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Brain cancer. Coloured magnetic resonance imaging (MRI) scan of an axial section through a brain with a cancerous tumour (white, lower left). This malignant area of abnormal growth may be putting pressure on surrounding brain tissue, resulting in sensory disturbances, headaches, nausea and epileptic fits. The brain's ventricles (white), its fluid-filled cavities, are at centre. Brain tumours are surgically removed unless they are inaccessible, in which case radiotherapy and chemotherapy are used. MRI scanners produce slice images of the body using radio signals and a magnetic field. The scans are useful in revealing brain tumours and in planning brain surgery. (KEYSTONE/SCIENCE PHOTO LIBRARY/GUSTOIMAGES)