1 From the Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115 (S.A.M.); Department of Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, Calif (T.K.); and Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, Mass (S.S.H.).