CT Radiation Exposure in Perspective: How It Compares, Why It’s a Risk

To appreciate the dose risks—and thus the cancer risks—posed by computed tomography (CT), it is important to compare CT doses to doses received from other sources. Tables 1 and 2 list representative effective dose values experienced by patients undergoing various radiographic procedures and by individuals in everyday life. (Effective dose is the best parameter for judging radiation risks; see Displayed Dose versus Effective Dose: Understanding What's on the CT Control Console for more information. )

Table 1 shows that CT delivers effective doses that are higher than those from many other common x-ray exams. And those figures don't tell the whole story. Two points need to be remembered:

Comparing the effective-dose values from CT scans to the figures in Table 2 gives a sense of how high CT doses are, relative to more routine exposures. For example, a single CT scan of the head (1.4 mSv) delivers nearly half the effective dose provided by natural background radiation in an entire year (3 mSv), and slightly more than the additional dose experienced by a uranium miner in a year (1.25 mSv). And a single chest CT scan (5.9 mSv) exposes a patient to more radiation than is received in a year by individuals who are exposed to ionizing radiation in the course of their work (listed under "occupational exposures" in the table).

Note that, although there is a regulatory limit on the annual occupational exposure to radiation (in the United States, this limit is set by the Nuclear Regulatory Commission), there are no regulatory limits for patients receiving doses from medical procedures. Instead, medical practitioners are charged with the responsibility to keep doses as low as reasonably achievable (the ALARA principle). The presumption is that the benefit to the patient will outweigh the risks.

**Radiation...