imaging occurs in hospitals and imaging centers, as well as free-standing private physician, dental, and chiropractor practices.

A report released in early 2009 by the National Council on Radiation Protection and Measurements (NCRP)1 titled Ionizing Radiation Exposure of the Population of the United States indicated that in 2006 Americans were exposed to more than six times as much ionizing radiation from medical diagnostic procedures than in 1980 (NCRP, 2009). The average effective radiation dose2 to which the U.S. population is now exposed is estimated to be 3 mSv,3 which is comparable to the annual exposure from natural background radiation which has remained unchanged for the past 20 years.

The most significant changes in medical diagnostic imaging were attributed to rapid increases in usage of higher-dose procedures particularly CT and nuclear medicine (especially nuclear cardiology [Mettler, 2009]). Close to 82 million CT exams are now performed annually in the United States (IMV, 2011), up from 46 million in 2000 and 13 million in 1990 (Brenner and Hall, 2007). Cardiac diagnostic nuclear procedures increased from 1 percent of the total number of diagnostic nuclear medicine examinations performed in 1973 to 57 percent in 2005 (Mettler et al., 2009).

Many factors have been suggested as explanations for the sharp increase in CT use (Baker et al., 2008; Iglehart, 2009), such as advances in CT technology that have increased ease of use for physicians and comfort for patients during testing; increased CT scanner availability; favorable financial reimbursements for imaging procedures; and shifts in the practice of medicine including more time constraints and promotion of defensive medicine. Newer radiographic imaging modalities such as positron-emission tomography/CT (PET/CT), single-photon emission CT (SPECT/CT), and potentially CT for screening of high-risk asymptomatic patients (for example, smokers screened for early lung cancer detection) are likely to further increase the population’s exposure (Brenner and Hricak, 2010).


1 The NCRP is a congressionally chartered organization that formulates and disseminates information and research data related to radiation exposure and protection.

2 Effective dose is a dose parameter used to normalize partial-body radiation exposures relative to whole-body exposures to facilitate radiation protection activities (ICRP, 1991). Effective dose can also be used to enable comparison of risks between procedures that utilize ionizing radiation. The International Commission on Radiological Protection (ICRP) does not recommend use of effective dose for estimating population or individual risks. Effective dose is expressed in sieverts (Sv).

3 The exposures of particular individuals could be higher or lower than these reported averages depending on how many medical imaging procedures that use ionizing radiation they undergo. As discussed in Section 2.2, a number of individuals undergo multiple imaging exams in their lifetime. Others may not undergo any. Therefore, their exposure would be higher or lower than the estimated average.

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