Brain's olfactory centre. Coloured positron emission tomography (PET) scan of the brain during olfactory (smell) 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. The scan shows the level of brain activity associated with olfactory activity from low (purple) to high (yellow). The highest activity is in the olfactory centres (green & yellow) of the sensory cortex. PET scans use radioactively- labelled substances introduced into the blood to view metabolic activity.