1. Wellcome/EPSRC Centre for Interventional and Surgical Sciences, University College London, London W1W 7TY, UK
2. Department of Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK
3. Victor Horsley Department of Neurosurgery, National Hospital for Neurology and Neurosurgery, London WC1N 3BG, UK
4. Nanoengineered Systems Laboratory, UCL Mechanical Engineering, University College London, London WC1E 7JE, UK
5. Department of Computer Science, University College London, London WC1E 6BT, UK