Ionizing Radiation and Medical Imaging

Medical diagnostic procedures that are used to define and diagnose medical conditions are 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. The US National Council on Radiation Protection and Measurement reported that medical x-rays and nuclear medicine account for 15% of all radiation exposures.[1, 2, 3, 4]

The use of ionizing radiation in medicine began with the discovery of x-rays by Roentgen in 1895. Ionizing radiation is the portion of the electromagnetic spectrum with sufficient energy to pass through matter and physically dislodge orbital electrons to form ions. These ions, in turn, can produce biological changes when introduced into tissue. Ionizing radiation can exist in 2 forms: as an electromagnetic wave, such as an x-ray or gamma ray, or as a particle, in the form of an alpha or beta particle, neutron, or proton.[5]

Table 1. Relative Mass and Radiation Weighting of Ionizing Radiation Types (Greatest Effect to Least Effect) (Open Table in a new window)

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.

Table 2. Comparison of Terms Used to Define Radiation and Dose (Open Table in a new window)

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 biological 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).[5]

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).[6]

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 epidemiologic 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.

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. Studies 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."[7] 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.[6]

Radiation-induced malformations during pregnancy are important illustrations of deterministic effect. Studies on atomic bomb survivors show that the period of organogenesis (3rd to 8th week) is a particularly vulnerable window. Exposure between the 8th and 15th weeks 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.[6] Of course, medical necessity may require x-ray imaging of pregnant women in some circumstances.[8]

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.[9]

According to the American Cancer Society, the average person in the United States is exposed to about 3 mSv (millisieverts) of radiation from natural sources over the course of a year. The largest source of background radiation (typically about 2 mSv per year) is radon, a natural gas found in homes.[10]

Radiation exposures from tests include the following[10] :

• A single chest x-ray exposes the patient to about 0.1 mSv, which is about the same amount of radiation people are exposed to naturally over the course of about 10 days.
• A mammogram exposes a person to 0.4 mSv.
• A lower GI series using x-rays of the large intestine exposes a person to about 8 mSv.
• A CT scan of the abdomen and pelvis exposes a person to about 10 mSv.
• A PET/CT exposes a person to about 25 mSv.

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 United States); 85% of this comes from natural sources. The remaining proportion (15%) of the annual ionizing radiation dose comes from artificial sources, which are almost exclusively provided by medical ionizing radiation. The combined radiation exposure from nuclear fuel, Chernobyl fallout, and nuclear testing fallout accounts for less than 0.3% of the annual radiation dose (see Table 3).[11]

Table 3. Average Annual Radiation Dose Sources (Open Table in a new window)

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.[12] 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.[13]

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.[14, 15] 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.

For a detailed listing of the radiation doses of medical imaging procedures, see Table 4 and the image 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.

Average radiation dose of common radiographic procedures.

Some authors have concluded, consistent with the analysis by the authors of this article, that CT scans of the abdomen vary in effective radiation dose by as much as 13-fold, depending upon the technique and device used. Smith-Bindman et al reported that "within each type of CT study, effective dose varied significantly within and across institutions, with a mean 13-fold variation between the highest and lowest dose for each study type."[16]

According to the American Cancer Society, radiation exposures from medical tests are as follows[10] :

• A single chest x-ray exposes the patient to about 0.1 mSv, which is about the same amount of radiation people are exposed to naturally over the course of about 10 days.
• A mammogram exposes a person to 0.4 mSv.
• A lower GI series using x-rays of the large intestine exposes a person to about 8 mSv.
• A CT scan of the abdomen and pelvis exposes a person to about 10 mSv.
• A PET/CT exposes a person to about 25 mSv.

Given the tremendous variability in dose depending upon the facility, machine, and technique used to perform the imaging, the resulting variations in radiation exposure and potential cancer risk are also great. Presumably using the lowest estimated chest x-ray dose and the highest segment of the range of dose for CT scans of the abdomen, the FDA  has stated that "the radiation dose associated with a CT abdomen scan is the same as the dose from approximately 400 chest x-rays."[17] Based on their analysis, the FDA launched an initiative to reduce unnecessary radiation exposure from CT, nuclear medicine studies, and fluoroscopy. The initiative focuses on these types of medical imaging because "these procedures are the greatest contributors to total radiation exposure within the U.S. population and use much higher radiation doses than other radiographic procedures."[17]

Through this initiative, the FDA strives to promote patient safety through two principles of radiation protection developed by the International Commission on Radiological Protection[18] :

• Justification: The imaging procedure should be judged to do more good than harm to the individual patient. Therefore, all examinations using ionizing radiation should be performed only when necessary to answer a medical question, help treat a disease, or guide a procedure. The clinical indication and patient medical history should be carefully considered before referring a patient for any imaging examination.
• Dose Optimization: Medical imaging examinations should use techniques that are adjusted to administer the lowest radiation dose that yields an image quality adequate for diagnosis or intervention (ie, radiation doses should be "As Low as Reasonably Achievable"). The technique factors used should be chosen based on the clinical indication, patient size, and anatomical area scanned, and the equipment should be properly maintained and tested.

