Computed Tomography (CT) scans are a valuable diagnostic tool in modern medicine, but like many medical procedures, they come with both benefits and risks. One of the primary concerns associated with CT scans is the exposure to X-ray radiation. This article explores the amount of radiation in a CT scan, the potential risks involved, and how these risks are managed.
Potential Risks Associated with CT Scans
The main risks associated with CT scans are:
- Incidental Findings: CT scans may reveal benign or incidental findings that lead to unnecessary and potentially invasive follow-up tests. These additional tests can present further risks.
- Increased Cancer Risk: Exposure to X-ray radiation, even at low doses, carries a small possibility of inducing cancer.
While the probability of inducing cancer or heritable mutations from the radiation doses associated with CT procedures is considered very small, there is still some scientific debate regarding the effects of very low doses.
Radiation Dose from CT Examinations
To assess the risk of cancer from a CT procedure, the most relevant measurement is the effective dose, measured in millisieverts (mSv). Effective dose allows for comparing risk estimates associated with both partial and whole-body radiation exposures, and it also accounts for the different radiation sensitivities of various organs in the body.
Factors Affecting Radiation Dose
Radiation dose from CT procedures varies significantly depending on several factors:
- Body Part Examined: The size and density of the body part being scanned affect the amount of radiation needed.
- Type of Procedure: Different CT procedures require different radiation levels.
- CT Equipment and Operation: The specific equipment used and how it is operated influence the radiation dose.
Therefore, typical values cited for radiation dose should be regarded as estimates rather than precise values for individual patients or examinations. The actual dose could be two to three times higher or lower than these estimates.
Low-Dose CT Scans
Facilities performing “screening” procedures might adjust the radiation dose to levels lower than those typically used for diagnostic CT procedures, sometimes using “low dose CT scans.” However, reducing the dose can negatively affect the image quality, though this may be acceptable for certain imaging applications.
Radiation Dose Comparison Table
Here’s a comparison of radiation doses from various diagnostic procedures:
| Diagnostic Procedure | Typical Effective Dose (mSv) |
|---|---|
| Chest x-ray (PA film) | 0.02 |
| Lumbar spine | 1.5 |
| I.V. urogram | 3 |
| Upper G.I. exam | 6 |
| Barium enema | 8 |
| CT head | 2 |
| CT chest | 7 |
| CT abdomen | 8 |
| Coronary artery calcification CT | 3 |
| Coronary CT angiogram | 16 |
Source: Average effective dose in millisieverts (mSv) from McCollough CH, Bushberg JT, Fletcher JG, Eckel LJ. Answers to common questions about the use and safety of CT scans. Mayo Clin Proc. 2015;90(10):1380-92.
Effective doses from diagnostic CT procedures are typically in the range of 1 to 10 mSv. To provide context, this range is comparable to the lower doses (5 to 20 mSv) received by some survivors of the atomic bombs in Japan. These survivors have shown a slightly increased risk of cancer mortality.
Risk Estimates and the Linear Non-Threshold Model
The risk of developing cancer from radiation exposure depends on the body part exposed, the individual’s age at exposure, and their sex. A conservative approach, known as the “linear non-threshold” model, assumes that any amount of radiation carries some risk, and that the risk of adverse health effects is proportional to the radiation dose absorbed.
For example, a CT examination with an effective dose of 10 mSv may increase the possibility of fatal cancer by approximately 1 in 2000. In comparison, the natural incidence of fatal cancer in the U.S. population is about 1 in 5 (or 400 in 2000). Thus, the risk from a 10 mSv CT scan increases the total risk from 400 in 2000 to 401 in 2000. While the individual risk is small, it can become a public health concern if many people undergo frequent CT screening procedures of uncertain benefit.
Uncertainty and Controversy
There is considerable uncertainty regarding risk estimates for low-level radiation exposure. The risk is quite low compared to the natural risk of cancer, making it difficult to obtain statistically valid estimates. Some scientists believe that low doses of radiation do not increase cancer risk at all, although this is a minority view.
Conclusion
Understanding the amount of radiation in a CT scan and the associated risks is essential for both patients and healthcare providers. While CT scans are valuable diagnostic tools, it’s important to weigh the benefits against the potential risks. By understanding these factors, informed decisions can be made about when and how to use CT scans, ensuring the best possible outcomes for patients. Always discuss your concerns and the necessity of a CT scan with your doctor.
