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The AAPM/RSNA Physics Tutorial for Residents¹

Radiation Safety Considerations for Diagnostic Radiology Personnel

¹ From the Department of Radiology, University of Florida College of Medicine, 1600 SW Archer Rd, Gainesville, FL 32610-0374. From the AAPM/RSNA Physics Tutorial at the 1998 RSNA scientific assembly. Received March 4, 1999; revision requested April 9 and received May 6; accepted May 11. Address reprint requests to the author.

	Abstract

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

 
In radiation protection, the guiding philosophy is ALARA (as low as reasonably achievable), and states have regulatory authority. Dose limits are in part based on effective dose equivalent and differences in tissue sensitivities. In diagnostic radiology, the main source of occupational dose is scattered radiation from the patient—particularly from fluoroscopically guided procedures. Personnel stand near patients for long times, and angulated geometries with C-arm equipment may result in high personnel doses from backscatter. For all procedures, judicious applications of time, distance, and shielding affect dose. Appropriate use includes collimating properly, optimizing beam-on time, minimizing distances between image intensifier and patient, ensuring sufficient distance between patient and x-ray tube, and optimizing exposure rates for image quality and dose. Although dose limits typically regulate maximum whole-body dose, protective clothing worn by fluoroscopists reduces personnel risks; weighting factors can be applied to estimate effective dose equivalent. Pregnant personnel have lower limits, which apply only with voluntary declaration of pregnancy. With appropriate precautions, fetal doses can typically remain within recommended limits without changes in occupational tasks. Radiation workers in each state must ensure that regulations are appropriate. Then, for protection of both employees and employers, the rules can and must be followed.

Index Terms: Radiations, exposure to patients and personnel • Radiations, protective and therapeutic agents and devices • Radiology and radiologists, design of radiological facilities

	INTRODUCTION

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

 
Radiation safety is important in diagnostic radiology, not only because of regulatory requirements but also because of personnel and patient considerations. This article, the first in a series on radiation safety in radiology, focuses on the major issues related to personnel x-ray safety in radiology, primarily in fluoroscopy; personnel shielding; and personnel dosimetry (occupational monitoring). The philosophy of radiation protection and associated regulations (with an overview of jurisdictions of various agencies), as well as the policies related to medical x-ray workers, including pregnant x-ray workers, are discussed. The article also introduces basic terminology for radiation safety from x rays, including definitions of quantities and units. (The Appendix lists several publications that are useful sources of information on their respective topics.)

	RADIATION SAFETY TERMINOLOGY

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

 
Quantities and Units for Quantifying Ionizing Radiation
The quantities of concern relate to (a) measurements of the ionizing radiation field, (b) the energy absorbed by objects (perhaps people) in the radiation field, and (c) the biologic effects associated with the type of ionizing radiation and the associated risk of the exposed individual. The rate of irradiation may also be a factor in addition to the energy absorbed.

Exposure.—Ionizing radiation interactions cause ionization in the medium being irradiated. For diagnostic x rays, the instrument typically used for quantifying the radiation field is an air ionization chamber, which consists of air inside a measurement volume. When the chamber is exposed to ionizing radiation, some of the air molecules are ionized and the charges are collected. Instruments may thus be calibrated to measure the radiation field intensity in the chamber volume by expressing the collected charge as coulombs per kilogram of air. Many instruments determine and display measurements of exposure in units of roentgen, where 1 R is equivalent to 2.58 x 10^-4 C/kg (1). Exposure is defined strictly for air as the interacting medium. However, the term entrance skin exposure is frequently used in comparing techniques for various radiologic procedures, and it refers to the exposure at the location in space at which the central ray of the radiation beam enters the patient. Entrance skin exposure is not equivalent to entrance skin dose, because it does not include the contributions from radiation scattered within the patient. It is, however, a quantity that can be easily measured and compared among facilities.

Kerma.—To characterize an x-ray field, the quantity exposure is being replaced by the quantity air kerma. Kerma (an acronym for kinetic energy released per unit mass) is defined for air as well as for other interacting media such as tissue (2) and quantifies the energy transferred to charged particles from ionizing photon radiation. The unit of kerma is the gray (formerly, rad; 1 Gy = 100 rad), and 1 Gy of kerma is equivalent to 1 J of energy transferred to charged particles per kilogram of irradiated medium. For air, 1 Gy of kerma is equivalent to 115 R of exposure; therefore, 1 cGy is approximately equal to 1 R.

