Is radiation from a CT or PET scan dangerous?


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Dr. Ananthakrishnan standing by the Spectral CT-20 scanner.

Every day, patients come to me for imaging tests, such as computed tomography (CT) scans and X-rays.

Occasionally, patients are concerned because they’ve heard that these types of tests expose them to radiation. Is the radiation going to hurt me or cause problems down the line? What about the contrast agents I have to drink for the test – are they safe?

These are valid questions, and the truth is, imaging tests do expose patients to a small amount of radiation. There is a potential, small risk that being exposed to radiation can induce cancer in a patient. But in most cases, the benefits of imaging tests far outweigh the risks from radiation exposure.

I’ve put together answers to some commonly asked questions about radiation exposure to ease patients’ minds the next time a doctor recommends an imaging test.

Which types of diagnostic imaging use radiation?

We use X-rays and CT scans most often, and those involve radiation. Mammograms also involve radiation, but it’s a very small dose – the average American gets seven to eight times more radiation every year from background radiation than a patient does from the radiation involved in a mammogram.

We also perform nuclear medicine studies, such as PET scans, stress tests for the heart, and iodine scans for the thyroid gland. These tests also involve radiation, and the amount varies depending on the test.

Ultrasound and magnetic resonance imaging (MRI), however, do not involve radiation.

How do we use radiation with imaging tests?

Imaging tests have helped revolutionize the practice of medicine. Fifty years ago, physicians didn’t have a way to see the internal organs very well. So if there was a problem with a patient’s liver or kidneys, doctors didn’t have a way to identify it. Often, many medical problems weren’t diagnosed until they were very advanced, and sometimes doctors had to resort to exploratory surgery to find the source of the problem.

With X-rays and CT scans, we can now essentially take a picture of the internal organs. We can see even more detail by using contrast agents that patients drink or that we deliver through an IV. These scans help us detect many types of cancer and organ injuries, and seeing what’s wrong helps determine the next step of treatment. For example, if a patient comes in with abdominal pain, we can do a CT scan to determine whether the patient has appendicitis and needs an emergency operation.

Think of it like this: Combining radiation with contrast agents is like shining a flashlight. If we shine a flashlight through the air, there is nothing to stop the light and it flows freely. But if we shine a flashlight at a piece of metal, the metal blocks the light.

Likewise, the IV and oral contrast agents help block radiation rays. The contrast that patients drink enters the intestines, and that helps us see them better. The contrast that we inject through an IV allows us to see the organs and any injuries or potential tumors.

For example, if there is a tumor on the liver, IV contrast reacts differently with the tumor than it does with the healthy liver tissue, which allows us to see the tumor more easily than we would otherwise.

We also use radiation in the form of X-rays. Taking X-ray images allows us to see different parts of the body in different ways. X-rays of the bones involve a very low radiation dose and allow us to see problems, such as fractures and arthritis.

Are the contrast agents and dyes harmful?

They are very safe for most people. We use two types of contrast agents: One patients drink, and the other we inject through an IV.

  • The type patients drink: We use this method often. Most people’s intestines are about 11 feet long, so we ask that patients drink several cups of contrast so fluid fills the intestines well and gives us the best picture. The drinks are made with either barium or iodine, and they help us see the stomach, colon, and intestines during a CT scan. Some people don’t like the taste of the contrast, but many places now offer different flavors which can make it go down a bit easier. Though these drinks are quite safe, it’s a good idea for patients to drink plenty of water after the test to flush the agent out of their system.
  • The IV type: For CT scans, the IV contrast dye we use is iodine-based. It’s safe for most people, but rarely can cause kidney problems in patients who have pre-existing kidney issues, diabetes, or high blood pressure. In these patients, we perform a blood test before we give the IV contrast to screen out patients who are at higher risk for kidney problems. This contrast leaves the patient’s system through urine, so staying hydrated after a CT scan with IV contrast dye helps flush it out of the system faster. There is a very small risk of allergic reaction to the contrast dye, so if a patient has had an allergic reaction to the dye before, it’s important to inform the technologist prior to the CT scan.

How much radiation exposure will I get from a CT scan?

Often when people hear the word “radiation,” they think of scary things like an atomic bomb or a meltdown at a nuclear power facility. But the radiation used for medical purposes isn't that scary. These tests can help identify acute problems and different types of cancer. In many cases, a CT scan can help a surgeon plan a surgery, and sometimes can help patients avoid unnecessary surgery.

However, radiation exposure certainly isn’t trivial, so we make every reasonable effort to minimize a patient’s exposure to radiation. Patients can help by letting us know if they had the same test recently at another hospital – those results could help avoid an unnecessary test. Our CT scanners keep radiation doses as low as possible while still obtaining high-quality images. But in reality, the amount of radiation from a single imaging test really is not that much.

In general, the benefits of imaging tests far outweigh the radiation risks. The risk of dying of cancer for the average American is one in five. The additional risk of getting cancer from one CT scan is estimated to be less than one in 2,000. In my opinion, that risk is tiny for a test that could save someone’s life.    

What about patients who need multiple CT scans?

People who have cancer often need CT scans every few months to determine if their treatment is working. Some patients worry that repeated CT scans will give them cancer again in the future. I try to reassure these patients that while there is theoretically a small risk of getting a new cancer in many years from radiation exposure, the risk of having a complication from their existing cancer is much larger.

We don’t want to frighten anyone, so it’s important to maintain some perspective. Most people don’t realize this, but we are exposed to some radiation every day. All Americans have small amounts of radon in our homes, which gives us small doses of background radiation. We get radiation every time we take a cross-country flight. People who live at higher altitudes, such as Denver, Colorado, are exposed to more radiation than people who live at lower altitudes, such as Dallas-Fort Worth. While no one should be exposed to medical radiation unnecessarily, if a patient needs a CT scan, the risk of radiation exposure should not hold him or her back from getting it.

If imaging tests are safe, what’s with the lead vests?

For technologists and doctors like me who administer radiation to patients all day, that exposure adds up. We do this all day, every day, whereas patients are usually getting tested every once in a while. We wear lead vests to protect ourselves from continual exposure to radiation.

Our goal when we turn on the imaging machine is for the radiation to go to good use. We want it to go into the patient to give us a good picture of the bones or organs. That’s why we ask anyone who attends an appointment with a patient to stay outside the room during the test. The amount of radiation associated with an imaging test is small, but we also want to avoid unnecessary radiation exposure to anyone who isn’t getting the test.

Who should not have imaging tests done?

Some people are more susceptible to complications from radiation exposure, and we take extra care to manage that risk. Though the risk of getting cancer from the radiation from imaging scans is extremely low, it’s a little higher for women and younger patients. Younger patients who have chronic medical problems may need repeated imaging throughout their lifetimes, so we are extra cautious with them. We try to minimize the number of CT scans we give those patients and instead opt for MRIs or ultrasounds when possible.

Kids’ growing bodies are especially susceptible to radiation, so our pediatric radiologists minimize radiation exposure to pediatric patients whenever possible. Similarly, pregnant women should not undergo CT scans and X-rays unless they are absolutely necessary. It’s important to remember that different imaging tests are better to look at different parts of the body, so depending on a patient’s medical issues, an MRI or ultrasound may not be the best test.

For any test, it’s always important to look at the risks and the benefits. In most cases, the benefit of finding cancer or confirming the need for surgery outweighs the risk from radiation exposure from a CT scan.

If you’re concerned about radiation exposure during an imaging test, or if you have questions about any of the tests recommended to you, don’t hesitate to ask your doctor or technologist. We want you to be comfortable, and it’s your right to know how the tests work and why they’re important.