How PET scans work

What's going on during a PET scan?...
26 January 2021


PET image of a human brain


Katie Haylor explains the basics of how a PET scanner works...

A PET scan, or a positron emission tomography scan (quite a mouthful) is a clever way of getting detailed 3D images of how things are working inside the body - rather than just what an organ looks like. In PET, you’re injecting a radioactive chemical that you can track - commonly a type of sugar - into someone and in detecting the radiation being emitted from the tracking chemical, you can see how well different body functions are working.

Once the chemical’s been injected, the radioactive sugar gives out a particle called a positron. When a positron hits an electron in the body, they both turn into high energy waves. The PET scanner scans the body part of interest and looks out for high energy waves. In detecting this energy (known as gamma radiation), you can build a map of what’s happening in the body.

It’s useful to look at cancers, because cancer cells are metabolically rather hungry and use sugar faster than normal cells. Taking up more of the radioactive sugar means more gamma radiation detected from that area. So on the image, the cancerous bits will appear brighter.

So if you’re injecting someone with radiation, how safe is a PET scan?

The National Health Service website explains that any exposure to radiation carries a very small risk of potential tissue damage that could lead to cancer at a later date. But the amount of radiation you're exposed to in a standard PET scan is small – about the same as the amount you get from natural sources, such as the sun, over 3 years. 


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