You are in: eMedicine Specialties > Clinical Procedures > Medications Ionizing Radiation Exposure, Medical ImagingArticle Last Updated: Nov 13, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 9
  Author: Edward B Holmes, MD, MPH, Associate Clinical Professor, Department of Family and Preventative Medicine, Rocky Mountain Center for Occupational Environmental Health, University of Utah; Chief Medical Consultant for the State of Utah, Disability Determination Services for Social Security, Holmes Company Consulting, LLC Edward B Holmes is a member of the following medical societies: American College of Occupational and Environmental Medicine and European Association of Poisons Centres and Clinical Toxicologists Coauthor(s): George L White Jr, PhD, MSPH, Professor and Director, Public Health Program, School of Nursing and Health Sciences, Westminster College; David K Gaffney, MD, PhD, Professor of Radiation Oncology, University of Utah Editors: Erik D Schraga, MD, Consulting Staff, Department of Emergency Medicine, Mills-Peninsula Emergency Medical Associates; Consulting Staff, Permanente Medical Group, Kaiser Permanente, Santa Clara Medical Center; Mary L Windle, PharmD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy, Pharmacy Editor, eMedicine; Rick Kulkarni, MD, Medical Director, Assistant Professor of Surgery, Section of Emergency Medicine, Yale-New Haven Hospital Author and Editor Disclosure Synonyms and related keywords: radiation, radiation exposure, CT scan radiation, CT radiation exposure, ionizing radiation, radioactivity, x-ray, X-ray, gamma ray, alpha particle, imaging, nuclear weapons, dirty bomb, radiological dispersal device, RDD, terrorism, medical imaging, REM, rem, RAD, rad, radiation absorbed dose, radiation average dose, average radiation exposure, x-ray exposure, Gray, gray, Sievert, sievert, Curie, radiograph, radioactivity by energy, erg, decay activity rate, curie, Ci, becquerel, Bq, effect in air, roentgen, R, radiation-absorbed dose, biologic effect of radiation, roentgen equivalent man BACKGROUNDSection 2 of 9
  Medical diagnostic procedures used to define and diagnose medical conditions are currently the greatest manmade source of ionizing radiation exposure to the general population. However, even these sources are generally quite limited compared to the general background radiation on Earth. The risks and benefits of radiation exposure due to medical imaging and other sources must be clearly defined for clinicians and their patients. This article is a general overview for the medical practitioner, who should understand the fundamentals of medical ionizing radiation and the general associated risks. This article also acquaints the practitioner with relative doses of common radiographic procedures as well as natural background radiation.
A clear understanding of the measurement units of radiation and radioactivity is required to better communicate with colleagues or patients. Different units are used to describe radioactivity by energy (erg), decay activity rate (curie [Ci] or becquerel [Bq]), effect in air (roentgen [R]), ability to be absorbed (radiation-absorbed dose [rad] or gray [Gy]), or biologic effect (roentgen equivalent man [rem] or sievert [Sv]). See Table 2 below for a comparison of these terms.
The rad is the amount of radiation absorbed per unit mass. The current preferred term for absorbed dose is gray (Gy). One rad equals 0.01 Gy or 1 centigray. However, different tissues can have different absorbed doses and, therefore, unequal biologic effects, depending on the tissue and the source of radiation. For example, 1 Gy of alpha radiation can be more harmful than 1 Gy of beta radiation because alpha particles are much larger than beta particles and carry a greater charge. The rem is a unit that describes the equivalent dose, which accounts for the actual biological effect of radiation. The rem is calculated by multiplying the absorbed dose (rad) by a quality (Q) factor or the radiation weighting factor (RWF), which reflects the differences in the amount of potential biological effect for each type of radiation. For example, beta particles, gamma rays, and x-rays have a RWF of 1.0, making their effects on tissue largely equivalent. Alpha particles, however, have a RWF of 20, which indicates a biologic effect that is potentially 20 times greater than that of beta particles, gamma rays, or x-rays. The sievert (Sv) is the unit for equivalent dose in the System International (SI) nomenclature. It indicates what is received by each irradiated organ and relative sensitivity. The equivalent dose expressed in rem or Sv gives an index of potential harm to a particular tissue or organ from exposure to different radiation types (see Table 2 above for comparison of terms).2, 3 BIOLOGICAL EFFECTS OF IONIZING RADIATIONSection 3 of 9
