Nuclear medicine explained: imaging how your body works

Nuclear medicine is the odd one out among scans. Where an X-ray or ultrasound shows what a part of the body looks like, nuclear medicine shows how it’s working — and it does it in a completely different way.

How it works

You’re given a small amount of a radioactive tracer (a radiopharmaceutical) — usually by injection, but sometimes swallowed or inhaled.^[1] The tracer travels to a particular part of the body and makes it slightly radioactive for a short time. A special gamma camera then detects the energy coming out of you and builds a picture of how the organ is functioning.^[1]

This is the key difference from X-ray and CT: with those, radiation passes through you to make the image; with nuclear medicine, the signal is emitted from within you by the tracer.^[1] Areas of high activity show up as “hot spots”, and areas of low activity as “cold spots”.^[1]

Because it reveals function — metabolism, blood flow, cell activity — nuclear medicine can find disease at an early stage, sometimes before it shows on other scans.^[1]

The tracers and the cameras

The tracer is chosen to travel to a specific target — one goes to the bones, another to the thyroid. The most common general tracer is Technetium-99m; for PET, it’s usually FDG, a glucose-like tracer that busy cells (including many cancers) take up.^[1] Only a tiny amount is used.

The images are captured by:^[1]

• A gamma camera — which detects the energy (it doesn’t emit any radiation itself).
• SPECT — a gamma camera that rotates around you for 3D images.
• PET — a donut-shaped scanner for the most detailed functional imaging.

Increasingly these are combined with a CT (SPECT-CT, PET-CT), so a functional finding can be matched to precise anatomy.^[1]

What it’s used for

Nuclear medicine is used across the body, including:^[1]

• Bones — fractures, infection, arthritis, and cancer spread (a bone scan)
• Heart — blood flow to the heart muscle (a myocardial perfusion scan)
• Thyroid — how the gland is functioning
• Kidneys — blood flow, function and drainage
• Cancer — staging, spread, response to treatment and recurrence (often with PET)
• Brain — seizures, and some causes of dementia or movement disorders

What to expect

After the tracer is given (a small pin-prick for the injection, sometimes a cool sensation up the arm), there’s often a waiting period while it travels to the target — anywhere from minutes to a few hours.^[1] The imaging itself is painless; the camera makes no noise and you feel nothing from the radiation. Afterwards, the tracer’s radioactivity fades within hours to a day and passes out mostly in your urine — you’ll be asked to drink plenty of water to help clear it.^[1]

Is it safe?

Yes. Nuclear medicine has been used for more than 60 years, the dose is small, and there are no known long-term adverse effects from these low-level exposures.^[1] Reactions to the tracer are extremely rare. Tell the team if you are, or might be, pregnant or breastfeeding. (See our guide to nuclear medicine risks.)

Frequently asked questions

Will the tracer make me radioactive?

Slightly, and only briefly — for hours up to about a day. You won’t feel anything, and the tracer is mostly cleared in your urine. Drinking water helps.^[1]

How is nuclear medicine different from a CT scan?

A CT shows what the body looks like (structure); nuclear medicine shows how an organ is working (function). They’re often used together — for example as a PET-CT.^[1]

Does the scan hurt?

No — apart from the small pin-prick of the injection, nuclear medicine imaging is painless. The main challenge is lying still, sometimes for a while.^[1]