A nuclear medicine injection is a shot of a mildly radioactive drug, called a radiopharmaceutical, into a vein. The drug travels through your body and collects in specific organs or tissues, giving off small amounts of radiation that a camera detects to create detailed images of how your body is functioning. Unlike a CT scan or MRI, which shows what your anatomy looks like, nuclear medicine imaging reveals how well organs and tissues are actually working.
What’s in the Injection
A radiopharmaceutical has two key parts: a radioactive atom (the radioisotope) that emits detectable energy, and a targeting molecule designed to travel to a specific organ or tissue. A chemical linker holds them together so the drug stays stable as it moves through your bloodstream. The targeting molecule is what makes the drug useful. One type might be drawn to bone tissue, another to heart muscle, and another to cancer cells. The radioisotope is just along for the ride, acting as a signal that imaging cameras can pick up from outside your body.
How the Tracer Reaches Its Target
After the injection enters your bloodstream, the tracer circulates through your body and gradually accumulates wherever its targeting molecule is designed to go. A tracer meant for heart imaging, for example, first appears in the right side of the heart, then the left side, and finally concentrates in the heart muscle wall itself. A brain tracer might start out distributed broadly through the brain’s blood supply, then over the course of a few hours bind specifically to receptors in a targeted brain region.
Areas with higher metabolic activity or more of the targeted receptors will absorb more tracer and appear brighter on the scan. That’s why nuclear medicine is so useful for spotting things like tumors, infections, or reduced blood flow. A “cold spot” where tracer doesn’t show up can be just as revealing as a bright one.
Common Types of Scans
The three most common scans that use these injections are bone scans, heart scans, and brain scans. Each uses a different radiopharmaceutical tailored to the tissue being studied. A bone scan can reveal stress fractures, infections, or cancer that has spread to the skeleton. A heart scan (often called a cardiac perfusion scan) shows how well blood flows through the heart muscle, which helps identify blockages or damage. Brain scans can map blood flow patterns to evaluate conditions like dementia, epilepsy, or traumatic brain injury.
PET scans, which use a sugar-based tracer that cancer cells absorb in large quantities, are another major category. A whole-body PET/CT scan is one of the most widely used tools for staging cancer and monitoring whether treatment is working.
Nuclear Medicine for Treatment
Not all nuclear medicine injections are for imaging. Some deliver higher doses of radiation directly to diseased tissue as a form of targeted therapy. Radioactive iodine has been used for decades to treat overactive thyroid glands and thyroid cancer, since the thyroid naturally absorbs iodine. More recently, the FDA approved a radiopharmaceutical therapy for metastatic prostate cancer that attaches a radioactive atom to a molecule that seeks out a protein found on prostate cancer cells. In clinical trials, patients receiving this treatment saw disease progression delayed to a median of 9.3 months compared to 5.6 months with alternative therapy. These therapeutic injections are typically given in a series of sessions spaced several weeks apart.
How Much Radiation You’ll Receive
The radiation dose from a diagnostic nuclear medicine injection varies by the type of scan. For context, the average person in the U.S. absorbs about 3 millisieverts (mSv) of natural background radiation per year just from everyday living. A single chest X-ray delivers roughly 0.1 mSv, equivalent to about 10 days of that natural background exposure. A whole-body PET/CT scan sits at the higher end, delivering around 22.7 mSv, comparable to about 7.6 years of natural background radiation. Most other nuclear medicine scans fall somewhere between those two extremes.
The radioactive tracers used in diagnostic scans are designed to decay quickly. Depending on the specific isotope, most of the radioactivity leaves your body within hours to a couple of days, partly through natural decay and partly through urination.
Side Effects and Safety
Adverse reactions to nuclear medicine injections are remarkably rare. A review of reports submitted to the British Nuclear Medicine Society over nearly a decade found a rate of roughly 2.5 to 3.1 reactions per 100,000 injections. When reactions did occur, the most common were rash, itching, and vomiting. Severe allergic reactions (anaphylaxis) were reported but extremely uncommon, with only 7 hospitalizations for anaphylaxis across the entire reporting period.
One practical concern is extravasation, where the tracer leaks out of the vein at the injection site instead of entering the bloodstream. This can cause localized discomfort and may affect the quality of the scan. The U.S. Nuclear Regulatory Commission is currently developing rules that would require facilities to report extravasations that cause suspected radiation injury, a sign that regulators are tightening oversight even in an area that is already considered low-risk.
What to Expect Before and During the Procedure
Preparation depends entirely on which part of your body is being scanned. Bone scans, brain scans, kidney scans, and lung scans generally require no special preparation at all. Scans involving the digestive system require fasting for at least four hours. Cardiac exams also require a four-hour fast, and if a stress test is part of the exam, you’ll need to avoid caffeine for 24 hours beforehand. Thyroid scans may require you to stop certain medications in advance.
The injection itself feels like a standard blood draw or IV placement. After the tracer is injected, you’ll wait for it to reach its target tissue. This waiting period varies depending on the scan. Some heart scans may begin within minutes, while a bone scan typically requires a wait of two to three hours. During this time, you can usually sit comfortably or leave the facility and return. The imaging portion involves lying still on a table while a camera rotates around you or you pass through a ring-shaped scanner. The camera detects the radiation emitted by the tracer and builds images from those signals. The scan itself is painless.
Drinking extra water after the procedure helps flush the remaining tracer from your system faster, especially since much of it is eliminated through urine.