1. Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK
2. Oxford Centre for Clinical Magnetic Resonance Research, Radcliffe Department of Medicine, University of Oxford, Level 0, John Radcliffe Hospital, Oxford, OX3 9DU, UK
3. The MR Research Centre, Aarhus University, Aarhus 8200, Denmark
4. Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK