In the past 15 years, since the introduction first of spiral CT and then of multidetector row CT, the number of CT examinations has increased enormously. The developments in technology have resulted in significant improvements in image quality and an increase in the number of applications, with replacement of various invasive examinations performed for diagnostic purposes. Above all, however, there has been a marked and clinically significant improvement in diagnostic accuracy.
These advances have, however, been accompanied by an enormous increase in the radiation dose to patients. In 2009 Dr. Mettler et al.  summarized the results of two large studies by the U.S. National Council on Radiation Protection and Measurements and the United Nations Scientific Committee on Effects of Atomic Radiation that respectively evaluated all radiation sources in the United States and worldwide. In the past fifty years the number of diagnostic radiologic examinations has increased approximately tenfold, but what is more interesting is that the average effective dose has approximately doubled in the past ten years. The highest effective dose, taking 2006 as a reference year, is due to CT, which accounts for almost half the collective effective dose from diagnostic examinations. Of all CT scans, approximately one-third are abdominal/pelvic examinations and these contribute almost half of the collective effective dose from CT scans.