Absorbed Dose.—Absorbed dose refers to the energy that is deposited locally in an absorbing medium from ionizing radiation. For the energies typically used in diagnostic x-ray procedures, absorbed dose and kerma are equivalent. By contrast, for high-energy photon interactions, the more energetic charged particles may deposit energy at sites distant from the initial interaction site, with a corresponding loss of electronic equilibrium. In this case, absorbed dose and kerma are not equivalent (2). The unit of absorbed dose is the gray, which is the same unit as for kerma, and 1 Gy of absorbed dose is equivalent to 1 J of energy absorbed by the medium per kilogram of absorbing medium. One gray of absorbed dose is equivalent to 100 rad, the traditional unit of absorbed dose. Doses, including skin dose, include the contributions from scattered radiation in addition to the primary radiation. Organ doses consist of the total energy absorbed by an organ per unit mass of the organ.

Quantities and Units for Quantifying Biologic Risks
Radiation Weighting Factor.—Biologic effects associated with ionizing radiation depend not only on the absorbed dose but also on the type of radiation that deposits its energy, that is, on the density of the energy deposited as the radiation passes through the absorbing medium. Technically, these values are somewhat different for different energies of x rays and electrons, as well as for other types of particulate radiations. For radiation protection purposes, such as for the determination of dose equivalent, equivalent dose, effective dose equivalent, and effective dose, these values have been standardized and simplified to the tabulated values for radiation weighting factor (W_R) (Table 1) (3). For ionizing photons (ie, x rays and gamma rays) and electrons, this value is equal to 1, but it is much higher for other particulate ionizing radiations.

Equivalent dose is the product of the average absorbed dose in a tissue or organ and its associated radiation weighting factor (W_R). The unit of equivalent dose is the joule per kilogram, with the special name of sievert (Sv).

Effective Dose Equivalent and Effective Dose.—In addition to weighting factors associated with different types of radiation (ie, W_R), additional weighting factors are associated with different tissues because of different tissue sensitivities to ionizing radiation. Both effective dose equivalent (H_E) and effective dose (E) have tissue weighting factors associated with them. These quantities are neither numerically equivalent—because of differences in their associated tissue weighting factors—nor interchangeable—because of differences in the underlying assumptions associated with their definitions.

Tissue Weighting Factor.—Different tissue weighting factors (W_T) have been defined for the determination of effective dose equivalent and effective dose. Both sets of weighting factors relate to the risks associated with cancer for different organs and the risks of creating heritable effects from future offspring as a result of radiation dose. Effective dose equivalent was defined earlier than effective dose, and rules of the Nuclear Regulatory Commission (NRC) were modified to incorporate some aspects of this quantity in its revisions to 10 CFR Part 20; therefore, this quantity is used as the basis for radiation protection limits in existing regulations (6). Effective dose equivalent includes consideration of the risks to a population of radiation workers, and hereditary effects are included for two generations of offspring of these radiation workers. Risks to extremities and deterministic effects, such as to the lens of the eye, were excluded from effective dose equivalent.

Reassessment of radiation risk was made in the late 1980s after a revised determination of the radiation doses and subsequent radiation biologic effects resulting from the atomic bombs in World War II in Japan. Analysis of these revised doses and their associated effects on survivors and their offspring led to revisions of the assessed risks associated with different organ doses. Effective dose was defined as a new quantity to take these revised risks into account (3,7,8). These risks for radiation workers include both fatal and nonfatal cancers, 4.0 x 10^-2 Sv^-1 (4.0 x 10^-4 rem^-1) and 0.8 x 10^-2 Sv^-1 (0.8 x 10^-4 rem^-1), respectively, and severe genetic risks of 0.8 x 10^-2 Sv^-1 (0.8 x 10^-4 rem^-1). In addition, risks for members of the general public are included so that these risks may be used for the assessment of patient risks, for example, in research protocols. The list of organs considered for tissue weighting factors to determine effective dose was expanded from the list for effective dose equivalent to include the esophagus, stomach, liver, and bladder and thereby reduced the fraction considered as "remainder." In both cases, the total risk is 1.00.

