1. Marconi Caswell Ltd., Caswell, Towcester, Northants NN12 8EQ, UK.
2. Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2BZ, UK.
3. Robert Steiner Magnetic Resonance Unit, Clinical Sciences Centre, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London W12 0HS, UK.