Brain activity. Coloured positron emission tomography (PET) scan showing brain activity. The PET scan has been superimposed onto a grey magnetic resonance imaging (MRI) scan. In this axial (horizontal) section the front of the brain is at top, with ventricles (cavities, black) at centre. The two hemispheres of the brain can be seen at left and right. The scan shows the levels of glucose (sugar) consumption (red) due to mental activity. PET scans use radioactively-labelled substances introduced into the blood to view metabolic activity.