1. Dept. of Neurosurg., Univ. of Marburg, Marburg, Germany
2. Dept. of Imaging & Visualization, Siemens Corp. Res., Princeton, NJ, USA
3. Inst. for Med. Image Comput., Fraunhofer MEVIS, Bremen, Germany
4. Dept. of Math & Comput. Sci., Univ. of Marburg, Marburg, Germany