What is a PET scan?
Positron emission tomography (PET)scans are used to produce detailed, three-dimensional images of the inside of the body.
The images clearly show the part of the body being investigated and can highlight how effectively certain functions of the body are working.
What PET scans are used for
PET scans are often used to help diagnose a range of different cancers and work out the best ways of treating them. The PET scan can show how far a cancer has spread or how well it is responding to treatment.
Sometimes, PET scans are used to help plan complex heart surgery, such as a heart transplant. They can also be used to help diagnose a number of conditions that affect the normal workings of the brain (neurological conditions), such as dementia.
Read more about why PET scans are used.
How PET scans work
PET scanners have a flat bed with a large, circular scanner at one end.
Before you have a PET scan, a radioactive substance (a radiotracer) will be passed into your body. This is usually by injection into a vein.
The tracer gives off particles called positrons that release gamma waves (a type of radiation), which can be detected by the PET scanner.
By tracking the movement of the tracer, the scanner can build up a detailed image of a number of the body’s functions, as well as identifying areas of the body that have been affected by disease.
As PET scanners are expensive, they are usually only found at larger hospitals and some specialised research centres.
Due to the lack of availability, PET scans tend to only be recommended for people with complex health problems. They are not routinely used to diagnose cancer, but are often used in confirmed cancer cases to see how far the cancer has spread and whether treatment has been effective.
Any exposure to radiation carries a very small risk of causing damage to tissues and the possibility of triggering a new cancer.
However, in a standard PET scan, the amount of radiation you are exposed to is the same as the amount received from natural sources, such as the sun, over the course of three years.
Increasingly, PET scans are being combined with computed tomography (CT) scans to provide more detailed images.
These new types of scanners, known as PET-CT scanners, use higher levels of radiation, which are equivalent to the recommended levels that someone working in a nuclear power plant should be exposed to over the course of a year. However, this dose is still well within the acceptable safety limits for radiation exposure.
Read more about the possible risks of PET scans.
PET scan risks
During a positron emission tomography (PET) scan, your body's tissue will be exposed to radiation.
Radiation is measured in units known as millisieverts, or mSv for short. A standard PET scan would expose you to about 8mSv, which is the same amount of radiation that you are exposed to from natural sources of radiation, such as the sun, over the course of three years.
A combined PET-CT scan would expose you to between 15-25mSv. The recommended annual limit that someone who works with radiation should be exposed to over the course of a year is 20mSv.
Most experts believe that the risk of cancer developing only becomes significant in people who are exposed to 100mSv or more.
However, as a precaution a PET scan is not recommended for pregnant or breastfeeding women unless the benefits of the scan clearly outweigh any risks to the baby. Close contact with pregnant women, babies or young children should be avoided for a few hours after having a PET scan.
Read more about radiation exposure.
How is a PET scan performed?
Positron emission tomography (PET) scans are usually carried out on an outpatient basis. This means you will not need to stay in hospital overnight.
Preparing for the scan
Your doctor will give you instructions about how to prepare for your scan. You will not usually be able to eat anything for four to six hours before the scan and you will be advised to drink plenty of water.
You may also be advised to avoid caffeine for 24 hours before your scan. Eating and consuming caffeine could change how your body responds to the radiotracer (radioactive substance), which could make the result of your scan unreliable.
The scan procedure
PET scanners have a flat bed with a large, circular scanner at one end.
Before having the scan, a radioactive substance known as a radiotracer will be passed into your body.
Depending on the part of your body being scanned, this can either be done by injecting the radiotracer into a blood vessel or breathing it in as a gas. The radiotracer can take 30-90 minutes to travel around your body.
When you are ready to have the scan, you will be taken to the room where the PET scanner is located. You will be asked to lie on the bed of the scanner, which will be moved into the centre of the scanner.
During the scan, you will need to stay still and will be asked not to talk. Depending on the part of your body being scanned, the procedure will take 30-60 minutes.
The scan should not be painful, but if you feel unwell there is a buzzer you can press to alert the medical team in charge of your care. The medical team will be able to see you throughout the scan.
