CT Safety

What is the risk of CT?

As we have learned in previous sections, Radiographs and CTs use radiation for image production. The diagnostic information provided by these techniques is often crucial in making diagnoses; however, as an ordering physician it is important to understand the risks and benefits of these imaging tests before ordering them on your patients.

There is a potential risk of radiation related cancer following CT and X-Ray. The additional lifetime risk of fatal cancer from a single head CT is considered to be very low (1/100,000-1/10,000), for a CTA of the head without and with contrast low (1/10,000-1/1,000) and for a spine CT low (1/10,000-1/1,000). This risk multiplies when patients receive multiple scans.

Your patients will ask you about this topic. There are several dose measurement units, including sievert (or millisievert), gray, rad, rem, and Roentgen. Millisievert (mSv) can be referred to as the "effective dose" and is often used to discuss dose in the context of radiation safety.

Humans are subject to background radiation from various sources, including cosmic, the air (ie radon), foods, and the ground. Radon is typically the largest source, which can vary depending on where you live. The cosmic radiation can vary as well depending on where you live, due to elevation (higher altitude = more radiation). The average effective dose a person in the United States experiences is about 2.5-3.0 mSv/year.

The following table illustrates a few equivalent effective doses, the time equivalent of naturally occurring radiation, equivalence to a chest radiograph, and the incremental risk one exam of a single exam causing a fatal cancer. For example, a single chest x-ray is equivalent to 10 days of background radiation exposure. The data in this table comes from radiologyinfo.org and the ACR. In many centers because of newer technologies the dose has been reduced, sometimes significantly. At UW many studies have drastically reduced dose. However, this data probably applies to many typical imaging centers and is what patients will see if they read articles online. Additionally, these numbers are not directly applicable to pediatric patients due to large variation in patient size and in doses prescribed by different CT protocols.