PET Scan

Authors: Dr Michael Hofman*
                            Dr Dee Nandurkar *

What is a PET scan?

PET stands for “positron emission tomography”. It is a nuclear medicine imaging test in which a small amount of liquid radioactive material is injected into the body and is used to diagnose a variety of diseases, including many types of cancers, and brain and heart disease.

The radioactive substance most commonly used in PET scanning is a simple sugar (like glucose) called FDG, which stands for “fluorodeoxyglucose”. It is injected into the bloodstream and accumulates in the body where it gives off energy in the form of gamma rays. These are detected by the PET scanner and a computer converts the signals into detailed pictures or images showing how tissue and organs are working. If you are having an FDG-PET, your sugar metabolism (how sugar is used by your body) is imaged. This is commonly used for cancer imaging, as the cancer cells need sugar to grow. FDG is also useful for imaging inflammatory or infective processes, and for imaging brain metabolism.

PET scanners are combined with computed tomography (CT) scanners, called PET-CT scanners. CT imaging uses X-ray equipment to create detailed images of slices of the inside of your body. The PET-CT combination allows any abnormality on the PET scan to be precisely located within the body, allowing for more accurate diagnosis of any problems. The PET or PET-CT scanner looks like a large box with a circular hole in the middle.

Why would my doctor refer me to have this procedure?

The reasons for having a PET-CT scan are continually evolving, with new ways of testing a broader range of conditions and symptoms, and using new radioactive substances. Nevertheless, most PET scans are carried out in patients with cancer. PET-CT is important for identifying certain cancers and assessing their spread through the body. This will allow your doctor to determine the most appropriate treatment for you and advise you on your options. Scans are also used at intervals to determine whether the cancers have responded to your treatment.

How do I prepare for a PET scan?

You will receive specific instructions based on the type of PET scan you are undergoing. If you are unsure about any aspect of preparation, contact the facility where your PET scan is going to be carried out.

It is important that you let staff at the hospital or radiology facility where you are having the scan done know if you are (or think you could be) pregnant or are breast-feeding.

Women who are breast-feeding and people who are the primary or sole carer for small children may need to make special preparations for after the test, to stop breast-feeding for a short time and to avoid close contact with young children. This is due to the small amount of radioactivity your body may release for a while after the test. Talk to your referring doctor or the nuclear medicine facility where you will have the test for details. See InsideRadiology: Nuclear Medicine for further information.

Take with you to your appointment any previous X-ray or radiology images you have, as comparison with these by the nuclear medicine physician (a specialist doctor), who looks at and interprets your PET scan, can be very helpful.

For an FDG PET-CT, you will be asked to not eat anything for several hours before the PET scan, because this may alter your sugar metabolism and may affect the quality of the images or pictures. Drinking water is both acceptable and advised so that you are not dehydrated. If you have diabetes, you will be provided with specific instructions and may need to stop taking some diabetes medications before having the scan.

You need to wear comfortable, loose clothing, and will generally be changed into a hospital gown. It is important that you are not wearing metal, including jewellery, watches, zips and bra hooks, as these can affect the quality of the images produced.

What happens during a PET scan?

After you arrive at the hospital or radiology facility, a nurse or nuclear medicine technologist will explain the procedure and prepare you for the PET scan. You will be asked to change into a gown. A small needle will be inserted into a vein, usually in your arm or the back of your hand, to fit an intravenous line (a thin plastic tube) through which the liquid radioactive material is injected. A brief medical history will be taken to ensure the optimal (or best) scanning method is used and to also help with subsequent image interpretation. Your blood sugar level will be checked, as high or low blood sugar levels can alter the appearance of the scan. The radioactive substance is then injected into your vein through the intravenous line.

If you are having an FDG-PET scan, you will be asked to rest quietly in a bed or arm chair, avoiding movement or talking for 90 minutes. During this time you will be alone. You may be asked to drink some contrast material that moves through your stomach and bowel, and helps to improve the quality of the images. Occasionally, depending on the medical condition or symptom, a catheter (a thin flexible tube) may be placed into your bladder to help improve image quality.

