Visualization and Data Analysis, Konrad-Zuse-Zentrum für Informationstechnik, Berlin, Germany Visualization and Virtual Reality Group, FhG IGD Darmstadt, Germany Institute of Medical Informatics, University of Hamburg, Hamburg, Germany Medical Imaging Group, DKFZ Heidelberg, Germany Computer Graphics Group, Technical University of Kaiserslautern, Kaiserslautern, Germany Image and Signal Processing