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Positron emission tomography (PET) scan
A PET 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 normal and abnormal biological function of cells and organs.
PET uses a radiopharmaceutical made up of a radioactive isotope attached to a natural body compound, usually glucose. The radiopharmaceutical concentrates in certain areas of the body and 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 analyzes 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.
PET scan machines are expensive to buy and operate, so they are not readily available. This test is only available at a very limited number of centres in Canada.
Why a PET scan is done
A PET scan may be done to:
- diagnose cancer (in certain cases)
- determine the stage (find out how far the cancer has spread and if it is present in other organs and tissues)
- PET scan is used if other imaging tests are unclear, inconclusive or surgical procedures are not possible.
- find out if cancer treatment is working
- check if cancer has come back (recurred) after treatment or spread to other locations
- diagnose nervous system and cardiovascular diseases
How a PET scan is done
A PET scan is usually done as an outpatient procedure in the nuclear medicine department of a hospital or specialized PET scan centre. The test takes 45 minutes to 2 hours depending on whether a single organ or the whole body is scanned.
- It is important for a woman to tell the nuclear medicine staff if she is breast-feeding, pregnant or thinks she may be pregnant before having any nuclear medicine test.
Some special preparation is required before the scan.
- The person usually fasts for 4–6 hours before the test:
- 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.
- A person 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.
- Fasting decreases the use of glucose by organs.
- A person 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 takes 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.
- They may feel slight pressure or tugging during injection of the isotope.
- The person is asked to drink at least 2 glasses of fluids after the injection and before the scan.
- The person 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.
- An empty bladder is less active and gives a clearer image.
About 1 hour after the injection, the PET scan is done.
- 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 (feces). 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.
What the results mean
PET scans detect areas of activity (metabolism or cell growth) in the body.
- More FDG collects in cancer cells than normal cells because cancer cells use (metabolize) more glucose.
- Cancerous tumours are usually more active, have a higher metabolic rate than normal tissue, and appear differently on a PET scan.
Further investigation may be necessary to rule out other abnormalities.
- Researchers are still studying the use of PET scans in cancer detection and staging.
- PET scans cannot tell the difference between a cancerous (malignant) tumour and a non-cancerous (benign) tumour.
- PET scans may not detect small tumours (less than 8 mm) and less aggressive tumours.
- PET scans are often combined with prior bone scan, x-ray, CT or MR imaging and laboratory test results.
- Combined PET/CT scanning joins computed tomography (CT) scan and PET scan into one procedure.
- It may provide a more complete image of a tumour’s location, growth or spread than either test alone.
What happens if a change or abnormality is found
The doctor will decide whether further tests, procedures, follow-up care or additional treatment are needed.
Special considerations for children
Being prepared for a test or procedure can reduce anxiety, increase cooperation and help the 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.
- Check with the doctor to find out if food or liquids are restricted.
- Explain to children that when the radiopharmaceutical is given they will feel:
- a sharp prick when the needle is inserted
- slight pressure or tugging when the radiopharmaceutical is injected
- Some children may find it difficult if they have to lie still during the test.
- Some children may need sedation to be able to lie still for the whole test.
- Let children know that they might hear a clicking noise during the scan.
- Some children feel closed-in when the scanner passes over their body.
- Let them know that the camera may touch their skin, but will not hurt.
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 the child uses it.
- Place diapers in the outside garbage.
The preparation for a PET scan depends on the age and experience of the child. Find out more age-specific information on helping children cope with tests and treatment.
Great progress has been made
Some cancers, such as thyroid and testicular, have survival rates of over 90%. Other cancers, such as pancreatic, brain and esophageal, continue to have very low survival rates.