What is the purpose of a PET scan and how does it differ from a CT or MRI?

PET scan measures metabolic activity by detecting a radioactive tracer, revealing how tissues function, while CT and MRI show structural anatomy. PET is especially useful for locating cancer spread, assessing heart muscle viability, and detecting early brain disorders.

How should I prepare for a PET scan, and are there any dietary restrictions?

Typically, you should fast for 4–6 hours before the scan, avoid strenuous exercise, and drink plenty of water. Inform your doctor about any medications, especially those that affect glucose levels, as they may need to be adjusted.

How long does a PET scan take, and what will I experience during the procedure?

The entire process takes about 30–60 minutes. After the tracer injection, you will lie still on a table while the scanner captures images. You may feel a brief injection sensation, but the scan itself is painless.===

What are the risks of the radioactive tracer used in a PET scan?

The tracer delivers a low dose of radiation, comparable to a few chest X‑rays. It is generally safe for most adults, but pregnant or breastfeeding women should discuss alternatives. Side effects are rare and usually mild, such as a brief feeling of warmth.

PET Scan

A PET scan is a nuclear imaging test that uses a radioactive tracer to visualize how your tissues and organs are functioning and to detect early signs of disease.

Overview

A PET (positron emission tomography) scan is a nuclear imaging test that looks at how well your tissues and organs are working. It also checks for signs of cancer. For the scan, you will get an injection of a small amount of a tracer, which is a safe radioactive substance.

The tracer settles in areas of your body that have high levels of chemical activity. This activity can be a sign of cancer or other diseases. The tracer can also help measure blood flow, oxygen use, and changes in the metabolism of a particular tissue or organ. Metabolism is the process your body uses to make energy from the food you eat.

Chemical changes in your body can show up before you even have symptoms of a disease. Since a PET scan can see how your organs are working in real-time, it can find signs of disease at an early stage, often before the disease can be seen on other imaging tests.

What is it used for?

A PET scan is most often used to:

Diagnose or monitor certain cancers, including breast, thyroid, and lung cancers
Find out how well your heart muscle is working
Check blood flow to your heart
Check for signs of certain brain disorders, such as Parkinson's disease, Huntington's disease, Alzheimer's disease, and other types of dementia

A PET scan is often done along with a CT scan, a type of x-ray that takes a series of pictures as it rotates around you.

Why do I need a PET scan?

You may need a PET scan to find out if you have cancer. If you've already been diagnosed with cancer, you may need this test to see if your cancer treatment is working. You may also need this test if you:

Have a family history of heart disease.
Are being treated for heart disease. This test can show if the treatment is working.
Have symptoms of a brain disease, such as tremors, memory problems, and/or seizures.

What happens during a PET scan?

Before the scan, you will change into a hospital gown. You may be asked to empty your bladder. During the scan:

A health care provider will inject the radioactive tracer into your vein through an intravenous (IV) line. The tracer sends out a form of energy called gamma rays. The scanner picks up the rays to create images of the inside of your body.
You will need to wait for your body to absorb the tracer, which takes 45 to 60 minutes.
You will then lie on a narrow, padded table, which will slide into a large, donut-shaped scanner.
The scanner will move slowly across your body to capture images. As this happens, you will need to be very still for about 30 minutes.
You will hear clicking or buzzing noises as the images are taken.
The scanner will send images to a computer monitor for your provider to review.
Your provider will then remove the IV line.

The test takes about two hours to complete, including the time you wait for the tracer to be absorbed by your body.

Preparation

Talk to your provider about how to prepare for the test:

You may be asked to fast (not eat or drink) for four to six hours before the test.
You may be asked to avoid tobacco products and food or drinks with caffeine or alcohol for 24 hours before the test.
If you have diabetes and use insulin, you may need to change the timing of your regular dose. Your provider will give you specific instructions about adjusting your insulin since people with diabetes may not absorb the sugar in the tracer.
You may need to stop taking other medicines or supplements before the test, so tell your provider about everything you take.
Tell your provider if you have claustrophobia (fear of enclosed spaces). Your provider may decide to give you a medicine to help you relax.

Risks

There is very little exposure to radiation in a PET scan. The radioactive substance will leave your body through your urine and stool within 2 to 10 hours. Drink more water after the test to help remove the remaining tracer.

Radiation exposure can be harmful to a developing baby, so tell your provider if you are pregnant or think you may be pregnant. Also, tell your provider if you are breastfeeding, since the tracer may contaminate your breast milk. Allergic reactions to the tracer are rare and usually mild.

What do the results mean?

Depending on which part of the body was being scanned, your results may show signs of:

Cancer: Cancer cells show up as bright spots on a PET scan.
Heart disease: The scan can show decreased blood flow to the heart.
A brain disorder: The scan may show changes in certain brain chemicals that can be a sign of a disease.

Some cancers don’t show up on a PET scan, so your provider may order additional tests. If you also had a CT scan, your provider will review the results of both to help make a diagnosis.