Table 2 presents the tissue weighting factors for both effective dose equivalent and effective dose. Current scientific understanding of the risks associated with exposure to radiation is described by effective dose. However, current radiation protection regulations are based on the older quantity, effective dose equivalent. Therefore, both sets of values need consideration at the present time.

	RADIATION SAFETY PHILOSOPHY AND ALARA

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

In most environments, radiation safety means minimizing all radiation exposure to all individuals; however, medical care presents a unique situation in which patients are intentionally irradiated. Protection philosophy requires consideration of the benefits of the radiation to which a person is exposed. The goal is to perform diagnostic procedures that optimize both radiation exposure and diagnostic information.

Patients and personnel must be considered separately: Regulatory dose limits for radiation workers do not apply to medical irradiation received by patients. Therefore, when a radiation worker becomes a patient, only patient considerations are applicable.

	BASIC PRINCIPLES OF X-RAY SAFETY

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

 
Beam-on Time Considerations
The duration of exposure of the individual is directly proportional to the occupationally received radiation dose. Therefore, personnel doses may be directly linked to patient doses. In fluoroscopy, typically both the peak kilovolt-age (kVp) and milliamperage (mA) are adjusted automatically, and the operator can control only the time that the x-ray beam is on. Reducing both personnel and patient doses may be accomplished effectively by minimizing the total "beam-on" time for a procedure through judicious use of the exposure switch to ensure that irradiation is occurring only when the fluoroscopist is actively viewing the image. Use of a last-image hold feature can be helpful in reducing the overall beam-on time, so that scrutiny of anatomic features can be accomplished without a competing concern about additional radiation dose. Teaching hospitals in particular should provide fluoroscopes with a last-image hold feature, and all new fluoroscopes should be purchased with this feature.

Another consideration in personnel dose is the time that the worker is inside the fluoroscopy room or near a mobile radiography unit when exposures are being made. Optimizing the number of images in an exposure sequence for serial radiography (including digital radiography) or cinefluorography by controlling both the exposure rate and duration of the sequence allows both patient and personnel doses to be optimized. Use of a control booth whenever possible is desirable to minimize occupational dose. Similarly, bringing patients to radiographic rooms with control booths rather than bringing mobile units to patient beds whenever possible is also desirable.

Definitions for X-ray Interactions and Safety
The x-ray beam and its interactions and their implications for safety considerations are illustrated in Figure 1. A small amount of radiation is emitted through the shielded x-ray tube housing (ie, leakage). The useful or primary beam exits the tube and interacts with the patient: Some radiation is absorbed (the source of patient dose), some is transmitted (the source of the image), and some is emitted from the patient as primary scatter (the source of potential hazard). The patient attenuates the primary beam so that 5%–15% is transmitted; the beam transmitted through the patient interacts with one or more barriers—preferably only the image receptor. Primary scatter is emitted from the patient in all directions, which is the major source of hazard to personnel in mobile radiography and fluoroscopy. The patient thus becomes the source of scattered radiation but only during the time of the primary radiation exposure. Primary scattered radiation may also interact with other objects (including people) and produce secondary scattered radiation. The intensity of secondary scatter is much lower than that of primary scatter. Stray radiation is defined as the sum of the scattered radiations and x-ray tube housing leakage, with scattered radiation from the patient being the major component.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 1.  Schematic provides a graphical representation of definitions of radiation safety terms as they are used in radiation protection (with an x-ray tube as the radiation source).

View larger version (16K): [in this window] [in a new window] [Download PPT slide]   Figure 2.  Schematic illustrates the inverse-square law. As the surface area of a sphere is proportional to the square of its radius, the radiation intensity decreases proportionately as the square of the distance from a point source. The ray spacing is related to the radiation intensity.

Keeping the image intensifier or imaging assembly as close to the patient as possible minimizes the overall distance between the focal spot and image receptor. This geometry keeps the fluoroscopic beam intensity as low as possible, allows the image intensifier to serve as a scatter barrier between the patient and operator, and minimizes image blur simultaneously. In Figure 3, appropriate fluoroscopic geometry is compared with two poor geometries.

