A positron emission tomography (PET) scan is an imaging test that allows your doctor to check for diseases in your body.

The scan uses a special dye containing radioactive tracers. These tracers are either swallowed, inhaled, or injected into a vein in your arm depending on what part of the body is being examined. Certain organs and tissues then absorb the tracer.

When detected by a PET scanner, the tracers help your doctor to see how well your organs and tissues are working.

The tracer will collect in areas of higher chemical activity, which is helpful because certain tissues of the body, and certain diseases, have a higher level of chemical activity. These areas of disease will show up as bright spots on the PET scan.

The scan can measure blood flow, oxygen use, how your body uses sugar, and much more.

A PET scan is typically an outpatient procedure, which means you can go about your day after the test is finished.

In the United States, around 2 million PET scans are performed each year, according to Berkley Lab.

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PET scan machine Henry Westheim Photography / Alamy Stock Photo

Your doctor may order a PET scan to inspect your blood flow, your oxygen intake, or the metabolism of your organs and tissues. PET scans show problems at the cellular level, giving your doctor the best view of complex systemic diseases.

PET scans are most commonly used to detect:

Cancer

Cancer cells can have a higher metabolic rate than noncancerous cells. Because of this high level of chemical activity, cancer cells often show up as bright spots on PET scans. For this reason, PET scans are useful both for detecting cancer and for:

  • seeing if the cancer has spread
  • seeing if a cancer treatment like chemotherapy is working
  • checking for a cancer recurrence

However, these scans should be read carefully and explained by a doctor, as it’s possible for noncancerous conditions to look like cancer on a scan. It’s also common for solid tumors to fail to appear on PET scans.

Heart problems

PET scans can reveal areas of decreased blood flow in the heart. This is because healthy heart tissue tends to take in more of the tracer than unhealthy tissue or tissue that has decreased blood flow.

Different colors and degrees of brightness on the scan show different levels of tissue function. This information can help you and your doctor decide how to move forward with treatment.

Learn more about the heart PET scan.

Brain disorders

Glucose is the main fuel of the brain. During PET scans, tracers are “attached” to compounds such as glucose. By detecting radioactive glucose, the PET scan can show which areas of the brain are using glucose at the highest rates.

When a specialist interprets the scan, they can see how the brain is working and check for any irregularities.

Learn more about the brain PET scan.

PET scans are used to help diagnose and manage many CNS disorders, including:

PET scans show metabolic changes occurring at the cellular level in an organ or tissue. This is important because diseases often begin at the cellular level. CT scans and MRIs cannot reveal problems at the cellular level.

PET scans can detect very early changes in your cells. CT scans and MRIs can only detect changes later, as a disease alters the structure of your organs or tissues.

Detection of illness at the cellular level gives your doctor the best view of complex systemic diseases, such as:

Sometimes these tests are used along with PET scans to give doctors a clearer picture of the inside of your body. This is often called PET/CT or a PET/MRI scan. In this case, the tests are performed at the same time.

  • On its own, a CT scan uses special X-ray equipment to produce pictures of the inside of the body.
  • MRI scans use magnetic fields and radio frequency pulses to create images of internal structures such as organs, soft tissues, and bone.

When either of these scans is performed in conjunction with a PET scan, they result in what’s called image fusion. A computer combines the images from the two scans to create a three-dimensional image, which provides more information and allows for a more precise diagnosis.

Gallium scans are similar to PET scans. They involve the injection of gallium citrate, a radioactive tracer. Gallium scans are a multiday process and are typically performed 1 to 3 days after the tracer is administered.

These scans aren’t as commonly performed for the detection of cancer, though some forms of the gallium scan are combined with newer tests such as the PET scan.

The PET scan uses radioactive tracers, but the exposure to harmful radiation is minimal. The amount of radiation in the tracer is small, so the risks to your body are low. Tracers are considered radiopharmaceuticals and must meet Food and Drug Administration (FDA) standards for both safety and performance.

Still, it’s a good idea to discuss possible risks with your doctor.

The risks of the test are also minimal in comparison to how beneficial the results can be in diagnosing serious medical conditions.

The tracer is essentially glucose with the radioactive component attached. This makes it very easy for your body to eliminate the tracers naturally after the test, even if you have a history of kidney disease or diabetes.

