1. Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104, USA. Tel.: , E-mail: conant@oasis.rad.upenn.edu
2. Department of Radiology, Thomas Jefferson University, 111 South 11th Street, 3390 Gibbon Building, Philadelphia, PA 19107, USA. Tel.: ; Fax: ; E-mail: Andrew.Maidment@mail.tju.edu