A PET scan, or Positron Emission Tomography Scan, is a nuclear medicine imaging test that uses a form of radioactive sugar to create images of body function and metabolism. PET imaging can be used to evaluate both normal and abnormal biological function of cells and organs.
PET uses a radiopharmaceutical made up of a radioactive isotope or tracer attached to a natural body compound, usually glucose. The tracer travels to places in the body where there is tumour activity and concentrates there, where it is detected by the PET scanner.
The PET scanner is made up of a circular arrangement of detectors. These detectors pick up the pattern of radioactivity from the radiopharmaceutical in the body. A computer analyses the patterns and creates 3-dimensional colour images of the area being scanned. Different colours or degrees of brightness on a PET image represent different levels of tissue or organ function.
Why a PET Scan is Done
A PET scan may be done to diagnose cancer (in certain cases) and determine the stage (find out how far the cancer has spread and if it is present in other organs and tissues).
A PET scan is used if other imaging tests are unclear, inconclusive or surgical procedures are not possible, to find out if cancer treatment is working, or to check if cancer has come back (recurred) after treatment or spread to other locations.
A PET scan can also diagnose nervous system and cardiovascular diseases.
Performing the Procedure
A PET scan is usually done as an outpatient procedure in the nuclear medicine department of a hospital or specialised PET scan centre. The test takes 45 minutes to 2 hours depending on whether a single organ or the whole body is scanned.
Some special preparation is required before the scan; the person usually fasts for 4–6 hours before the test because fasting decreases the use of glucose by organs. Special instructions about diet and medications are often given to people with diabetes or abnormal blood sugar levels. Blood sugar levels are checked before the radiopharmaceutical is given.
One may have to avoid certain medications, caffeine, tobacco and alcohol because these substances can affect the results. Talk to your doctor or nuclear medicine department about what you should avoid.
An individual booked to have a scan may be asked to wear clothing that has no metal zippers, belts or buttons. They may be asked to change into a gown during the test and remove glasses, jewellery or objects that could interfere with the test.
The doctor will take a history of recent surgery, biopsy, therapy (such as chemotherapy or radiation therapy), current symptoms and physical findings.
A radiopharmaceutical is injected into a vein in the hand or arm vein. FDG is the radiopharmaceutical given most often, although others may be used. The radiopharmaceutical contains a small amount of a radioactive isotope mixed with glucose (a form of sugar). Tissues in the body absorb varying amounts of the radiopharmaceutical, depending on their rate of metabolic activity (growth rate).
The person will feel some discomfort from the intravenous injection; they will feel a sharp prick when the needle is inserted and may feel slight pressure or tugging during injection of the isotope.
The patient may be asked to drink at least 2 glasses of fluids after the injection and before the scan and should urinate as often as necessary and empty their bladder just before the scan. Depending on the area being studied, a urinary bladder catheter or medication (diuretic) may be used to help get rid of urine because an empty bladder is less active and gives a clearer image.
The PET scan is done approximately 1 hour after the injection has been given.
The PET scanner looks like a large, doughnut-shaped scanner, similar to a CT scanner. The person sits or lies down on the exam table and is asked to stay very still. Sometimes PET scan can be done along with CT or MRI.
After the scan, the radioactive material quickly loses its radioactivity. It passes out of the body through the urine or stool (faeces). Depending on the type of radiopharmaceutical used, it may take a few hours or days to completely pass out of the body.
Drinking fluids after the procedure helps flush the radiopharmaceutical from the body. Special instructions may be given to take special precautions after urinating, to flush the toilet twice and to wash the hands thoroughly.
Potential Side Effects
The dose of x-rays or radioactive materials used in nuclear medicine imaging can vary widely. Dose depends on the type of procedure and body part being examined. In general, the dose of radiopharmaceutical given is small and people are exposed to low levels of radiation during the test. The potential health risks from radiation exposure are low compared with the potential benefits. There are no known long-term adverse effects from such low-dose exposure.
Some potential side effects that might occur include:
- Bleeding, soreness or swelling may develop at injection site.
- Allergic reactions to the radiopharmaceutical may occur, but are extremely rare.
Special Considerations for Children
Being prepared for a test or procedure can reduce anxiety, increase cooperation and help your child develop coping skills. Parents or caregivers can help prepare children by explaining to them what will happen, including what they will see, feel and hear during the test.
Explain to your child that when the radiopharmaceutical is given they will feel a sharp prick when the needle is inserted then some slight pressure or tugging when the radiopharmaceutical is injected
Children need to lie still on the exam table during the scan, which may be unpleasant for them. Some children may need sedation to lie still for the whole test. Some children may like to hold a special toy or blanket during the scan or listen to music or a story during the scan.
You can stay with your child while they have their scan to calm them and help them lie very still. Pregnant women cannot stay in the room during the scan.
If your child feels closed-in when the scanner passes over their body, reassure them that the scan does not hurt and will not last very long.
Instructions may be given for special precautions that need to be taken when caring for children during the first 6–24 hours after the test:
- If the caregiver is pregnant, someone else should do most of the child care.
- Wear disposable, waterproof gloves when handling the child’s urine, stool or vomit, including diaper changes.
- Change sheets or clothing that has vomit, urine or stool smears on it. Wear disposable, waterproof gloves when handling sheets or clothing. Sheets and clothing can be washed in the regular laundry.
- Flush the toilet immediately after use by the child.
- Place diapers in the outside garbage.