1. Physikalisch-Technische Bundesanstalt (PTB); Abbestr. 2-12 10587 Berlin Germany
2. Present address: Max Planck Research Group NMR Signal Enhancement; Max Planck Institute for Biophysical Chemistry; Am Fassberg 11 37077 Göttingen Germany
3. Medical Department Institute of Medical Physics and Biophysics; University of Leipzig; Härtelstr. 16-18 04107 Leipzig Germany