1. From the Departments of Radiology (N.J., M.T.M., C.J., B.F., H.L., Y.H., F.K., P.E.B., W.R.W.), Bioengineering (B.F.M., E.H., S.K.I.), and Medicine (Y.H., P.B.), Perelman School of Medicine, University of Pennsylvania, 3400 Civic Center Blvd, South Pavilion, Room 11-155, Philadelphia, PA 19104.