Describe how a PET scan works?

A PET scan which is known as Positron Emission Topography involves passing a positron emitting source through a patient. The source used for a PET scan is FDG which contains Fluorine-18. A positron emitted by the FDG will annihilate an electron in the body of the patient. This produces 2 gamma ray photons which travel in opposite directions. There is a ring of detectors located around the patient to detect the gamma ray photons produced. The machine measures the delay time between the arrival of each gamma ray photon to find the area of increased activity where the annihilation took place. PET scans are useful for locating the presence of things like tumours because the FDG source is made up of glucose. Hence it is likely to flow to an area which requires large amounts of energy such as a tumour. Hence this allows cancerous tumours to be found and located using PET scans.