Table 4. Radiation Doses of Medical Imaging Procedures ^{[12, 13, 19, 20, 21, 22, 23, 24, 25, 26]} (Open Table in a new window)

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 (Open Table in a new window)

One growing concern in the field of medical imaging is the current trend in patient-procured whole-body CT scans.[27] These scans are sometimes routinely repeated. The positive and negative predictive values of these whole-body scans for disease detection have not been definitevely determined. The American College of Radiology has stated that radiological procedures are medically prescriptive and should be used for specific purposes when patient benefit outweighs potential risk.[2]

In a study by the Mayo Clinic of cumulative radiation doses from CT scanning in Olmsted County, Minnesota, performed between January 1, 2004, and December 31, 2013, of 54,447 adults (median age, 44.0 years at inclusion), 26,377 (48.4%) underwent at least one CT. Ten-year radiation doses from CT were 0.1 to 9.9 mSv in 15.8% of the population (8593 patients), 10 to 24.9 mSv in 16.9% (9502), 25 to 99.9 mSv in 13.8% (7492), and 100 mSv or greater in 1.9% (1041). Computed tomography of the abdomen and pelvis accounted for 67.2% of the estimated dose.[28]

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.[29] 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. 

In addition to the radiation exposure risk to patients undergoing radiologic procedures, physicians and medical staff in facilities performing imaging can be exposed to ionizing radiation. The Occupational Safety and Health Administration (OSHA) has exposure standards for employees, and various professional organizations have recommended exposure limits for health care workers potentially exposed to radiation. One study involving an analysis of ionizing radiation exposure dose among emergency physicians revealed very low levels of exposure.[30] On the other hand, radiologists using fluoroscopy and other techniques may have much higher exposure doses in the work setting.

According to one study, ionizing radiation exposure from medical procedures is rising sharply in the United States, with a per-capita annual effective dose of 3.0 millisieverts (mSv). Hemodialyzed and kidney transplanted patients may receive still higher doses of ionizing radiation due to the presence of multiple comorbidities.[31]  Patients admitted to the medical ICU (MICU) are also often subjected to multiple radiologic studies.[3]

Pediatric heart transplant patients are exposed to significant amounts of ionizing radiation during the first post-transplant year, most during scheduled catheterization. As survival improves, considering the long-term risks associated with these levels of exposure is important. In a study of 31 patients who underwent heart transplantation at a median age of 13.6 years, the median number of radiologic tests performed was 38 (range, 18-154), including 8 catheterizations (range, 2-12) and 28 x-ray images (range 11-135). Median cumulative effective dose was 53.5 mSv (range 10.6-153.5mSv), of which 91% was derived from catheterizations. These wide ranges indicate dosages and risks vary dramatically depending upon the number of imaging studies performed, and a mean, or even a median, may not represent actual individual patient risk. Older age at transplant is a statistically significant risk factor for increased exposure.[32]

Medical ionizing radiation has great benefits and should not be feared, especially in urgent situations. Radiologic 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.[33]

Cancer is a central public health problem. It is the leading cause of death in persons in the United States younger than 85 years. The lifetime incidence of cancer in the United States is 45% for males and 38% for females.[34] The overall spontaneous risk of fatal cancer in a lifetime in industrialized countries is 1 in 4 (25%). In pediatric populations, the potential for the medical uses of radiation to do harm is much greater than for adults because of children's more radiosensitive tissue and longer life expectancies.[6]

Using complex modeling, some authors have concluded that cancer risk from medical imaging can be estimated. Although clearly not as conclusive or exact as risk estimates that could be obtained from a prospective exposure study, the estimates are concerning and should be considered. After analysis of their cancer risk modeling studies, Berrington de González et al estimated that "approximately 29,000 (95% uncertainty limits [UL]; 15,000-45,000) future cancers could be related to CT scans performed in the US in 2007."[35]

Table 6 indicates the number of days of natural background radiation necessary to expose a person to the same amount of radiation in various numbers of chest x-rays.

Table 6. Equivalent Doses of Background Radiation and Chest X-rays (Open Table in a new window)

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 (Open Table in a new window)

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. A study involving over 400,000 nuclear radiation workers showed a dose-related increase in all cancer mortality from radiation.[36] The explosions at a Japanese nuclear power plant after the 2011 massive earthquake increased fear of contamination from radiation and possible health risks.

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 few 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.

X-rays (including CT scans) should be ordered judiciously. An article in the New England Journal of Medicine notes that the evidence is "convincing" that the radiation dose from CT scans can lead to cancer induction in adults and "very convincing" in the case of children.[27] Clinicians need to realize that doses from a typical CT scan can range from 6-35 times higher than the dose of a standard chest x-ray examination (see Table 4 for comparisons).

In a study comparing cancerogenesis risks posed by the 64-row detector and the 320-row detector CT scanners during coronary CT angiography (CCTA), the lifetime attributable risks (LAR) for 50-, 60-, and 70-year-old patients who underwent scanning with the 320-row detector was 30% lower for lung and over 50% lower for female breast than that with the 64-row detector. According to the study, the use of 320-row detector CT would result in a combined cumulative cancer incidence of less than 1/500 for breast in women and less than 1/1000 for lung in men. These authors concluded that lung and female breast cancer LAR reductions with a 320-row detector CT scanner compared with a 64-row detector are substantial.[37]

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.

In March 2015, filgrastim (Neupogen) was approved by the FDA to increase survival in patients acutely exposed to myelosuppressive doses of radiation (suspected or confirmed exposure to radiation >2 gray [Gy]).[38]

For more information on radiation injuries and decontamination, see the Medscape topics CBRNE - Radiation Emergencies, Radiation-Exposure Injuries, and External Decontamination for Radiation Exposure.