Positron emission tomography

Positron emission tomography (PET) is a special way to see inside the body. It uses tiny amounts of special substances called radiotracers that give off a kind of energy. These substances help doctors see how different parts of the body are working, such as how cells are using energy or how blood is flowing.

PET is a common tool in hospitals. A special kind of medicine called a radiopharmaceutical is put into the body. This medicine has a tiny bit of a radioactive material attached to it. When this material breaks down, it gives off very small particles. These particles then bump into other tiny parts in the body, creating pairs of energy waves that go in opposite directions. Special cameras can pick up these waves and use them to make a three-dimensional picture of what is happening inside.

PET scanners are sometimes combined with CT scanners, which are good at making detailed pictures of structures in the body. When used together, they are called PET-CT scanners. One downside of PET scanning is that the machines can be very expensive to buy and keep running.

Uses

PET is a tool used in both medicine and research. It helps doctors look at tumors and find out if cancer has spread. It is also used to study diseases of the brain, such as different types of dementia.

In research, PET helps scientists understand how the brain and heart work normally. It is also used in studies with animals, allowing scientists to do more tests without needing many animals.

Physiological processes in the body often lead to changes in anatomy. PET can detect biochemical activity and the presence of certain proteins before these anatomical changes become visible. This is achieved by using radiolabelled molecular probes, which are taken up at different rates depending on the tissue. The regional uptake of these tracers in different anatomical structures can then be visualized and approximately quantified based on the amount of injected positron-emitting tracer detected in the scan.

PET can also be done with a special camera fitted with a coincidence detector, though the quality of these scans is lower and they take longer.

Other methods of medical imaging include single-photon emission computed tomography (SPECT), computed tomography (CT), magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI), and ultrasound. SPECT is similar to PET but uses different materials to detect molecules in the body.

Oncology

PET scanning with a special material called FDG is commonly used in cancer care. FDG is taken up by cells that use a lot of glucose, such as cancer cells. This helps doctors see if cancer has spread to other parts of the body. FDG PET scans are the most common type of scan used in medical care.

Neuroimaging

PET can also be used to study the brain. For example, it can measure blood flow to the brain and help diagnose Alzheimer’s disease. It can also be used to find the area of the brain that causes seizures.

Cardiology

PET is used in heart care to check blood flow to the heart and diagnose conditions like coronary artery disease.

Infectious diseases

PET can help doctors see infections in the body by using special materials that light up areas where there is inflammation.

Bio-distribution studies

In research, PET is used to study how new drugs move through the body in animals, without needing to harm the animals.

Small animal imaging

Special small PET scanners have been made to study animals like rats and mice without putting them to sleep.

Musculo-skeletal imaging

PET can also be used to study muscles and bones. For muscles, it helps see which ones are working during exercise. For bones, it helps measure bone metabolism and blood flow.

Safety

PET scanning is a safe way to look inside the body, but it does use a tiny bit of radiation. This radiation helps doctors see how the body is working. The amount of radiation used is very small and is similar to what some people get naturally from the environment in a year.

When PET scanning is done together with a CT scan, the radiation amount can be a little higher. But even then, it is still carefully controlled to keep it as safe as possible. Doctors use these scans to help find and manage illnesses like cancer.

Operation

Radionuclides and radiotracers

Main article: List of PET radiotracers

The radioactive substance used in PET scanning is a simple sugar called fluorodeoxyglucose (FDG). Many different tracers can be used for research or clinical work, depending on what part of the body is being studied, such as:

• Fluorodeoxyglucose (¹⁸FFDG or FDG) is commonly used to detect cancer, as cancer cells need sugar to grow.
• [¹⁸F]Sodium fluoride (Na¹⁸F) is widely used for detecting bone formation.
• Oxygen-15 (¹⁵O)-water is used to measure blood flow in the heart.
• Carbon-11 (¹¹C)-methionine is used to image brain tumors.

