1. From the Experimental Neuroimaging Section, Laboratory of Diagnostic Radiology Research, National Institutes of Health, Bethesda, MD; National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD; National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD; and Clinical Center, Department of Transfusion Medicine, National Institutes of Health, Bethesda, MD.