White matter fibres. Coloured diffusion tensor imaging (DTI) scans of sections through a healthy brain showing the orientation of white matter fibres. The front of the brain is at top. Fibres running between right and left are red, those running between the front and back are blue and those running between the interior and exterior are green. White matter consists of bundles of nerve fibres. The fibres transmit nerve signals between brain regions and between the brain and the spinal cord. DTI scans show the diffusion of water along white matter fibres, allowing their orientations and the connections between brain regions to be mapped.