Brain in head. Coloured three-dimensional magnetic resonance imaging (MRI) scan of a sagittal (side) view of a human brain inside a head cross-section. The front of the head is at left. The main part of the brain is the cerebrum (at upper centre), where conscious brain activity takes place. Its surface is folded into gyri. Below is the elongated brainstem and, to its right, the oval-shaped cerebellum. Both of these subconsciously control the body's vital functions. MRI scanners build images of the human body using pulses of radio waves. Successive slices" are compiled into a three-dimensional image by computer."