[1] Univ Cambridge, Addenbrookes Hosp, Dept Radiol, Cambridge CB2 0QQ, England
[2] Univ Aberdeen, Sch Med & Dent, Aberdeen AB25 2ZD, Scotland
[3] Aberdeen Biomed Imaging Ctr, Sch Med, Aberdeen AB25 2ZD, Scotland
[4] Univ Minnesota, Ctr Magnet Resonance Res, Dept Radiol, Minneapolis, MN 55455 USA