[1] Nalt Hosp Neurol & Neurosurg, Inst Neurol, Dementia Res Ctr, London WC10 3BG, England
[2] Hammersmith Hosp, Imperial Coll London, Cyclotron Unit, Med Res Ctr Clin Sci, London, England
[3] Univ London London Sch Hyg & Trop Med, Med Stat Unit, London WC1E 7HT, England