The use of medical imaging has greatly improved our ability to accurately diagnose disease and manage patient care. However, as in all aspects of health care, it is important that the benefits of a procedure outweigh the risks involved. Whereas the benefits of medical imaging, while often difficult to quantify, are widely appreciated, optimizing this benefit-to-risk ratio requires that we know something about the risks involved. Most radiologic and all nuclear medicine procedures expose patients and operators to ionizing radiation. Radiation exposure can result in detrimental health effects, including skin injury, genetic defects, and cancer. Thus, it is important that physicians both avoid inappropriate procedures and limit exposure to levels that are “as low as reasonably achievable” (ALARA principle) (International Commission on Radiological Protection [ICRP] 103, 2007).