A tracer is a substance introduced into the body (or an environment) that can be detected and followed as it moves, revealing information that would otherwise be invisible. In medicine, tracers are most commonly small amounts of radioactive material used in imaging scans to show how organs function, where blood flows, and whether cancer is present. Outside of medicine, tracers help scientists track everything from groundwater movement to nutrient cycles in ecosystems.
How Medical Tracers Work
A medical tracer, often called a radiopharmaceutical, is made of two parts: a carrier molecule and a radioactive atom bonded tightly to it. The carrier molecule is chosen because it naturally travels to a specific organ or participates in a specific biological process. The radioactive atom acts as a signal flare, emitting energy that imaging equipment can detect from outside the body.
You receive a tracer by injection into a vein, by swallowing it, or by inhaling it, depending on what your doctor needs to see. An inhaled tracer might be used for lung function testing, while an injected one could map blood flow through your heart. Once inside, the tracer follows the same pathways as the natural substance it mimics, but the radioactive tag lets a scanner pinpoint exactly where it goes and how much accumulates in each area.
What Tracers Reveal in Imaging Scans
The two main imaging technologies that use tracers are SPECT and PET scans. Both detect gamma rays, but they work differently.
In a SPECT scan, the tracer emits gamma rays directly, and a rotating camera captures them from multiple angles to build a 3D image. The most widely used SPECT tracer is technetium-99m, which has a six-hour half-life, meaning half its radioactivity fades every six hours. That’s long enough to complete imaging but short enough to limit radiation exposure. Technetium-99m is used to image the brain, bones, lungs, kidneys, thyroid, heart, liver, and more, making it the single most common radioactive tracer in medicine.
PET scans use a different type of radioactive decay. The tracer releases a tiny particle called a positron, which almost immediately collides with an electron in your tissue. The two particles annihilate each other and produce a pair of gamma ray photons that shoot off in exactly opposite directions. The scanner detects both photons simultaneously, and that paired detection allows for extremely precise localization of the tracer.
How Tracers Detect Cancer
The most widely used PET tracer is a modified form of glucose tagged with a radioactive fluorine atom. Cancer cells consume far more glucose than normal cells because they need extra energy to divide rapidly and survive in harsh conditions. They pull in this glucose look-alike through the same channels they use for regular sugar, and at much higher rates.
Here’s the clever part: once inside a cell, the tracer gets partially processed but then becomes stuck. Unlike real glucose, it can’t be broken down further or expelled. So it accumulates, and cancer cells, which are consuming the most, light up brightest on the scan. This allows doctors to see not just a tumor’s location but also how metabolically active it is, which helps guide treatment decisions and monitor whether therapy is working.
Radiation Exposure From Tracers
The amount of radioactive material in a tracer is very small. A standard bone scan using technetium-99m delivers an effective dose of about 3 to 4 millisieverts. For comparison, the average person absorbs roughly 3 millisieverts per year just from natural background radiation: cosmic rays, radon in soil, and trace radioactivity in food. So a typical tracer scan exposes you to roughly one year’s worth of natural background radiation in a single session.
Tracers are also designed to leave the body quickly. The radioactive atoms decay on their own (technetium-99m loses half its radioactivity every six hours), and the kidneys and urinary tract flush the chemical remnants. Within a day or two, most of the tracer is effectively gone. Drinking extra water after a scan helps speed this process along.
Newer Tracers for Heart Imaging
Tracer development continues to expand what scans can detect. In September 2024, the FDA approved a new PET tracer specifically designed for heart imaging. It helps identify areas of the heart muscle that aren’t getting enough blood flow, a condition that can signal blocked arteries or damage from a heart attack. Previously, heart imaging relied heavily on SPECT tracers, so dedicated PET options represent a step toward higher-resolution cardiac scans.
Tracers Outside of Medicine
The same basic concept, introducing a detectable substance and following where it goes, applies well beyond hospitals. Hydrologists use stable isotopes of oxygen, hydrogen, carbon, nitrogen, and sulfur as tracers to map how water moves underground, how long it has been stored in an aquifer, and where contamination enters a water supply. The U.S. Geological Survey has used oxygen isotope ratios in groundwater to estimate how long water has been underground and to build regional water quality monitoring programs.
Environmental tracers don’t need to be radioactive. Fluorescent dyes injected into streams can reveal hidden connections between surface water and underground cave systems. Nitrogen isotopes in rivers can pinpoint whether pollution is coming from agricultural fertilizer, sewage, or natural sources. In each case, the principle is identical to a medical scan: put something detectable into a system you can’t see inside, then watch where it shows up.