3-D MRI brain scan. Coloured three-dimensional magnetic resonance imaging (MRI) scan of a healthy human brain seen from the front. The cerebrum, the largest section of the brain, is divided into two hemispheres (at left and right). Conscious thoughts, emotions and movements are controlled within the cerebrum. The surface of the cerebrum is folded into gyri and fissured with sulci. At the bottom of the brain is part of the rod-like brainstem, site of unconscious reflex coordination. MRI scanners build images of the human body using pulses of radio waves. Successive slices" are compiled into a three-dimensional image by computer."