PET stands for positron emission tomography, and it is a way of creating 3D images of the inside of your body as well as images to show how well certain parts of the body are working. During the scan a slightly radioactive substance (known as a radiotracer) is injected into your bloodstream that can then be detected as it moves around your body and is taken up by different cells or parts of your body. It shows which cells are more active and which are less. It is usually used in combination with CT or MRI scanning to provide detailed images. Your body then excretes the radiotracer over the course of a couple of hours after the scan so it does not stay in your system. PET scanning is particularly useful for helping diagnose cancer, looking at its spread or response to treatment as well as looking at conditions that affect the brain.
The scanner is large and white with a hole in the middle where the bed (and patient) passes through. You are laid down on the bed for a PET scan and the bed will move you through the scanner. The radiotracer will be injected into your arm and should not be painful. The scan usually takes up to an hour to complete and you will need to try to lie as still as possible during this time.
The amount of radiation from the radioactive tracer is very small and your body will excrete the substance over the course of a couple of hours but it can give an extremely small increased risk of developing cancer in the future. It is also best avoided in people who are pregnant unless it is an emergency situation.