Brain activity causes an increase blood flow to certain areas of the brain. The oxygen carried by the flood to the brain is labelled with a small amount of radioactive isotope oxygen-15. This has a short half life decaying by emitting antimatter called a positron.
The positron emitted then annihilates with a nearby electron. Due to E = mc², the mass vanishes and is destroyed but the energy is still there in the form of a pair of gamma-ray photons travelling in opposite directions.
The detectors around the brain pick up these photons and generate a computer generated map of the activity of the brain
The detectors detect the gamma-ray photons almost simultaneously (as they are a pair). The change in time will help work out the location of the source of gamma rays.
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