Radiation damages the cell by damaging DNA molecules directly through ionizing effects on DNA molecules or indirectly through free radical formation. A lower dose delivered through a long period of time theoretically allows the body the opportunity to repair itself. Radiation damage may not cause any outward signs of injury in the short term; effects may appear much later in life. Deterministic effects, such as cell killing, can be more immediate and have a threshold above which severity increases with radiation dose. However, the threshold is not necessarily the same in each individual or tissue. While healing may ensue, necrosis and fibrotic changes in internal organs, acute radiation sickness, cataracts, and sterility may also occur. For acute deterministic effects, large doses are usually required, such as 1-2 Gy or 1-2 Sv (with x-ray exposure RWF of 1).4 (Click here to complete a Medscape CME activity on the risk of CT radiation exposure to certain patients with Crohn disease.) Stochastic effects, such as mutations, can result in cancer and hereditary effects. Cancer induction can have a long latency period. Estimating cancer risks associated with diagnostic x-rays using epidemiological tools is difficult because of extrapolation to low radiation doses, recall bias, and different x-ray energies used at various institutions. Most low-dose human ionizing radiation risk estimates come from the atomic bomb survivors in Japan. Other sources of information include laboratory cellular mutation studies and studies on various strains of mice; of course, the applicability to humans remains to be seen. Significant debate is ongoing in the scientific community regarding the effects of low-dose radiation, whether the dose-response curve is linear or nonlinear at low doses, and whether or not a threshold of adverse effect exists. Recent studies have led the Committee on Health Effects of Exposure to Low levels of Ionizing Radiations (BEIR VII) to conclude that "biologic data are emerging on phenomena that could affect the shape of the dose-response curve at low doses."5 The latency period to cancer induction from human ionizing radiation exposure varies from several years to more than 20 years, if it occurs at all.4 Radiation-induced malformations during pregnancy are important illustrations of deterministic effect. Studies on atomic bomb survivors show that the period of organogenesis (3rd-8th week) is a particularly vulnerable window. Exposure between the 8th and 15th week can lead to malformations of the forebrain, resulting in mental retardation. The threshold dose during these periods of pregnancy is much lower, potentially at 100-200 mSv. However, high doses to the embryo or fetus can result in death or gross malformations at 0.1 Sv to 1 Sv. Fetal radiation exposure can increase the risk of cancer in later childhood. Pregnant women should avoid all ionizing radiation, if possible, since x-rays to one site on the body provide some scatter dose to the fetus.4 Of course, medical necessity may require x-ray imaging of pregnant women in some circumstances. The other main sequelae of radiation are hereditary effects. Radiation damage to the gonads during the reproductive period of life produces mutations to the gametes. Inherited diseases can encompass a range of mild disorders to serious consequences, including death or severe mental defects. However, no human population studies have shown hereditary effects from typical background ionizing radiation doses. Furthermore, some studies of the offspring of atomic bomb survivors have not shown statistically significant increases in hereditary defects or cancers.6 SOURCES OF IONIZING RADIATIONSection 4 of 9
Most human exposure to ionizing radiation comes from natural sources inherent to life on Earth. The annual average dose for the world population is approximately 2.8 mSv (3.0 mSv in the
MEDICAL USES OF RADIATIONSection 5 of 9
The vast majority of artificial exposure to ionizing radiation in the general population comes from uses in medicine or allied health for diagnosis and therapy. Medical ionizing radiation contributes 0.4 mSv to the annual average dose of radiation (>14%). The most frequently used modality of radiation is diagnostic x-ray examinations. Examinations of the chest account for over 25% of all x-ray examinations.10 The most common radiographic test is the chest x-ray, and it has a wide range of effective dose—approximately 0.02-0.67 mSv, depending upon the individual and equipment settings. In conventional radiography, the effective dose that a patient receives depends on several factors. First, it depends on beam energy and filtration, which increase the average energy to result in an acceptable image. Second, collimation in radiography allows exposure to the area of interest and reduces scatter and unnecessary exposure to other tissues. Third, grids are also used to reduce scatter. Both collimation and grids act to improve radiographic images. Fourth, patient size dictates the amount of incident radiation, because the thicker the tissue in the area of interest, the higher the x-ray energy required for penetration.11 With these factors in mind, the fact that different people may have varying doses for the same commonly performed test is not surprising. Furthermore, different institutions were shown to have a wide range of doses for various diagnostic tests.12, 13 In Table 4, doses for common radiographic procedures are given in ranges, which are due to variations in technique and body habitus, as reported in the literature. Interventional radiology has the highest doses of radiation, followed by computed tomography (CT) and then plain-film radiography. Foradetailed listing of the radiation doses of medical imaging procedures, see Table 4 and Media file 1 below. The effective dose associated with most diagnostic imaging modalities in medicine covers a wide range, from less than 0.03 to more than 70 mSv.
