A single full body CT scan typically exposes a person to a radiation dose in the range of about **20 to 25 millisieverts (mSv)**, though this can vary depending on the scanner, protocol, and patient positioning. This dose is significantly higher than that from standard X-rays or natural background radiation, but it is carefully controlled to balance diagnostic benefit against radiation risk.
To understand this better, it helps to know what a CT scan is and how radiation is involved. CT, or computed tomography, uses X-rays to create detailed cross-sectional images of the body. Unlike a regular X-ray that captures a flat image, a CT scan rotates around the body, taking multiple images from different angles, which a computer then assembles into a 3D picture. This process requires a higher dose of ionizing radiation compared to a single X-ray because it collects more data for detailed visualization.
Radiation dose in CT scans is measured in millisieverts (mSv), which quantifies the effect of radiation on human tissue. For context, the average person receives about 3 mSv of natural background radiation annually from cosmic rays, soil, and other sources. A chest X-ray delivers about 0.1 mSv, roughly equivalent to 10 days of natural background radiation. In contrast, a full body CT scan can deliver around 20 to 25 mSv, which is equivalent to several years’ worth of natural background exposure compressed into a single exam.
The exact radiation dose from a full body CT depends on several factors:
– **Scan protocol and area covered:** A full body scan covers multiple regions—head, chest, abdomen, pelvis, and sometimes limbs—each contributing to the total dose.
– **Patient size and positioning:** Larger patients may require higher doses for image clarity. Positioning also matters; for example, raising both arms during torso scans can reduce radiation dose by minimizing scatter, lowering exposure from about 24.7 mSv (arms down) to approximately 19.2 mSv (both arms up).
– **Scanner technology and settings:** Modern CT machines use dose reduction technologies and optimized protocols to minimize radiation while maintaining image quality.
– **Use of contrast agents:** While contrast dye itself doesn’t add radiation, scans with contrast may involve longer or multiple phases, potentially increasing total exposure.
Radiation from CT scans is ionizing, meaning it has enough energy to remove tightly bound electrons from atoms, which can damage DNA and potentially increase cancer risk. However, the risk from a single CT scan is generally low. The lifetime risk of developing cancer from one full body CT scan is small but not zero, and it varies by age, sex, and individual sensitivity. Younger people and women tend to have higher estimated risks because their tissues are more radiosensitive and they have longer life expectancy for potential effects to manifest.
Because of these risks, medical professionals follow the ALARA principle—”As Low As Reasonably Achievable”—to minimize radiation exposure. This means CT scans are ordered only when necessary, protocols are optimized to use the lowest dose that still yields diagnostic-quality images, and protective measures like shielding are employed when possible.
Repeated CT scans increase cumulative radiation exposure, so patients and providers are encouraged to keep track of imaging history and consider alternative imaging methods like MRI or ultrasound when appropriate, as these do not use ionizing radiation.
In summary, a full body CT scan involves a radiation dose roughly equivalent to 20 to 25 mSv, which is substantially higher than typical X-rays but carefully managed to maximize diagnostic benefit while minimizing risk. Advances in technology and technique continue to reduce doses, but awareness and judicious use remain essential to patient safety.
