[1] Harvard Univ, Sch Med, Dept Radiol, Massachusetts Gen Hosp, Boston, MA 02115 USA
[2] Harvard Univ, Massachusetts Gen Hosp, Sch Med, Dept Neurosurg, Boston, MA USA
[3] Univ Cologne, Fac Med, Inst Neurophysiol, D-50931 Cologne, Germany
[4] Univ Dresden, Dept Neurosurg, Dresden, Germany