Coloured diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) scans of nerve pathways (red, blue and green) in the brain of a 20 year old male patient with a tumour (purple) in the left temporoparietal region. Diffusion tensor imaging measures the direction of water diffusion, which in the brain reveals the orientation of nerve fibres. At top left, red fibres have a left to right orientation, green a front to back orientation and blue an up and down orientation. The tumour interrupts the course of the pyramidal tracts (red) in the left hemisphere. The pyramidal tracts are responsible for voluntary control of the muscles.