1. School of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK
2. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA
3. Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
4. School of Mathematics and Statistics, University of St Andrews, St Andrews KY16 9SS, UK