3 Principles Of Radiation Safety

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There are many types - or modalities - of medical imaging procedures, each of which uses different technologies and techniques. Computed tomography (CT), fluoroscopy, and radiography ("conventional X-ray" including mammography) all use ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA and may elevate a person's lifetime risk of developing cancer.

CT, radiography, and fluoroscopy all work on the same basic principle: an X-ray beam is passed through the body where a portion of the X-rays are either absorbed or scattered by the internal structures, and the remaining X-ray pattern is transmitted to a detector (e.g., film or a computer screen) for recording or further processing by a computer. These exams differ in their purpose:

The discovery of X-rays and the invention of CT represented major advances in medicine. X-ray imaging exams are recognized as a valuable medical tool for a wide variety of examinations and procedures. They are used to:

As in many aspects of medicine, there are risks associated with the use of X-ray imaging, which uses ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA. Risks from exposure to ionizing radiation include:

The above statements are generalizations based on scientific analyses of large population data sets, such as survivors exposed to radiation from the atomic bomb. One of the reports of such analyses is Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 (Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council). While specific individuals or cases may not fit into such generalizations, they are still useful in developing an overall approach to medical imaging radiation safety by identifying probable at-risk populations or higher-risk procedures.

Because radiation risks are dependent on exposure to radiation, an awareness of the typical radiation exposures involved in different imaging exams is useful for communication between the physician and patient. (For a comparison of radiation doses associated with different imaging procedures see: Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog)

The medical community has emphasized radiation dose reduction in CT because of the relatively high radiation dose for CT exams (as compared to radiography) and their increased use, as reported in the National Council on Radiation Protection and Measurements (NCRP) Report No. 160. Because tissue effects are extremely rare for typical use of many X-ray imaging devices (including CT), the primary radiation risk concern for most imaging studies is cancer; however, the long exposure times needed for complex interventional fluoroscopy exams and resulting high skin doses may result in tissue effects, even when the equipment is used appropriately. For more information about risks associated with particular types of X-ray imaging studies, please see the CT, Fluoroscopy, Radiography, and Mammography web pages.

While the benefit of a clinically appropriate X-ray imaging exam generally far outweighs the risk, efforts should be made to minimize this risk by reducing unnecessary exposure to ionizing radiation. To help reduce risk to the patient, all exams using ionizing radiation should be performed only when necessary to answer a medical question, treat a disease, or guide a procedure. If there is a medical need for a particular imaging procedure and other exams using no or less radiation are less appropriate, then the benefits exceed the risks, and radiation risk considerations should not influence the physician's decision to perform the study or the patient's decision to have the procedure. However, the "As Low as Reasonably Achievable" (ALARA) principle should be followed when choosing equipment settings to minimize radiation exposure to the patient.

X-ray imaging (CT, fluoroscopy, and radiography) exams should be performed only after careful consideration of the patient's health needs. They should be performed only when the referring healthcare provider judges them to be necessary to answer a clinical question or to guide treatment of a disease. The clinical benefit of a medically appropriate X-ray imaging exam outweighs the small radiation risk. However, efforts should be made to help minimize this risk.

Extensive information is available on types of X-ray imaging exams, diseases and conditions where different types of X-ray imaging is used, and on the benefits and risks of X-ray imaging. The following web sites are not maintained by FDA and provided for illustrative purposes only:

As highlighted in its Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging, the FDA recommends that imaging professionals follow two principles of radiation protection of patients developed by the International Commission on Radiological Protection (Publication 103, The 2007 Recommendations of the International Commission on Radiological Protection; Publication 105, Radiological Protection in Medicine):

While the referring healthcare provider has the primary responsibility for justification and the imaging team (for example. imaging physician, technologist, and medical physicist) has the primary responsibility for exam optimization, communication between the referring healthcare provider and imaging team can help ensure that the patient receives an appropriate exam at an optimal radiation dose. Facility quality assurance and personnel training with a focus on radiation safety are crucial for applying the principles of radiation protection to X-ray imaging exams.

Unnecessary radiation exposure may result from medical imaging procedures that are not medically justified given a patient's signs and symptoms, or when an alternative lower-dose examination is possible. Even when an exam is medically justified, without sufficient information about a patient's medical imaging history, a referring physician might unnecessarily prescribe a repeat of an imaging procedure that has already been conducted.

Clinicians can manage justification through the use of evidence-based referral criteria to select the most appropriate imaging procedure for the particular symptoms or medical condition of a patient. Referral criteria for all types of imaging in general and for cardiac imaging in particular are provided, respectively, by the American College of Radiology and the American College of Cardiology. In addition, the Centers for Medicare & Medicaid Services is assessing the impact of appropriate use of advanced imaging services through use of decision support systems in its Medicare Imaging Demonstration, which is testing the use of automated decision support systems that incorporate referral criteria. The International Atomic Energy Agency has published information for Referring Medical Practitioners.

Patient radiation dose is considered to be optimized when images of adequate quality for the desired clinical task are produced with the lowest amount of radiation considered to be reasonably necessary. A facility can use its quality assurance (QA) program to optimize radiation dose for each kind of X-ray imaging exam, procedure, and medical imaging task it performs. Patient size is an important factor to consider in optimization, as larger patients generally require a higher radiation dose than smaller patients to generate images of the same quality.

Note that there may be a range of optimized exposure settings, depending on the capabilities of the imaging equipment and the image quality requirements of the physician. Radiation exposure may be optimized properly for the same exam and patient size at two facilities (or on two different models of imaging equipment) even though the radiation exposures are not identical.

One important aspect of a QA program entails routine and systematic monitoring of radiation dose and implementation of follow-up actions when doses are considered to be anomalously high (or low). Here are the rudiments of QA dose monitoring and follow-up:

Norms are referred to as "diagnostic reference levels" (DRLs), or simply "reference levels" for interventional fluoroscopy exams. They are established by national, state, regional, or local authorities, and by professional organizations. For a particular medical-imaging task and patient size group, a DRL is typically set at the 75th percentile (third quartile) of the distribution of dose-index values associated with clinical practice. DRLs are neither dose limits nor thresholds. Rather, they serve as a guide to good practice without guaranteeing optimum performance. Higher than expected radiation doses are not the only concern; radiation doses that are substantially lower than expected may be associated with poor image quality or inadequate diagnostic information. The FDA encourages the establishment of DRLs through the development of national dose registries.

Facilities can characterize their own radiation dose practices in terms of "local" reference levels, such as medians or means of the distributions dose-index values associated with the respective protocols they carry out. Local reference levels should be compared to regional or national diagnostic reference levels, where available, as part of a comprehensive quality assurance program. Such comparisons are essential to quality improvement activities. However, even when regional or national DRLs are not available for comparison, tracking dose indices within a facility can be of value in helping to identify exams with doses that fall far outside their usual ranges.

Because imaging practice and the patient population can vary amongst and within countries, each country or region should establish its own DRLs. While the focus of the list of resources below is on U.S. or more general guidelines from international radiation protection organizations, the references include a few examples of how other countries establish and use DRLs. Note that while the use of DRLs is voluntary in the U.S., it is a regulatory requirement in many European countries.

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