1. From the Department of Radiology, Massachusetts General Hospital, Boston, Mass (R.V.G., M.S.G., P.J.H.); Department of Diagnostic Imaging, the Hospital for Sick Children, 555 University Ave, Toronto, ON, Canada M5G 1X8 (M.L.C.G.); and Department of Medical Imaging, University of Toronto, Toronto, Ont, Canada (M.L.C.G.).