Positron Emission Tomography (PET)

Overview

Nuclear medicine is a medical specialty that uses small amounts of radioactive material to check how the body is working, and to help diagnose and treat diseases. PET and SPECT scans are the two most common types of tests used in nuclear medicine. Both are painless, and both use only a small amount of injected radioactivity, much less than you might think.

PET and SPECT scans are different from tests like computed tomography (CT) or magnetic resonance imaging (MRI) scans, which mostly show what organs and tissues look like. Instead, PET and SPECT show how organs and tissues are actually working. Sometimes a PET or SPECT scan is combined with a CT scan in one machine (called a PET/CT or SPECT/CT scanner). These combo scans are now the main way doctors figure out how advanced a cancer is, all over the world. 

How PET Scans Work

PET scans use a special substance called a radioactive tracer, which is injected into the body. This tracer helps create detailed, 3-dimensional (3D) pictures of organs and how they're functioning at a molecular level. Most of the time, the tracer is given through an IV (a needle in a vein), but sometimes it can be breathed in, swallowed, or injected directly into a specific area of the body. SPECT scans work in a similar way, also using radioactive tracers.

A radioactive tracer is made of one radioactive atom attached to a "carrier" molecule. The carrier molecule can be different depending on what part of the body or what kind of metabolic activity the doctor wants to look at. When these tracers are officially approved by the FDA for medical use, they're called radiopharmaceuticals, meaning they've passed strict safety and quality tests. A nuclear medicine doctor picks whichever tracer will give the clearest and most useful information for that patient's specific health question.

The main difference between PET and SPECT is the type of radioactivity each one uses. SPECT tracers give off gamma rays directly as they break down. PET tracers give off positrons instead, tiny particles that have the same mass as electrons, but with an opposite electrical charge. When a positron meets an electron in the body, they destroy each other and turn into two gamma-ray photons that fly off in opposite directions. No matter which specific PET tracer is used, the photons it creates are always the same type. The PET scanner's detectors pick up these photons and use them to build images of what's going on inside your organs.

You can watch a short video about how PET scans work:

What Happens During a PET Scan

Uses for PET Scans in Diagnosis and Treatment

Cancer Detection and FDG

One of the biggest uses of PET scans is finding cancer and tracking how it changes over time, whether it's spreading, or how well it's responding to treatment. Cancer cells divide and grow quickly, and this requires a lot of energy from glucose (a type of sugar). Because of this, cancer cells tend to absorb much more glucose than normal cells; and generally, the more aggressive the cancer, the more glucose it uses. Scientists created a modified, radioactive version of glucose called FDG (fluorodeoxyglucose), which has become the most widely used tracer for finding cancer and detecting whether and where it has spread.

Nuclear Medicine Therapy (Radiotheranostics)

Radioactive tracers aren't just used to diagnose problems, they can also be used to treat them, similar to how regular (non-radioactive) medicines treat everything from headaches to cancer. Here's how it typically works: doctors first give a very small dose of a radioactive tracer and take images to see how and where the body absorbs it. If it looks like the tracer is being taken up the right way, the patient is then given a much larger dose of that same tracer (or a closely related version, called its "radiotheranostic pair") so the radiation itself can treat the condition. This treatment version is almost identical to the diagnostic imaging version, except for the exact type of radioactive atom used. A medical physicist can use the earlier diagnostic images to calculate exactly how much radiation dose different organs will receive, a process called dosimetry. After treatment, doctors can do more low-dose scans to see how well the patient is responding to the therapy. 

Radiation Safety

The amount of radiation used in most nuclear medicine scans is similar to a routine chest X-ray, or even less than what you'd get from or CT scan. There is some concern that repeated radiation exposure from medical scans over time could slightly raise cancer risk, but this risk is very small compared to the benefits of getting a needed diagnostic scan. It's worth noting that radiotheranostic treatments purposely use larger doses of radioactive tracers, aimed at the affected area, in order to treat the disease.

Just like radiologists, nuclear medicine doctors work hard to keep radiation exposure as low as possible while still getting a scan that's clear enough to be useful.

Find Clinical Trials

Clinical trials uncover better ways to prevent, diagnose, treat, and understand diseases and conditions. To ensure results apply to everyone, volunteers of all ages, sexes, and backgrounds, including both healthy individuals and those with specific medical conditions, are needed. Find clinical trials on PET scans and clinical trials on nuclear medicine.

Research Information

The National Institutes of Health (NIH) is the largest public funder of biomedical research in the world. NIH invests most of its budget in medical research seeking to enhance life and to reduce illness and disability. NIH-funded research has led to breakthroughs and new treatments helping people live longer, healthier lives, and building the research foundation that drives discovery.

NIH supports research into creating new, more precise radioactive tracers for both diagnosing and treating diseases, as well as developing technologies that let doctors get clearer images while using less radiation, or get results faster.

NIH-funded research is also working on improved screening methods to help confirm diagnoses more accurately and quickly. As imaging technology keeps advancing, researchers will be able to see inside the body more clearly while making the process safer for patients.

NIH also uses PET scans and other imaging tools to study the biological causes of mental health disorders. Serious mental illnesses, such as schizophrenia and certain mood disorders, are linked to, and sometimes caused by chemical imbalances in the brain. NIH researchers are working to develop new tracers that can help measure these chemical imbalances, in order to better understand these disorders and improve how they're treated.

Find NIH-funded research projects using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies.