1. Johns Hopkins University School of Medicine Department of Radiology, 217 Traylor Building 720 Rutland Avenue, Baltimore, Maryland 21205 National Institutes of Health, Clinical Center Laboratory of Diagnostic Radiology Research National Institute of Neurologic Disorders and Stroke Laboratory of Functional and Molecular Imaging Bethesda, Maryland 20892 F.M. Kirby Research Center for Functional Brain Imaging Kennedy Krieger Institute, Baltimore, Maryland 21205