View larger version (13K): [in this window] [in a new window] [Download PPT slide]   Figure 3.  Schematics illustrate three fluoroscopic geometries. In (a), the x-ray tube is too close to the patient, which is indicated by the large change in ray spacing between the entrance to and exit from the patient. (Ray spacing is related to the dose rate.) In (b), the x-ray tube is farther from the patient, and the dose distribution is more uniform; however, the image intensifier tube is too far from the patient, which causes the patient entrance dose to be increased. In (c), both tubes are placed appropriately.

Diagnostic radiology health care personnel typically receive little to no radiation dose while working in control booths, a small amount while working with mobile radiographic units, and most of their radiation dose from fluoroscopy. The main source of this dose is scattered radiation emitted from the patient. Thick body parts and large patients, as compared with thinner anatomy, require more primary radiation to allow sufficient transmitted radiation for the formation of an adequate image, because the requirement for adequate image quality is based on the radiation that enters the image intensifier. This requirement depends on the quality of the imaging system components, as well as the type of study being performed and the fluoroscopist's willingness to accept noisier images from a larger aperture in the camera optics (10).

Several clinical use choices impact scattered radiation dose. Exposure rates at the location of the beam entrance to the patient depend on the patient's anatomy and the amount of contrast medium in the imaged volume. Maximum limits are set by federal regulations at 2.6 mC/kg/min (corresponding to air kerma rates of approximately 4 Gy/min) for normal image-intensified fluoroscopy and twice that for equipment with optional high-level controls manufactured after May 1995 (11). Although the high dose rate mode in fluoroscopy produces images with lower levels of noise, use of this mode may potentially increase radiation exposures to personnel and patients, because higher levels of primary radiation correspond to production of more scattered radiation. For older equipment, federal regulations have no limit to the high dose rate mode, and some units have been set by manufacturers to allow very high dose rates (five to 10 times the new equipment limit).

On the other hand, pulse-mode fluoroscopy typically corresponds to an overall decrease in cumulative beam-on time by limiting the irradiation time to a portion of the procedure. The reduction in dose rate is likely not to be as great as one might expect from the fraction of the beam-on time, because the tube current is typically increased to reduce the noise level because of perception considerations; however, significant reduction in dose, perhaps by a factor of two, is achievable, compared with that incurred in continuous fluoroscopy (10).

Use of photofluorospot film, cinefluorographic film, and digital recording of fluoroscopic images requires higher dose rates per image than doses required for fluoroscopic viewing. The maximum exposure rates described above for fluoroscopy do not apply for these uses, nor do they apply for conventional spot film (ie, radiographic) imaging. Each image requires a patient entrance air kerma of 30–400 mGy, and serial imaging involves many images taken at a high repetition rate (12). Clearly, the total dose depends on the number of images, which is lowered by a reduction in the repetition rate. As for all radiologic procedures, only essential personnel should be in the room during the exposures.

The effect of equipment quality control is of particular significance in fluoroscopy. As the image intensifier ages and loses efficiency, the automatic brightness control automatically increases the primary beam technique factors and exposure rates to keep an adequate light level to the camera. Monitoring equipment performance over time is necessary to identify this source of potential hazard.

When the collimation is inadequate, the radiation field can be larger than suspected, with an increase to patient and personnel doses. Collimation to a smaller beam area reduces two dimensions of the scattering volume and also improves image contrast. Figure 4 shows the effects of field size (ie, collimation) and distance on scattered radiation intensity for a constant x-ray tube current and a fixed phantom. It also shows the effect of peak tube potential, which typically increases automatically with patient thickness. As peak tube potential increases, this effect is large because (a) the primary beam intensity increases and (b) the scattered radiation is more penetrating. The effect of collimation is dramatic (Fig 4).

View larger version (17K): [in this window] [in a new window] [Download PPT slide]   Figure 4.  Graph depicts the effects of collimation, distance, and peak tube potential (kVp) on scattered radiation intensity. Measurements were made at 3 mA with a thin plastic cube phantom 22 cm on each side and filled with water for scattering. The location of measurements was at a height of 1.5 m (5 ft) above the floor to simulate dose rates to the head of many individuals. The scattered radiation from the smaller field, shown as stars for the three peak tube potentials, is much lower at all distances from the table and particularly so at the table side. The fluoroscopist can back away one-third of a meter (approximately 1 foot) from the table by taking one step backward.

