Healthy brain. Coloured magnetic resonance imaging (MRI) scan of an axial section of the human head showing a healthy brain. The front of the head is at top. The cerebrum of the brain is divided into left and right hemispheres. It has a folded appearance. It is the largest part of the brain, and controls conscious thoughts. The dark areas in the brain are the ventricles, spaces filled with cerebro- spinal fluid. MRI provides slice images of the body using radio waves and a magnetic field to provoke tissues to emit signals depending on their composition. It is a useful diagnostic aid, particularly for studying nervous tissue.