1. Cyclotron Laboratory, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands, Department of Physiology and Department of Pathology, Cardiovascular Research Institute Maastricht (CARIM), Maastricht University, P.O. Box 616, 6200 MD Maastricht, the Netherlands, and Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, the Netherlands