View larger version (14K): [in this window] [in a new window] [Download PPT slide]   Figure 5.  Schematics show the effects of clinical field location on radiation scatter. In (a), the primary beam is close to the midline of the patient and is associated with less intense radiation scattered to the fluoroscopist because it is attenuated more through a greater thickness of overlying tissue and is farther away. In (b), there is more scatter to the fluoroscopist, because the primary beam is closer to the operator and less attenuated by the patient. Because of these factors, the fluoroscopist may not be fully aware of the intensity of either the primary beam or the scattered field.

View larger version (43K): [in this window] [in a new window] [Download PPT slide]   Figure 6.  Schematics show the effect of C-arm geometry on backscatter. In (a), the fluoroscope has an overhead x-ray beam orientation, with the image intensifier below the patient. The face of the fluoroscopist is not shielded from scattered radiation and is fairly close to the x-ray tube housing, from which leakage radiation emerges (typically, but not always, a small amount). In (b), the overhead image intensifier acts as a barrier to protect the face of the fluoroscopist, with the backscattered radiation directed toward the floor.

Stray Radiation.—Stray radiation fields for two orientations of an x-ray unit in a cardiac catheterization suite are illustrated in Figures 7 and 8 (14). Figure 7 shows the stray radiation measured for a phantom with isokerma curves for the 90° left anterior oblique cardiac projection, that is, with the image intensifier to the patient's left and with the x-ray beam horizontally directed approximately 1 and 1.5 m above the floor. Figure 8 shows the stray radiation with isokerma curves for the 60° left anterior oblique cardiac projection for the same two heights above the floor. It is clear from Figure 8 that both the x-ray tube and image intensifier provide shielding and that the angulation of the x-ray tube to the floor provides additional protection.

View larger version (53K): [in this window] [in a new window] [Download PPT slide]   Figures 7, 8.  (7) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 90° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.) (8) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 60° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.)

View larger version (54K): [in this window] [in a new window] [Download PPT slide]   Figures 7, 8.  (7) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 90° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.) (8) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 60° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.)

View larger version (49K): [in this window] [in a new window] [Download PPT slide]   Figures 7, 8.  (7) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 90° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.) (8) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 60° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.)

View larger version (48K): [in this window] [in a new window] [Download PPT slide]   Figures 7, 8.  (7) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 90° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.) (8) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 60° left anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.)

The most recent NCRP report on shielding was written in 1976 (15), before the current guidelines for occupational exposure to radiation were written. NCRP Report No. 116, written in 1993, recommends that new facilities be designed to limit occupational doses to a fraction of the annual cumulative dose limit of 10 mSv (1 rem) and to limit doses to others (ie, not occupationally exposed persons) so that they do not exceed 1 mSv (100 mrem) annual effective dose (8,16). This guidance is in stark contrast to the design criteria in NCRP Report No. 49 (15) of 50 mSv (5 rem) per year for controlled areas occupied by radiation workers and 5 mSv (500 mrem) per year for uncontrolled areas occupied by others. In addition, the ionizing radiation workload in diagnostic radiology has changed substantially since 1976, particularly in interventional procedures and computed tomography. Therefore, guidance for shielding design is in transition, and revisions to the guidelines are under development.

Types of Shielding.—One aspect of shielding design that should be considered for all modern work areas is control booths that are sufficiently large to shield all the personnel who do not need to be in the examination area during spot radiographic, fluorospot radiographic, cine radiographic, or digital exposures. Booths with large windows and additional monitors for remote viewing are particularly useful in teaching hospitals.

Mobile barriers may be rolled into position inside radiography-fluoroscopy suites to protect individuals (eg, anesthesiologists and nurses) who need to be near patients for extended periods during the procedures. Ceiling-suspended barriers are available as transparent shielding during interventional procedures to protect the entire upper body of the fluoroscopist. In addition, the x-ray equipment may offer shielding options: Fluoroscopic tables may be purchased with optional shielding and leaded drapes that protect personnel from patient scatter.

Flexible protective clothing such as aprons, vests, skirts, thyroid shields, and gloves should be available for the times when an individual must work in an unshielded environment. The necessity for wearing protective items depends on an individual's work habits and environment, facility policies, and applicable regulations. The typical flexible material for protective clothing is lead-impregnated rubber. Special composite materials that may be lightweight and easier to wear are also available but usually more expensive. Eyeglasses with side shields are available with or without prescription lenses for eye protection for personnel who need them.

