[1] Framingham Heart Dis Epidemiol Study, Framingham, MA 01702 USA
[2] Harvard Univ, Sch Med, Div Cardiol, Massachusetts Gen Hosp, Boston, MA 02115 USA
[3] Harvard Univ, Sch Med, Dept Radiol, Brigham & Womens Hosp, Boston, MA 02115 USA
[4] Boston Univ, Dept Math, Boston, MA 02215 USA
[5] Boston Univ, Sch Med, Dept Neurol, Boston, MA 02215 USA
[6] Boston Univ, Sch Med, Dept Epidemiol & Prevent Med, Boston, MA 02215 USA