1Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, United States, 2UNC/NCSU Joint Department of Biomedical Engineering, University of North Carolina of Chapel Hill, Chapel Hill, NC, United States, 3Biomedical Research Imaging Center, University of North Carolina of Chapel Hill, Chapel Hill, NC, United States, 4Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC, United States