[1] Univ Minnesota, Sch Med, Minneapolis, MN 55455 USA
[2] Mayo Clin, Dept Neurol Surg, Rochester, MN USA
[3] Univ Toronto, Dept Med Imaging, Toronto, ON, Canada
[4] Univ Toronto, Dept Neurol Surg, Toronto, ON, Canada
[5] Mayo Clin, Dept Radiol, 200 1st St SW, Rochester, MN 55902 USA