A CT scan of the lungs exposes a person to significantly more radiation than a standard chest X-ray. To understand why, it helps to first look at what each imaging method involves and how radiation doses compare.
A chest X-ray is a quick, simple imaging test that uses a small amount of ionizing radiation to produce a two-dimensional image of the lungs and chest area. The radiation dose from a typical chest X-ray is very low, roughly equivalent to about 0.1 millisieverts (mSv). This amount is comparable to the natural background radiation a person receives over about 10 days from the environment. Because the radiation dose is so low, chest X-rays are considered very safe for routine use.
In contrast, a CT (computed tomography) scan of the lungs uses a series of X-ray images taken from multiple angles around the body, which a computer then processes to create detailed cross-sectional images of the lungs and surrounding tissues. This technique provides much more detailed and comprehensive information than a chest X-ray, allowing doctors to detect smaller abnormalities such as early lung nodules that might not be visible on an X-ray.
However, this increased detail comes with a higher radiation dose. A standard chest CT scan typically delivers about 4 millisieverts of radiation, which is roughly 40 times the dose of a chest X-ray. If contrast dye is used during the CT scan, the dose can increase further, sometimes up to 6 millisieverts or more. Even low-dose CT scans, which are specially designed to minimize radiation exposure while still providing useful images for lung cancer screening, expose patients to radiation equivalent to about 6 to 8 chest X-rays.
The higher radiation dose from CT scans is important because ionizing radiation carries a small risk of causing damage to DNA, which can potentially lead to cancer over time. While the risk from a single CT scan is low, it is higher than that from a chest X-ray. This is why medical professionals carefully weigh the benefits and risks before recommending a CT scan, especially for repeated use.
Despite the higher radiation, CT scans are invaluable in detecting lung diseases early, particularly lung cancer. Studies have shown that low-dose CT screening in high-risk groups, such as long-term smokers, can reduce lung cancer mortality by detecting tumors at an earlier, more treatable stage. This benefit often outweighs the small increased risk from radiation exposure.
In summary, a CT scan of the lungs delivers substantially more radiation than a chest X-ray—about 30 to 40 times more in standard scans, and still several times more even in low-dose CT scans. The choice between these imaging tests depends on the clinical need for detailed lung images versus minimizing radiation exposure. Medical guidelines emphasize using CT scans judiciously and applying dose-reduction techniques to keep radiation “as low as reasonably achievable” while obtaining the necessary diagnostic information.