Coloured diffusion tensor imaging (DTI) magnetic resonance imaging (MRI) scan showing white matter fibres of the substantia nigra in Parkinson's disease. This area controls smooth muscle movement. In Parkinson's disease the nerve cells here die, reducing the amount of the neurotransmitter dopamine produced. This causes tremor, jerky movements and a lack of coordination. Th 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.