A radiotracer is a small amount of radioactive material that, once introduced into your body, emits signals a scanner can detect to create detailed images of how your organs and tissues are functioning. Unlike X-rays or CT scans that show anatomy, radiotracers reveal what’s actually happening inside your body: how blood is flowing, how actively cells are consuming energy, and whether tissues are working normally or showing signs of disease.
How a Radiotracer Is Built
Every radiotracer has two parts: a carrier molecule and a radioactive atom. The carrier molecule is chosen specifically to interact with whatever biological process doctors want to observe. It might mimic glucose to track energy use, bind to bone tissue to find fractures, or even attach to a sample of your own red blood cells to locate the source of internal bleeding. The radioactive atom is bonded tightly to that carrier, acting as a beacon that imaging equipment can pick up from outside your body.
The most widely used radiotracer is a modified form of glucose called FDG, tagged with a radioactive form of fluorine. Cancer cells consume glucose at a much higher rate than normal cells, a quirk of tumor biology known as the Warburg effect. FDG enters cells the same way glucose does, through sugar transporters on the cell surface. Once inside, the cell processes it just enough to trap it there, but not enough to use it for energy. The radioactive signal builds up in cells that are hungriest for sugar, making tumors light up on the scan. This same principle lets doctors monitor whether a cancer is responding to treatment: if the glow fades, the tumor’s metabolism is slowing.
What Radiotracers Detect
Radiotracers are used in two main types of imaging. PET scans (positron emission tomography) measure metabolic activity, blood flow, and chemical absorption. SPECT scans (single-photon emission computed tomography) gauge blood flow and how radioactive material distributes through tissues and organs. The choice between them depends on what question your doctor is trying to answer.
In oncology, PET scans help stage cancers across nearly every part of the body, including lung, breast, prostate, colorectal, and brain tumors. They can detect metastases that have spread to bone and catch early signs of cancer recurrence. In neurology, radiotracers help distinguish Alzheimer’s disease from other forms of dementia by tracking the buildup of abnormal protein plaques, identify seizure origins in epilepsy, and observe changes in brain chemistry associated with Parkinson’s disease. For the heart, specialized tracers measure how well blood reaches the heart muscle, assess damage after a heart attack, and detect coronary artery disease. SPECT scans also play a role in diagnosing strokes, bone diseases, and infections of unknown origin.
How the Scanner Reads the Signal
In a PET scan, the radioactive atom on the tracer emits a tiny particle called a positron. That positron travels a fraction of a millimeter through surrounding tissue before colliding with an electron. When the two meet, they annihilate each other and release two packets of energy (photons) that shoot off in exactly opposite directions. Rings of detectors surrounding your body register both photons at nearly the same instant. By mapping millions of these simultaneous hits, the scanner pinpoints where the tracer has concentrated and builds a three-dimensional image of activity inside your body.
SPECT works on a simpler principle. The radioactive atom emits a single gamma ray, and a camera rotating around you captures those rays from multiple angles to construct an image.
What to Expect as a Patient
Most radiotracers are injected into a vein, though some can be swallowed or inhaled depending on the exam. For a standard FDG-PET scan, you’ll fast for at least six hours beforehand so your normal blood sugar doesn’t interfere with the tracer’s uptake. Your blood glucose will be checked before injection. Guidelines recommend drinking several glasses of water during the fasting period and continuing to hydrate after injection, which helps flush excess tracer from your system.
After the injection, you’ll wait about an hour for the tracer to circulate and accumulate in the target tissues. During this uptake period, you’ll typically sit quietly in a dimly lit room, since physical activity and even talking can cause muscles to absorb the tracer and muddy the images. If the tracer is given orally instead, the wait extends to roughly two hours because absorption through the gut is slower. The scan itself usually takes 20 to 45 minutes, during which you lie still on a table that slides through the scanner.
Radiation Exposure and Safety
The radiation dose from a nuclear medicine scan typically ranges from 0.3 to 20 millisieverts (mSv), depending on which tracer is used and what’s being imaged. For context, the average person absorbs about 3 mSv per year from natural background radiation: cosmic rays, radon in soil, and trace radioactivity in food. A standard PET scan falls in a similar range to a CT scan.
The radioactive atoms used in tracers are chosen partly because they decay quickly. Technetium-99m, the workhorse isotope for SPECT imaging, has a physical half-life of about six hours, meaning half its radioactivity is gone in that time. Inside the body, it clears even faster: the effective half-life drops to roughly one and a half to two hours because your kidneys are flushing it out simultaneously. Fluorine-18, used in FDG-PET, has a half-life of about two hours. By the next day, the tracer in your body is essentially gone.
Side Effects and Reactions
Adverse reactions to radiotracers are exceptionally rare. Data from the British Nuclear Medicine Society found only about 2.5 to 3.1 reactions per 100,000 doses administered. When reactions do occur, they’re usually minor: a skin rash, itching, nausea, or brief flushing. Serious allergic responses like swelling or anaphylaxis have been documented but remain isolated case reports across decades of use. Some people notice mild discomfort or redness at the injection site, which resolves on its own.
The French national database recorded 304 adverse reaction reports over a 24-year period, roughly a dozen per year in an entire country’s nuclear medicine practice. About 43% of those were classified as serious, though that category included any reaction requiring a hospital stay, not just life-threatening events. For the vast majority of patients, the experience amounts to a needle stick, a quiet wait, and a painless scan.