A positron emission tomography (PET) scan is an imaging test that uses a radioactive substance called a tracer to look for disease in the body.
A PET scan shows how organs and tissues are working. This is different than magnetic resonance imaging (MRI) and computed tomography (CT), which show the structure of and blood flow to and from organs.
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A PET SCAN CAN HELP TO
Show up a cancer
Find out the stage of a cancer
Show whether a lump is cancer or not
Show whether a cancer has spread to other parts of the body
Decide the best treatment for your cancer
Show how well cancer drug treatment is working
Show the difference between scar tissue and active cancer tissue
After you have had treatment for cancer, a CT scan may show that there are still some signs of the cancer left. But this may not be active disease. It could be scar tissue left over from cancer killed off by your treatment. A PET scan can show whether this tissue is active cancer or not.
In lung cancer, PET scans are sometimes used to look for cancer in the lymph nodes in the centre of the chest. Or to show whether the cancer has spread to other areas. This can help your specialist to decide whether it is possible to remove the cancer with surgery.
HOW THE TEST IS PERFORMED
A PET scan uses a small amount of radioactive material (tracer). The tracer is given through a vein (IV), most often on the inside of your elbow. The tracer travels through your blood and collects in organs and tissues. This helps the radiologist see certain areas of concern more clearly.
You will need to wait nearby as the tracer is absorbed by your body. This takes about 1 hour.
Then, you will lie on a narrow table that slides into a large tunnel-shaped scanner. The PET detects signals from the tracer. A computer changes the signals into 3-D pictures. The images are displayed on a monitor for your doctor to read.
You must lie still during test. Too much movement can blur images and cause errors.
How long the test takes depends on what part of the body is being scanned.
HOW TO PREPARE FOR THE TEST
You may be asked not to eat anything for 4 – 6 hours before the scan. You will be able to drink water.
Tell your health care provider if:
You are afraid of close spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious.
You are pregnant or think you might be pregnant.
You have any allergies to injected dye (contrast).
Always tell your health care provider about the medicines you are taking, including those bought without a prescription. Sometimes, medications may interfere with the test results.
HOW THE TEST WILL FEEL
You may feel a sharp sting when the needle with the tracer is placed into your vein.
A PET scan causes no pain. The table may be hard or cold, but you can request a blanket or pillow.
An intercom in the room allows you to speak to someone at any time.
There is no recovery time, unless you were given a medicine to relax.
WHY THE TEST IS PERFORMED
A PET scan can reveal the size, shape, position, and some function of organs.
This test can be used to:
Check brain function
Diagnose cancer, heart problems, and brain disorders
See how far cancer has spread
Show areas in which there is poor blood flow to the heart
Several PET scans may be taken over time to check how well you are responding to treatment for cancer or another illness.
A normal result means there were no problems seen in the size, shape, or position of an organ. There are no areas in which the tracer has abnormally collected.
WHAT ABNORMAL RESULTS MEAN
Abnormal results depend on the part of the body being studied. Abnormal results may be due to:
Change in the size, shape, or position of an organ
Problem with organ function
The amount of radiation used in a PET scan about the same amount as for most CT scans. Short-lived tracers are used so the radiation is gone from your body in about 2-10 hours.
Tell your doctor before having this test if you are pregnant or breast feeding. Infants and babies developing in the womb are more sensitive to radiation because their organs are still growing.
Rarely, people may have an allergic reaction to the tracer material. Some people have pain, redness, or swelling at the injection site.
It is possible to have false results on a PET scan. Blood sugar or insulin levels may affect the test results in people with diabetes.
Most PET scans are now performed along with a CT scan. This combination scan is called a PET/CT.