1. From the Intramural Research Program (L.L.B.H., M.T., J.S., A.B.Z., L.F., S.M.R.), National Institute on Aging, National Institutes of Health, Bethesda, MD; Department of Radiology (G.D., J.S., M.A.K.), Johns Hopkins University School of Medicine, Baltimore, MD; Department of Electrical Engineering (B.A.L.), Vanderbilt University, Nashville, TN.