Having a PET scan

If you have a PET scan, what can you expect?

An antimatter drug

Before you arrive at the hospital, the medical technicians have to make the radiopharmaceutical. This is the drug which will carry positrons into your body.

FDG molecule

FDG is a glucose molecule, with fluorine-18 attached

They start by using a cyclotron (a small particle accelerator, usually in or near the hospital) to make a chemical element called fluorine-18.

Fluorine-18 is unstable: left to itself it decays into oxygen-18, by emitting positrons.

The technicians attach the fluorine-18 to ordinary glucose molecules, to make a drug called fluorodeoxyglucose (FDG).

The fluorine-18 in the FDG goes on emitting positrons.

When you arrive at the hospital the FDG is given to you as an injection.

The drug carries positrons around your body

Ordinary glucose travels to areas in your body that are working hard: your heart, lungs, brain, and other organs. The fluorodeoxyglucose (FDG) does the same.
The fluorine-18 in the FDG is continually emitting high-energy positrons, which interact with molecules in your body. With each interaction, the positrons lose energy and slow down, until very soon they are able to pair up with an electron, form positronium, and annihilate.

The scanner detects where the positrons go

When a positron and an electron annihilate, all their mass is converted to energy, and given off in the form of two gamma rays. We know that the gamma rays are always emitted in opposite directions to each other, and that they always have an energy of 511 thousand electron volts.

This is where the scanner comes in.

The PET scanner is a giant cylinder, big enough to fit around your body.

There are gamma-ray detectors all around the cylinder, so the scanner can detect any gamma rays that leave your body.

Each time the scanner detects a matching pair of gamma rays, its software works out their point of origin: the exact location where the positron and electron annihilated. This will be only a very short distance from the place where the FDG emitted the positron.

The positrons are drawn to sites of activity

So, by detecting where in your body the FDG is emitting positrons, the PET scanner can trace exactly where your body is using up glucose.
In a healthy body this means organs like the heart, liver and brain. But glucose will also be drawn to unhealthy sites such as fast-growing tumours, so they show up in the scan.

The PET scanner’s software builds a three-dimensional image of every point where a positron and electron annihilated. This gives doctors a detailed picture of the activity inside your body, without any need for anaesthetics or surgery.

After the scan

Fluorine-18 has a half-life of just 110 minutes. This means it stays in your body just long enough for the PET scan, then disappears.
The fluorine-18 lasts for such a short time that it has to be used as soon as it is made. We don’t want to waste time, or lose some of the drug, transporting it across the country. That’s why cyclotrons are often in, or very close to, hospitals.

Next: why have a PET scan? >>


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