PET scan

Page last reviewed: 13/07/2011

A PET scan produces three-dimensional, colour images that show how the tissues inside your body work. PET stands for positron emission tomography.

A PET scan works by detecting a radioactive substance inside the body and making images that show where the radiation is concentrated. Radiation is introduced into the body before a PET scan using a medicine called a radiotracer. This builds up in the part of your body that will be examined (see How it works, above).

A PET scan can be used to diagnose a condition or to see how a condition is developing. It can also measure how well a treatment is working (see Why it is necessary, above). 


The amount of radiation used is very small, so it will not damage your body (see Risks, above).

However, tell your doctor or specialist if you are pregnant or breastfeeding in advance of your appointment. Radiation could be harmful to your baby unless you take precautions.

A PET scan is painless and you should be able to go home on the same day. You are unlikely to have any side effects and you can carry on with daily activities as normal.

Women who are pregnant and children should NOT accompany you to or collect you from the PET/CT department, as you will still be mildly radioactive for a few hours after the scan.


Currently, only a few hospitals have the facilities to carry out PET scanning. You may have to travel to have a PET scan, depending on what facilities are available in your area.

Page last reviewed: 13/07/2011

A PET scan is a complementary test. This means that it is usually used with other tests, such as X-rays or magnetic resonance imaging (MRI) scans, to diagnose or investigate a health condition. It can also be used to find out how well a treatment is working.

The main advantage of a PET scan over other scans, such as an X-ray or MRI scan, is that it can show how a part of your body is working, rather than simply what it looks like. This can also be useful in medical research.

Investigating health conditions

PET scanning can be used to diagnose a health condition or to find out how it is developing or whether it has spread to other parts of the body. Common conditions that a PET scan can investigate include:


PET scanning can tell which part of your brain is affected by epilepsy and whether certain treatments are suitable for you.

Alzheimer's disease

PET scanning can be used to diagnose Alzheimer's disease.


Cancer specialists use PET scanning to:

  • show up a cancer,
  • determine the stage of cancer,
  • show if the cancer has spread to other parts of the body,
  • decide the best treatment for cancer, and
  • show how well chemotherapy is working.

Heart disease

PET scans of the heart can identify whether parts of the heart have been scarred or damaged, and if the heart is working properly.

Medical research

PET scanning can be used to research how the body works and to understand what happens when something goes wrong with the way that the body functions.

Areas that can be studied include brain function, and the effects of stress, drug abuse and ageing.


The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
An X-ray is a painless way of producing pictures of inside the body using radiation.
MRI stands for magnetic resonance imaging. It uses magnets and radiowaves to take detailed pictures of inside the body.
The heart is a muscular organ that pumps blood around the body.

When not to have a PET scan

A PET scan may not be suitable if you have chemical imbalances in your body. For example, the scan may give false results if you have eaten a few hours before the scan or if you are diabetic.

Page last reviewed: 13/07/2011


Before a PET scan is carried out, a radioactive medicine is produced in a machine called a cyclotron. This is then attached to a natural chemical, such as glucose, water or ammonia, to make a radioactive version of the chemical. This is known as a radiotracer. A small amount of radiotracer is either injected into your arm, or you will breathe it in as a gas.

Once inside the body, the radiotracer will travel to the parts of the body that use the natural chemical. For example, a radioactive drug called fluorodeoxyglucose (FDG) is attached to glucose to make a radiotracer. When this enters the body, it goes to the parts that use glucose for energy.

FDG is used to show up cancers because cancers process glucose in a different way from normal body tissue.  

Detecting Positrons

A PET scan works by detecting the distribution of radiation energy given out from the reactions of positrons (positively charged particlesproduced by the radiotracer). Positrons are made as the radiotracer is broken down inside your body. The energy shows up as a three-dimensional image on a computer screen.

The image:

The image shows how certain parts of your body break down the radiotracer. Different concentrations of radiotracer show up as areas of different colour and brightness on a PET image.

A radiologist will look at the images that a PET scan produces and report the results to your doctor. A radiologist is a person who has training in interpreting pictures of inside the body.


Glucose (or dextrose) is a type of sugar that is used by the body to produce energy.

Page last reviewed: 13/07/2011

A PET scan is usually an outpatient treatment, which means you will not need to stay in hospital overnight.


Your doctor will give you instructions on how to prepare for your scan.

Normally, you will not be able to eat anything for four to six hours before your scan and you will have to drink plenty of water beforehand. It is important that your body’s glucose levels are low for the scan to be effective. You may be advised not to drink any caffeine in the 24 hours before your scan.

If you are diabetic, please contact the PET/CT department a few days in advance of your appointment. You will be questioned regarding your diabetes and given specific instructions for fasting and medication prior to the test.

You should not undertake any strenuous exercise on the day prior to your appointment.Your doctor will give you instructions on how to prepare for your scan.

The scan

A small amount of radiotracer is either injected into your arm or you will breathe it in as a gas.

It can take around 30 to 90 minutes to travel around the body. During this time, you must rest and not move or talk. You may be given a drug, such as diazepam, to help you relax.

When you are ready for your scan, you will be taken to the examination room where the PET scanner is. You will be asked to lie on a cushioned examination table, which is then moved into the scanning machine. The scanner has a large ‘hole’ through which you will pass 2-4 times as the scan is being taken. It is not an enclosed tunnel.

You will be asked to stay still during scanning. A scan normally lasts around 30 to 60 minutes, depending on the part of the body being scanned.

You should not feel any pain during the scan, but if you feel unwell or need some help, you can alert the medical staff as they will be listening and guiding you through the test. The staff can see you throughout the scan.

At some hospitals, you may have access to a CD player while you are lying in the scanner, so check beforehand if you can bring some CDs. You may be able to bring a friend to keep you company.


You are only exposed to a small amount of radiation, so you will not have any side effects and can usually go home soon after your scan is completed. Please avoid any close contact with pregnant women or children for a few hours after your test.

Drink plenty of fluids afterwards to flush the radioactive drugs from your body. All traces of the radiotracer should leave your body naturally around three hours after it was given.

The results of your PET scan may take a couple of weeks to arrive. A report will be sent from the radiology department to your specialist, who will give you the results.

Page last reviewed: 13/07/2011

If you are pregnant or breastfeeding, there may be a risk to your baby if you have a PET scan. This is because even small amounts of radiation can be harmful to a baby.

If you are pregnant or breastfeeding, speak to the staff at the hospital before the scan is carried out. They can advise you on what precautions to take.

You are advised not to have close contact with pregnant women, babies or young children for a few hours after a PET scan.    

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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