[1] Harvard Univ, Sch Med, Dept Hlth Care Policy, Boston, MA 02115 USA
[2] Brigham & Womens Hosp, Dept Med, Div Gen Med & Primary Care, Sect Hlth Serv & Policy Res, Boston, MA 02115 USA
[3] Brigham & Womens Hosp, Dept Radiol, Boston, MA 02115 USA
[4] Harvard Univ, Sch Publ Hlth, Dept Biostat, Boston, MA 02115 USA