The average medical radiation effective dose to the U.S. population in 2006 was estimated at approximately 3.0 mSv, an increase of 600% in a single generation. Computed tomography (CT) alone accounts for approximately half of this medical radiation dose. Ongoing advances suggest that CT will continue to be the most important contributor, by far, to medical doses in the United States. The use of ionizing radiation in medical imaging, including CT, provides valuable diagnostic information that undoubtedly benefits many patients. Exposure to radiation, however, is currently believed to carry a small, but nonzero, risk.