Positron emission tomography is an imaging technique that uses radiation to show activity within the body on a cellular level. People more commonly refer to it as a PET scan.

PET scans help a doctor diagnose certain health conditions, plan treatment, find out how an existing condition is developing, or check the effectiveness of a treatment. Doctors commonly use PET scans for people who are undergoing cancer treatment and those who may have neurological or radiological problems.

Please keep reading to learn more about PET scans and how they work.

A PET scan is a test that can show multidimensional, color images of the inside workings of the human body.

It shows not only what an organ looks like, but also how it is functioning.

Doctors may use a PET scan in combination with a CT scan or MRI scan to confirm a diagnosis for an individual.

In a PET scan, a machine detects the radiation that a radiotracer emits. A radiotracer consists of radioactive material tagged to a natural chemical, such as glucose.

The radiologist injects this radiotracer into the person’s body, where it travels to cells that use glucose for energy. The more energy a group of cells needs, the more the radiotracer will build up in that location. This will show up on images that a computer reconstructs.

The cells, or activity, will show up as “hot spots” or “cold spots.” Active areas are bright on a PET scan. They are known as “hot spots.” Where cells need less energy, the areas will be less bright. These are “cold spots.”

Compared with healthy cells, cancer cells are very active in using glucose, so a radiotracer made with glucose will light up areas of cancer. The radiologist will examine the image that the computer produces and report the findings to a doctor.

An example of a glucose-based radiotracer is fluorodeoxyglucose (FDG), in which radioactive fluoride molecules are tagged to glucose to make a radiotracer. FDG is the most commonly used radiotracer today. Oxygen can also work in place of glucose.

While an MRI or CT scan shows how part of the body looks, a PET scan can reveal how it is functioning.

Doctors commonly use PET scans to investigate a number of conditions. These include:

  • Epilepsy: A PET scan can reveal which part of the brain epilepsy is affecting. This can help doctors decide on the most suitable treatment, and it can be useful if surgery is necessary.
  • Alzheimer’s disease: PET scans can help diagnose Alzheimer’s disease by measuring the uptake of sugar in specific parts of the brain. Affected brain cells tend to have reduced use of glucose compared with healthy cells.
  • Cancer: PET scans can reveal the presence and stage of a cancer, show whether and where it has spread, and help doctors decide on treatment. A PET scan can give an idea of how well chemotherapy is working, and it can detect a recurring tumor sooner than other techniques.
  • Heart disease: A PET scan can help detect which parts of the heart have become damaged or scarred, and it can help identify circulation problems in the working of the heart. This information can help doctors plan treatment options for heart disease.
  • Medical research: Researchers can learn vital information, especially about the workings of the brain, by using PET scans.

A CT or MRI scan can assess the size and shape of body organs and tissue, but neither type can show how these are functioning.

A PET scan can show how an organ works, but without a CT or MRI image, it can be difficult to pinpoint the exact location of activity within the body. For this reason, a person may need to do two or more of these tests.

A PET scan is usually an outpatient procedure. Typically, the person should not consume any food for at least 4-6 hours before the scan, but they should drink plenty of water. They may have to avoid caffeine for at least 24 hours before the scan.

In most cases, the doctor will inject a small amount of radiotracer into a vein. However, it is also possible to breathe in the tracer as a gas, take it by mouth, or receive an injection directly into an organ.

Depending on which organ is involved, it may take 30–90 minutes for the radiotracer to reach the targeted part of the body.

The scan will take place in a special designated room, where the individual will lie down on a cushioned examination table. The doctor will ask the individual to stay still and not talk, and they might offer the person medication to relax. The person will probably need to wear a gown, and they may have to remove any jewelry.

The table slides into a large hole to surround the person with the machine. During the scan, the machine takes images.

Depending on which part of the body the machine needs to scan, a PET scan should take about 30–60 minutes.

The scan is not painful. If the person feels unwell, they can press a buzzer to alert the staff. A qualified practitioner will watch them during the scan.

Most people can go home as soon as the scan is finished. It is important to consume plenty of liquid to flush the radioactive drugs out of the body’s system more quickly.

As a PET scan uses radioactive material, there is a risk of radiation exposure.

Although for most people, the benefits of having a PET scan outweigh the risks, it is not suitable for everyone. For instance, a pregnant person should not have a PET scan, as the radioactive material may affect the fetus.

If a person is nursing, they should follow directions from the doctor on when it is safe to resume breastfeeding or pumping. They may need to wait until any radioactive tracers from the test leave their milk, which may take about 1–4 hours.

Anyone who is pregnant or breastfeeding should tell their doctor before having a PET scan.

Following a PET scan, a person may need to stay away from pregnant people, infants, and young children for a few hours, as the radioactivity poses a small risk to these individuals. Very rarely, an individual may have an allergic reaction to the tracer.

A PET scan is a radiation-based test that can help a doctor see the inside of a person’s body and assist in the diagnosis of a variety of conditions.

Although an MRI or CT scan can show the size and shape of structures, as well as the location of activity in the body, a PET scan can show a doctor how a person’s organs are actually working.

Often, a doctor will ask a person to undergo both a PET scan and an MRI or CT scan to get a more thorough overview of the suspected problem.