Radiation Doses from CT Scans

Medically Reviewed by Sabrina Felson, MD on November 11, 2022
Written by Shawna Seed
4 min read

Computed tomography -- also known as computed axial tomography, a CT scan, or a CAT scan -- is a tool doctors use to diagnose many types of health problems. Special X-ray equipment creates images of the structures inside your body. These pictures allow doctors to get a look at your internal organs, tissues, and blood vessels.

A CT scan exposes your body to some radiation. That involves a small amount of risk, and it’s important to understand the issues.

A CT scan uses what’s called “ionizing” radiation. It’s powerful enough to pass through your body to create clear images on a computer. This type of radiation could raise your chances of cancer at some point in the future.

But it’s important to remember that ionizing radiation is around you every day. It’s naturally in the environment. Cosmic rays from outer space and radon gas from rocks and soil, for instance, expose you to radiation. That’s called “background” radiation.

Experts use the phrase “effective dose” to describe how much radiation your body absorbs. Different types of tissue are more sensitive than others. The amount you absorb during a CT scan of your belly, for example, is different than the amount during a scan of your head.

The unit doctors use to measure the dose is the millisievert (mSv). To give an idea of the risk from different types of X-ray tests, doctors compare the mSv of a procedure to how much time it would take to absorb the same amount of background radiation from the environment.

Common types of CT scans and the amount of radiation you would absorb from them include:

  • Belly and pelvis: 10 mSv, equal to about 3 years of background radiation
  • Colonography: 6 mSv, equal to about 2 years of background radiation
  • Head: 2 mSv, equal to about 8 months of background radiation
  • Spine: 6 mSv, equal to about 2 years of background radiation
  • Chest: 7 mSv, equal to about 2 years of background radiation
  • Lung cancer screening: 1.5 mSv, equal to about 6 months of background radiation
  • Coronary angiography (CTA): 12 mSv, equal to about 4 years of background radiation
  • Cardiac (for calcium scoring): 3 mSv, equal to about 1 year of background radiation

Sometimes, you need a separate CT scan with an added substance called “contrast.” This helps some parts of your body show up more clearly on the images. You might need to drink it as a liquid or get a shot of it into your veins. Here are radiation doses for common procedures if you get a scan with contrast, and then one without contrast:

  • Belly and pelvis: 20 mSv, equal to about 7 years of background radiation
  • Head: 4 mSv, equal to about 16 months of background radiation

 

For most people, a CT scan doesn’t seem to boost the risk for cancer in a major way. Generally, the medical benefit you get from the scan outweighs the odds of any problem you might have in the future. A CT scan can give your doctor valuable information they need to treat you. In many cases, it means they can avoid using surgery to diagnose your problem.

Some situations need extra caution, though. Children’s bodies are more likely to be affected by radiation, and because they are young, they have more years ahead of them for the effects to show up.

Some people have repeated scans to help manage a health condition, such as kidney stones or Crohn’s disease. Doctors don’t have a specific limit on the number of CT scans you can have safely. But your cancer risk does go up the more CT sessions you have.

The best way to lower your risk of any issues is to try to keep your CT scans to a minimum. Some things to think about before you get one:

  • Ask your doctor to explain why you need the exam. Is there another test that doesn’t use radiation -- MRI or ultrasound, for instance -- that you could have instead?
  • If you need to see a different doctor or get care at another facility, ask your doctor’s office to forward your CT results or take copies yourself. It’s one way to avoid repeating scans when you don’t need to.
  • If you need to have multiple CT scans, keep a chart of them so your doctors know how often you’re exposed to radiation.
  • If you need scans to manage a health condition, ask your doctor if you can space them further apart.
  • Don’t push for a scan “just to be sure.” CT is a powerful tool that you should use only when it’s necessary. Experts don’t believe there’s any benefit to “whole body” scans when you don’t have symptoms of a health problem.