[1] Johns Hopkins Univ Hosp, Dept Radiol, 601 N Caroline St, Baltimore, MD 21231 USA
[2] Univ Calif San Francisco, Dept Radiol & Biomed Imaging, San Francisco, CA 94143 USA
[3] Harvard Med Sch, Massachusetts Gen Hosp, Dept Radiol, Boston, MA 02115 USA
[4] Univ Pittsburgh, Dept Radiol, Pittsburgh, PA 15260 USA