1. Department of Medical Physics and Bioengineering, University College London, London, UK
2. Martinos Center for Biomedical Imaging, Harvard Medical School, Boston, MA, USA
3. Department of Biological Sciences, University of Essex, Essex, UK
4. Engineering and Physical Sciences Research Council, Swindon, UK
5. Department of Health Sciences, University of L'Aquila, L'Aquila, Italy
6. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, USA