[1] Imperial Coll Healthcare NHS Trust, Dept Stroke Med, London W6 8RF, England
[2] UCL, Dept Appl Hlth Res, London, England
[3] Boehringer Ingelheim Ltd, Dept Market Access Pricing & Outcomes Res, Bracknell, Berks, England
[4] Imperial Coll Healthcare NHS Trust, Charing Cross Hosp, Dept Imaging, London W6 8RF, England