Laser beams are used to align a person's head prior to their undergoing a Positron Emission Tomography (PET) brain scan, A colour-coded PET scan is obtained by first injecting a positron- emitting radioisotope (produced in a cyclotron) into the bloodstream: the detector is sensitive to gamma rays produced when positrons emitted annihilate with electrons from the molecules in the brain (known as tracers) to which the radioisotope has been dedicated. Applications of PET include the use of radioactive oxygen to measure oxygen consumption in senile dementia, cerebrovascular disease and brain tumours.

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