, Germany; Institute of Diabetes Research and Metabolic Diseases of the Helmholtz Centre Munich at the University of Tübingen (IDM), Tübingen, Germany; German Center for Diabetes Research (DZD), Neuherberg, Germany. Electronic address: robert.wagner@med.uni-tuebingen.de.