You will then be moved to the scanning room and positioned on the PET scanning bed. It is important to remain as still as possible during the scan, as movement can result in reduced image quality and the images may be blurry. If you are uncomfortable after being positioned on the bed, please tell the nurse or technologist.

If you are having a PET-CT, the CT scan is done first and takes less than 2 minutes. The PET scan takes approximately 15–20 minutes, but the time will vary depending on the areas of your body being scanned.

The intravenous line will be removed before you leave. You should drink plenty of fluids after the test is finished. This will flush the radioactive substance out of the body through the kidneys and into the bladder.

Are there any after effects of a PET scan?

A PET scan is a very safe and routine procedure. Millions of PET scans have been done around the world without complication. The substances used in PET scanning are not associated with any side effects, so you should feel no different after the scan. Unless your doctor tells you otherwise, you can resume normal activities after a PET scan.

If you are breast-feeding, see the “How do I prepare” section for more information about special precautions you may need to take.

How long does a PET scan take?

This will depend on the type of scan you are having, but you can expect to be in the PET imaging facility for between 2–3 hours. The time on the PET scanner is typically around 20 minutes, but time is also needed for preparation.

When the scan is completed, you will be asked to wait while the images are checked to make sure they are clear. Occasionally, there is a need to obtain more images after this check.

What are the risks of a PET scan?

Nuclear medicine procedures, including PET scanning, are very safe. The scan involves an injection of a very small amount of a radioactive material or tracer, which will only remain in your body for a few hours. The radiation dose you receive is equivalent to several years of natural background radiation from the normal environment. This small amount of additional radiation does not cause any side effects.

Some people experience claustrophobia (fear of being confined in a small space) when inside the scanner machine. If you have experienced claustrophobia in the past, please inform the technologist, nurse or doctor when you arrive, as they can take steps to minimise your feeling of claustrophobia. A mild sedative can be injected into your vein to relieve your anxiety and claustrophobia. This will allow the examination to be completed without any discomfort or movement. If you require some sedation, it is important that you arrange for someone to take you home after the test. Driving is not advisable.

Occasionally, other drugs will be given as part of a PET scan and any possible side effects will be discussed with you.

If there is any possibility that you are pregnant or if you are breast-feeding you should inform the nurse, technologist or doctor.

What are the benefits of a PET scan?

PET scanning is a powerful diagnostic test that is having a major impact on the diagnosis and treatment of disease. It provides unique information that may assist in making a diagnosis, making decisions about treatment or providing a prognosis, that is, the likely outcome of any disease.

Nuclear medicine tests, including PET scanning, can provide information on how tissue or organs are working, which cannot be obtained from other imaging techniques. PET scans may detect disease earlier than other types of scanning by identifying early changes to tissue and organs.

Who does the PET scan?

A nuclear medicine technologist operates the equipment during the procedure. The technologist, or a nurse, will also be involved in preparing you for the scan. A doctor who has received highly specialised training in nuclear medicine will oversee the procedure, interpret the images and provide your doctor with a report of the scan.

Where is a PET scan done?

PET scanners are usually located within public or private hospitals or private radiology practices with nuclear medicine facilities.

When can I expect the results of my PET scan?

A doctor who has received specialised training in nuclear medicine will interpret the images and forward a report to your referring doctor, who will provide you with the results.
The time it takes your doctor to receive a written report on the test or procedure will vary depending on:

• the urgency with which the results are required by your doctor;
• the complexity of the test or procedure;
• whether more information is needed from your doctor before the test or procedure can be interpreted by the nuclear medicine specialist;
• whether you have had previous X-rays or other medical imaging that need to be compared with this test or procedure (this is commonly the case if you have a disease or condition that is being assessed as to its progress);
• how the report is sent to your doctor (i.e. phone, email, fax or mail).

Please feel free to ask the private practice, clinic, or hospital when the written report will be provided to your doctor.

It is important that you discuss the results with your doctor, either in person or on the telephone, so that they can explain what the results mean for you.

*The author has no conflict of interest with this topic.

Page last modified on 5/3/2018.