What is a Positron Emission Tomography (PET) Scan? | Neurology

PET scan

Overview of positron emission tomography (PET) scan

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 the disease will show up as bright spots on the PET scan.

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

A PET scan is typically an outpatient procedure. This 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.

Why is a PET scan performed?

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
  • Heart problems
  • Brain disorders, including problems with the central nervous system (CNS)


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

  • Seeing if cancer has spread
  • Seeing if a cancer treatment is working
  • Checking for a cancer recurrence

However, these scans should be read carefully by your doctor, as noncancerous conditions can look like cancer on a scan. It’s also common for solid tumours to fail to appear on PET scans.

Heart problems

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

Different colours and degrees of brightness on the scan will indicate different levels of tissue function, helping you and your doctor decide how best to move forward.

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 detect which areas of the brain are utilizing glucose at the highest rates.

Your doctor will look at the scan to see how the brain is working and to check for any abnormalities. PET scans are used to help diagnose and manage many central nervous system (CNS) disorders, including:

  • Alzheimer’s disease
  • Depression
  • Epilepsy
  • Head trauma
  • Parkinson’s disease

How does the PET scan compare to other tests?

PET scans show metabolic changes occurring at the cellular level in an organ or tissue. This is important because the disease often begins at the cellular level. CT scans and MRIs can’t 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:

  • Coronary artery disease (CAD)
  • Brain tumours
  • Memory disorders
  • Seizure disorders

What risks are involved with a PET scan?

The PET scan involves radioactive tracers, but exposure to harmful radiation is minimal. The amount of radiation in the tracer is small, so the risks to your body are low. 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, even if you have a history of kidney disease or diabetes.

How do you prepare for a PET scan?

Your doctor will give you complete instructions on how to prepare for your pet’s scan. Tell your doctor about any prescription, over-the-counter (OTC), or over-the-counter medications you are taking.

A few days before

You may be asked to refrain from strenuous physical activity, such as exercise, in the 24 to 48 hours preceding the test.

The day before

Twenty-four hours before your appointment, you will be asked to follow a low-carbohydrate, sugar-free diet. Foods and drinks to avoid:

  • Grain
  • Pasta
  • Bread
  • Rice
  • Milk and yoghurt, whether dairy or non-dairy
  • Fruit and fruit juices
  • Alcohol
  • Caffeinated drinks
  • Confectionery products, including gum and mint

Hours before

If you’re receiving anaesthesia for the procedure, don’t 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 anaesthesia, you’ll still want to refrain from eating anything for six 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 jewellery you’re wearing, including body-piercing jewellery.

If you’re undergoing a PET–CT, medical devices such as pacemakers and artificial hips won’t affect your results.

However, you cannot undergo a PET–MRI with nonapproved medical devices or metal implants.

How is a PET scan performed?

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.

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.

What happens after a PET scan?

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

However, because radioactive material will remain in your body for about 12 hours, you’ll want to limit your contact with both pregnant women and infants during this time.

Drink plenty of fluids after the test to help flush the tracers out of your system. Generally, all tracers leave your body after two days.

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 two business days, and your doctor will go over the results with you at your follow-up appointment.

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