[1] Univ Penn, Div Gastroenterol, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA
[2] Univ Penn, Sch Med, Philadelphia, PA 19104 USA
[3] Univ Penn, Dept Emergency Med, Philadelphia, PA 19104 USA
[4] Univ Penn, Off Human Res, Philadelphia, PA 19104 USA
[5] Univ Penn, Dept Radiol, Philadelphia, PA 19104 USA
[6] Univ Penn, Dept Med, Philadelphia, PA 19104 USA