Coloured diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) scan showing white matter fibres in the brain. At right are the eyeballs (spheres) and the optic nerves (grey tubes). Diffusion tensor imaging measures the direction of water diffusion, which in the brain reveals the orientation of nerve fibres. Red fibres have a left to right orientation, green a front to back orientation and blue an up and down orientation. Blood vessels (red) supplying the brain are also shown.