1Department of Radiology and Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Boston, Mass
2Intervention Centre, Oslo University Hospital, N-0027 Sognsvannsveien 20, 0372 Oslo, Norway
3Department of Radiology, University of Texas Southwestern Medical Center, Dallas, Tex
4Department of Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Heidelberg, Germany
5Department of Radiology, Oslo University Hospital, N-0027 Sognsvannsveien 20, 0372 Oslo, Norway
6Department of Neurosurgery, Oslo University Hospital, N-0027 Sognsvannsveien 20, 0372 Oslo, Norway
7Department of Physics, University of Oslo, Oslo, Norway