1 From the Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, White Bldg, Rm 270, Boston, MA 02114 (S.I.L., B.R.R.); Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pa (C.E.K.); and University of Michigan, Ann Arbor, Mich (G.D.L.).