1 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
2 Department of Radiology, University of Michigan School of Medicine, Ann Arbor, MI 48109, USA. Electronic address: mkilbour@med.umich.edu.
3 Institute of Neuroscience and Medicine, INM-5: Nuclear Chemistry, Research Centre Juelich, 52425 Juelich, Germany. Electronic address: h.h.coenen@fz-juelich.de.
4 Department of Radiology, University of Missouri, Columbia, MO 65211, USA.