1. Department of Radiology, University of Chicago, Chicago, IL 60637, USA
2. Xian Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Shaanxi, Xi’an 710119, China
3. IBM Almaden Research Center, San Jose, CA 95120, USA
4. Department of Radiology and BRIC, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA