1. University of Potsdam, Department of Physics, Postfach 601553, 14415 Potsdam, Germany
2. University of Karlsruhe, Institute for Applied Computer Science/Automation, Kaiserstr. 12, 76128 Karlsruhe, Germany
3. German Heart Center Munich, Clinic for Cardiovascular Surgery, Lazarettstr. 36, 80636 Munich, Germany