Radionuclides are added to compounds that the body uses normally, such as glucose (or glucose-like substances), water, or ammonia. These labeled compounds are known as radiotracers. PET technology can trace the path of any compound in living humans (and many other animals) if it can be labeled with a PET isotope. As of this writing, there are already dozens of radiotracers in clinical use and hundreds used in research. In 2020, the most commonly used radiotracer in clinical PET scanning was FDG. This radiotracer is used in almost all scans for cancer and most scans for the brain, making up over 95% of radiotracers used in PET and PET–CT scanning.

Emission

To do the scan, a short-lived radioactive tracer isotope is injected into the person (usually into the bloodstream). Each tracer atom has been chemically added to a molecule that the body uses normally. There is a waiting period while this molecule builds up in the tissues being studied. Then the person is placed in the imaging scanner. The molecule most often used is FDG, a sugar, and the waiting period is usually about an hour. During the scan, a record of how much of the tracer is in the tissues is made as the tracer breaks down.

As the radioisotope breaks down, it releases a positron, a particle that is like the opposite of an electron. The positron travels a short distance in the tissue before it meets an electron. When they meet, they disappear and produce a pair of annihilation (gamma) photons moving in almost opposite directions. These are detected by a device in the scanner, creating a flash of light that is recorded.

Combination of PET with CT or MRI

Main articles: PET–CT and PET–MRI

PET scans are often looked at together with CT or MRI scans. Combining them gives both details about the structure and what that structure is doing. Because PET imaging works best when combined with pictures of the body’s structure, modern PET scanners often include high-end CT scanners (PET–CT). Because the two scans can be done one after the other with the person staying in the same spot, the pictures line up better. This helps show detailed views of moving parts or areas that change shape more clearly, which is common outside the brain.

At the Jülich Institute of Neurosciences and Biophysics, the world’s largest PET–MRI device started working in April 2009. It is a 9.4-tesla magnetic resonance tomograph (MRT) combined with a PET scanner. Right now, only the head and brain can be imaged at these strong magnetic fields.

For brain imaging, CT, MRI, and PET scans can be lined up without needing a combined PET-CT or PET-MRI scanner by using a device called the N-localizer.

History

The idea of using special cameras to see inside the body was first suggested by David E. Kuhl, Luke Chapman, and Roy Edwards in the late 1950s. They worked at Washington University School of Medicine and later at the University of Pennsylvania. In the 1960s and 1970s, more people like Michel Ter-Pogossian, Michael E. Phelps, and Edward J. Hoffman helped improve these imaging tools at the same university.

Scientists at Massachusetts General Hospital, such as Gordon Brownell and Charles Burnham, also helped develop this technology. They showed how special light could be used to see inside the body. In 1961, James Robertson and his team created the first machine to scan the head, called the “head-shrinker.”

A big step forward came when scientists created a special sugar called FDG. This sugar could light up areas of the body that were active. It was first used in tests in 1976 at the University of Pennsylvania. Later, new machines were built to use this sugar better.

Over time, scientists designed machines with circles of detectors around the body. James Robertson and Zang-Hee Cho were the first to suggest this circle design, which is still used today. In 2000, a new machine combining PET and CT scanning was named one of the best medical inventions of the year.

Cost

In Canada, PET scans in Ontario are free for patients as of October 2025 through the PET Scans Ontario Program. This program pays for routine PET scans when they are proven to help patients more than other imaging tests.

In the United States, the cost of PET scans changes depending on where you live, the type of facility, and your insurance. For example, Medicare data from 2026 shows that a whole-body PET scan costs about $872 in total, with patients paying around $174 after Medicare help.

In the United Kingdom, the National Health Service offers PET-CT scans for free to eligible patients. Australia also has Medicare subsidies to help cover the cost of eligible PET services.

Quality control

The performance of PET systems can be checked using special tools, one of which is called the Jaszczak phantom.