A typical CT scan exposes a person to a dose of ionizing radiation that is roughly equivalent to a few months to a few years of natural background cosmic radiation, depending on the type of scan and the body part being imaged. In general, the radiation from a single CT scan is often compared to the amount of cosmic radiation one would receive over several months to about three years at ground level.
Cosmic radiation is the natural ionizing radiation that constantly bombards the Earth from outer space. It comes mainly from high-energy particles originating from the sun and distant galaxies. The amount of cosmic radiation exposure varies with altitude and latitude but averages about 2 to 3 millisieverts (mSv) per year for people living at sea level. At higher altitudes, such as in mountainous regions or during air travel, exposure increases significantly.
A typical CT scan, depending on the type, delivers a radiation dose ranging from about 1 to 10 mSv. For example, a head CT scan might expose a patient to around 2 mSv, which is roughly equivalent to the cosmic radiation exposure accumulated over 8 to 12 months at sea level. A full-body CT scan or abdominal CT scan can be higher, sometimes approaching or exceeding 10 mSv, which corresponds to about three to five years of cosmic radiation exposure at ground level.
The reason CT scans involve more radiation than a standard X-ray is that CT imaging uses multiple X-ray beams rotating around the body to create detailed cross-sectional images. This requires a higher dose of radiation to produce clear, high-resolution images. However, modern CT scanners and protocols aim to minimize radiation exposure by using the lowest dose necessary to achieve diagnostic quality images.
While the radiation dose from a single CT scan is higher than the average annual dose from cosmic radiation, it is important to understand that the exposure is brief and localized to the scanned area. The risk from this radiation is generally considered low but not zero. Repeated CT scans or scans in children, who are more sensitive to radiation, can increase the cumulative risk of radiation-induced effects, including a small increase in lifetime cancer risk.
In contrast, cosmic radiation exposure is continuous but at a much lower dose rate spread evenly over time and the whole body. The biological effects of chronic low-dose exposure from cosmic rays differ from the acute exposure during a CT scan, but both contribute to the total radiation dose a person receives in their lifetime.
In summary, a single CT scan typically delivers more radiation than one year of cosmic radiation exposure at sea level, often equivalent to several months up to a few years of cosmic radiation depending on the scan type. This comparison helps contextualize the radiation dose from medical imaging relative to natural background radiation, highlighting the importance of using CT scans judiciously and optimizing protocols to minimize unnecessary radiation exposure.