For some people, being inside the scanner can be an unpleasant and claustrophobic experience. Inform your care team before the day of the scan if you have a previous history of claustrophobia (a fear of confined spaces). They may be able to arrange for you to have medication known as a sedative to help you relax during the scan.
After the scan
As a PET scan involves being exposed to a very small amount of radiation, you will not experience any side effects and will usually be able to go home soon after the scan has been completed. If you have had a sedative, you will need to arrange for someone to drive you home after the scan.
You should drink plenty of fluids after the scan to help flush the radioactive medication from your body. The radiotracer should leave your body naturally around three hours after it was given.
The results of your PET scan will be sent from the PET scan department to your specialist. However, it may take a couple of weeks for your specialist to receive the results.
Why is a PET scan needed?
PET scans can also be used to find out how well a treatment is working.
The main advantage of a PET scan compared with other types of scans, is it can indicate how well a particular part of the body is working, rather than simply showing what it looks like.
PET scans can highlight how a condition has affected the body's functions, and enable healthcare professionals to accurately track how far a condition, such as cancer, has spread.
One of the most common uses of a PET scan is to investigate confirmed cases of cancer. In particular, a PET scan can usually provide three important pieces of information. These are:
- how far the cancer has spread (this is known as staging - the higher the stage, the further the cancer has spread)
- how well the cancer is responding to treatment
- whether any cancerous cells remain after a course of treatment has been completed
Less commonly, PET scans are used to help diagnose conditions that can affect the brain and nervous system (neurological conditions).
For example, a PET scan can be used to help plan brain surgery in cases of epilepsy that have not responded to medication.
The PET scanner can sometimes locate damage to the brain that is responsible for triggering seizures (disruption of normal brain activity). The damaged areas of the brain can be removed using surgery.
PET scans can also help diagnose dementia (a group of symptoms related to a decline in brain function) and Parkinson’s disease (a condition where part of the brain becomes progressively more damaged, resulting in involuntary shaking).
PET scans are a very effective way of tracking the blood flow in and around the heart. Therefore, they are often used to determine whether someone could benefit from different types of heart surgery such as a:
- heart transplant - where a diseased heart is replaced with a healthy heart taken from a donor who has died
- coronary artery bypass graft - where a blood vessel that is taken from another part of the body is used to bypass a blocked or narrowed section of artery
- angioplasty - where a blocked or narrowed section of artery is widened using a small metal tube called a stent
PET scans can also be used to look at how the body works and to help understand what happens when something goes wrong with the way the body functions.
For example, researchers are currently using PET scanners to study the brain function of people with autism (a range of related developmental disorders that affect language development and social interaction).
How does a PET scan work?
During a positron emission tomography (PET) scan, a substance known as a radiotracer will be passed into your body.
A radiotracer is a radioactive chemical that releases tiny particles called positrons (see below). It is usually injected into a vein or, rarely, it can be inhaled as a gas.
Most PET scanners use a radiotracer called fluorodeoxyglucose (FDG), which is similar to naturally occurring glucose except that it is radioactive. The advantage of using FDG is that your body will treat it in a similar way to normal glucose.
Studying how different parts of the body respond to glucose can provide a great deal of information about the body's processes. For example, cancerous tissue processes glucose in a different way to normal tissue, making FDG an effective way of detecting cancers.
As FDG moves through your body it releases positrons. The positrons quickly break down, releasing energy waves called gamma waves. The PET scanner can detect gamma waves, which show up on a computer screen as a three-dimensional image.
PET scan images can highlight how certain parts of your body break down the radiotracer. Different concentrations of positrons show up as areas of different colours and brightness.
A radiologist is a healthcare professional who is trained to interpret images of the inside of the body. They will look at the images produced by your PET scan and report their findings to your specialist doctor.
PET-CT scanners are a newer generation of PET scanners that also incorporate a computed tomography (CT) scanner. CT scanners take a series of X-ray images and a computer then assembles each scan into a more detailed image.
Combining both types of scanner enables the PET scanner to provide information about how well a certain part of the body is functioning, while the CT scanner can provide information about the anatomy of the body part (its appearance).
This is very useful because some conditions, such as cancer, can disrupt both the function and appearance of the affected body part.