[1] St Olavs Univ Hosp, Dept Neurosurg, N-7005 Trondheim, Norway
[2] Norwegian Univ Sci & Technol, N-7034 Trondheim, Norway
[3] SINTEF, Hlth Res, N-7034 Trondheim, Norway
[4] St Olavs Univ Hosp, Natl Ctr 3D Ultrasound Surg, N-7005 Trondheim, Norway
[5] St Olavs Univ Hosp, Natl Ctr Spinal Disorders, N-7005 Trondheim, Norway