People with allergies and other health conditions

It’s possible to have an allergic reaction to the tracer. People who are allergic to iodine, aspartame, or saccharin should let their doctor know.

Those who can’t have an iodine tracer typically receive a tracer made of diluted barium sweetened with saccharin.

Those most likely to have an allergic reaction to the iodine tracer include people with:

People who are pregnant

Radiation is not considered safe for developing fetuses. If you’re pregnant or think you may be pregnant, you should not get a PET scan.

People getting a PET/CT scan

If you’re receiving a PET/CT scan, additional tracer will be needed. This can be harmful to people who have kidney disease or who have elevated creatinine levels from medications they’re already taking.

Other risks

Other risks of the test include discomfort if you’re claustrophobic or uncomfortable with needles.

The injection may also lead to symptoms such as bleeding, bruising, or swelling.

Your doctor will provide complete instructions on how to prepare for your PET scan. Tell your doctor about any prescription, over-the-counter (OTC), or supplemental medications you’re taking.

A few days before

You may be asked to avoid strenuous physical activity, such as exercise, and deep-tissue massages in the 24 to 48 hours before the test.

The day before

In the 24 hours before your appointment, you’ll be asked to stick to a low carbohydrate, no sugar diet. Foods and beverages you should avoid include:

  • cereal
  • pasta
  • bread
  • rice
  • milk and yogurt, whether dairy or nondairy
  • fruit and fruit juices
  • alcohol
  • caffeinated beverages
  • candy, including chewing gum and mints

Foods you can eat include meat, tofu, nuts, and nonstarchy vegetables like carrots, asparagus, broccoli, salad greens, squash, and more.

Hours before

If you’re receiving anesthesia for the procedure, do not eat or drink anything the entire morning of your PET scan. Drink only a few sips of water if you need to take any medications.

If you’re not receiving anesthesia, you’ll still want to refrain from eating anything for 6 hours before your scan. Remember to avoid chewing gum or sucking on hard candy, cough drops, or mints.

You’ll be able to drink water, however, and take any medications as recommended.

You may be asked to change into a hospital gown. Because metal can interfere with the testing equipment, you’ll also need to remove any jewelry you’re wearing, including body-piercing jewelry.

If you’re undergoing a PET/CT scan, medical devices such as pacemakers and artificial hips will not affect your results.

However, you cannot undergo a PET/MRI scan with unapproved medical devices or metal implants.

Other considerations

You should also tell your doctor about any medical conditions you have, including:

  • If you’re pregnant or believe you could be pregnant, tell your doctor. This test may be unsafe for your baby.
  • If you’re breastfeeding, you may need to pump and store your breast milk 24 hours prior to the procedure — you will not be able to breastfeed for 24 hours after the test.
  • If you have diabetes, you’ll get special instructions for test preparation because fasting beforehand could affect your blood sugar levels. You’ll likely be told to take your normal dose of insulin and eat a light meal 4 hours before you’re scheduled to receive your scan.

Before the scan, you’ll get tracers through a vein in your arm, through a solution you drink, or in a gas you inhale. Your body needs time to absorb the tracers, so you’ll wait about an hour before the scan begins.

How long it takes for your body to fully absorb the tracer will depend on the area of the body being scanned.

While you wait, you’ll want to limit any movement, relax, and try to stay warm. If you’re undergoing a brain scan, you’ll want to avoid television, music, and reading.

Next, you’ll undergo the scan, which can last anywhere from 30 to 45 minutes. This involves lying on a narrow table attached to a PET machine, which looks like a giant letter “O.” The table glides slowly into the machine so that the scan can be conducted.

If you are undergoing multiple tests, this may add additional time, up to about 3 hours.

You’ll need to lie still during the scan. The technician will let you know when you need to remain still. You may be asked to hold your breath for several seconds. You’ll hear buzzing and clicking noises during the test.

When all the necessary images have been recorded, you’ll slide out of the machine. The test is then complete.

After the test, you can usually go about your day unless your doctor gives you other instructions.

However, because radioactive material can remain in your body for a few hours to days, you’ll want to limit your contact with both pregnant people and infants during this time.

Drink plenty of fluids after the test to help flush the tracers out of your system.

Meanwhile, a trained specialist will interpret the PET scan images and share the information with your doctor. The results are usually ready for your doctor within a few days. Your doctor will go over the results with you at your follow-up appointment.