CT has seen increased use, encompassing up to 40% of all radiographic studies. Nuclear medicine is used for treatment as well as diagnostic studies. The radionuclide technetium-99m in nuclear medicine has a short half-life of 6 hours. As shown in Media file 1 above and Table 5 below, the radiation doses from technetium scans are comparable to those of CT scans. Radiotherapy specifically uses radiation to kill cancer cells when trying to cure the cancer. To be effective, such doses typically require 20-60 Gy (or 20-60 Sv for x-ray equivalent). Table 5. Technetium Scan Radiation Doses
One growing concern in the field of medical imaging is the current trend in patient-procured whole-body CT scans.21 These scans are marketed in shopping centers directly to the general public as screening tests. These scans are sometimes routinely repeated. The positive and negative predictive values of these whole-body scans for disease detection have not been determined by quality studies to date. The American College of Radiology currently condemns the screening of healthy patients with whole-body CT scans. Radiological procedures are medically prescriptive and should "only be used for specific purposes when patient benefit outweighs potential risk."22 Studies have consistently shown that physicians who are not radiologists but who operate their own imaging equipment and have the opportunity to self-refer use imaging substantially more than do physicians who refer their patients to radiologists for imaging.23 A viable concern has been raised by many practitioners regarding the routine and repeated use by chiropractors of relatively high gonadal dose lumbar spine x-rays. Many chiropractors regularly perform repeat spine imaging on young healthy individuals, including women and children. The practice of routinely performing x-rays on women of childbearing age and children should be highly discouraged in this setting. PERSPECTIVESection 6 of 9
Medical ionizing radiation has great benefits and should not be feared, especially in urgent situations. Radiological dose and risk depends on good methodology and quality control. Obviously, using the lowest possible dose is desired. In fact, a central principle in radiation protection is "as low as reasonably achievable." Therefore, the prescribing physician must justify the examination and determine relevant clinical information before referring the patient to a radiologist. Indications and decisions should reflect the possibility of using non-ionizing radiation examinations, such as MRI or ultrasonography. Repetition of examinations should be avoided at other clinics or sites. The International Commission on Radiological Protection (ICRP) estimates that the average person has an approximately 4-5% increased relative risk of fatal cancer after a whole-body dose of 1 Sv. However, other studies on multiple cohorts of radiation workers have largely failed to establish statistically significant cancer risks. When multiple occupational cohorts were combined and evaluated in a somewhat systematic way, a combined excess relative risk of cancer death of just less than 1% was estimated.24 Table 6. Equivalent Doses of Background Radiation and Chest X-rays
Table 7 shows the ionizing radiation doses to which passengers may be subjected during air travel between various cities. Table 7. Typical Ionizing Radiation Dose From Air Travel
Debate continues over the health consequences of exposure to low levels of ionizing radiation. Most of the data were derived from estimates of exposure to the Japanese population after the atomic bombing. Recently, a study involving over 400,000 nuclear radiation workers showed a dose-related increase in all cancer mortality from radiation.26 Although the average annual radiation dose to the public from medical sources continues to be low (see Table 3), the use of medical x-rays has increased dramatically over the past couple of decades. In 1980, 3 million CT scans were performed in the United States; this has grown to more than 62 million CT scans per year. More than 4 million CT scans are performed annually on children. Some authors have estimated that one third of these scans may be medically unnecessary. In some emergency departments, an increasingly large number of patients with abdominal pain or headache are evaluated with CT scanning. Of further national and international concern is the ever-increasing threat of nuclear weapons or radiological dispersal devices (RDDs) to potentially spread ionizing radiation sources over large population areas. A basic understanding of ionizing radiation terms and relative dosages of various exposure sources may ultimately prove useful for medical practitioners faced with such exposure situations. Health physicists are trained in estimating exposure. These professionals would be highly valuable in the event of a radiation emergency but may not be readily available. Radiation exposure cannot be entirely avoided on this planet. Taking into account how much radiation people receive from natural sources, medical ionizing radiation accounts for only a small proportion of the annual average dose for the average patient. The proper use of medical ionizing radiation can greatly benefit patients. A better understanding of medical ionizing radiation allows practitioners to better communicate the risks and benefits to their patients. ACKNOWLEDGMENTSSection 7 of 9
The authors would like to acknowledge the assistance of occupational medicine resident Kathy Chang, MD, who provided input and assistance in compiling sources and tables and reviewing material as this article was being developed. MULTIMEDIASection 8 of 9
REFERENCESSection 9 of 9
Ionizing Radiation Exposure, Medical Imaging excerpt Article Last Updated: Nov 13, 2008 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