Personnel who may have their backs exposed to the x-ray beam should wear wraparound shielding. In wraparound garments, the back shielding is typically only one-half the thickness of the front shielding. For skirts and vests, the shielding is typically one thickness and is doubled in front and held in place with straps. Frequently, wraparound items are not completely doubled when they are worn, and the shielding may not be as protective as expected. That is, for a wraparound apron that is specified to be equivalent to 0.5 mm of lead, one thickness of the material is only 0.25 mm lead equivalent in the back and wherever the material is not overlapped. Personnel who need to wear wraparound aprons or their equivalent are typically support personnel who move around the room during procedures and therefore cannot be protected by mobile barriers. Others may wish to wear them for reasons of comfort.

Figure 9 shows an operator working in a typical radiography-fluoroscopy suite. The radiographic table and its side shield protect personnel from stray radiation that originates below the table; this shielding is absent for C-arm units. Figure 10 shows a fluoroscopist wearing leaded eyeglasses with side shields in a neuro-angiography suite with a biplane C-arm unit. In this room are a mobile floor-standing (mostly transparent) barrier for use by other personnel such as anesthesiologists and a ceiling-suspended transparent barrier for interventionalists. Because this shield is not always used, the fluoroscopist is wearing protective eyeglasses.

View larger version (123K): [in this window] [in a new window] [Download PPT slide]   Figure 9.  Photograph shows a typical set-up in a radiography-fluoroscopy suite. The table provides a flip-up side shield to cover the Bucky slot, and the imaging assembly has protective drapes to protect the fluoroscopist from patient scatter. The operator is wearing a protective skirt and vest and a heavy leaded glove to protect her hand when it is exposed to the primary beam.

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 10.  Photograph shows a biplane C-arm set-up in a neuroangiography suite. The room contains a mobile floor-standing (mostly transparent) barrier for use by other personnel and a ceiling-suspended transparent barrier with a suspended drape to protect the upper body of the interventionalist from scattered radiation (mostly backscatter). This shield is not always used; therefore, the physician is wearing protective eyeglasses. The importance of side shields for eyeglasses is shown: As the fluoroscopist observes the monitor, his face is rotated so that the lens of his left eye would be exposed to scatter if the side shield were absent. He is also wearing a thyroid shield to protect important organs and tissue (eg, thyroid and sternum).

Principles of Attenuation.—The amount of radiation attenuated by a material depends on the elemental composition of the material, its thickness, and the energy of the radiation passing through it. Each element has attenuating properties that are related to its atomic number and the binding energies of its electrons. Lead has historically been used as an effective shield because of its high attenuation. However, all elements, including lead, have discontinuities that create transmissive notches in their attenuating properties as a function of energy; these discontinuities (most frequently, the K-edge) have to do with the binding energies of the electrons in various orbitals. Therefore, the amount of lead required for safety purposes (typically 0.5 mm of lead equivalent thickness for aprons) is related to the overall transmission of scattered radiation, and the discontinuities affect the overall effectiveness of the attenuation. The transmission through 0.5 mm of lead is 3.2% at 100 kVp and 0.36% at 70 kVp (17).

Lead aprons are somewhat heavy to wear, and they can become uncomfortable at times for individuals wearing them. Aprons of 0.5 mm lead equivalent have been reported to range from 2.5 to 7 kg (18). Aprons can be custom made, so that they are only as long or as wide as they need to be for protection purposes. Well-fitting aprons cover much of the red bone marrow, and for women they should cover breast tissues (thyroid shields can also be used for this purpose). Different styles (eg, belted, two-piece, shoulder-supported) are available and were designed to improve comfort by taking some of the weight off the shoulders and upper back and sharing it with the hips and lower back.

Some aprons are made of composite materials of lead combined with elements of somewhat lower atomic numbers so that the weight of the apron is less than for lead alone (17,19). The elements that make up these composite materials are chosen so that their discontinuities in attenuation occur at somewhat lower energies than lead and so that they absorb more effectively than lead in the transmissive notches for lead. Because the combination provides a more uniform attenuation than for one element alone, composite materials can be essentially equivalent in effectiveness to lead with a lower total mass or weight by 15%–25%. Figure 11 shows the attenuating properties of three elements (tungsten, barium, and lead) used in a composite material described by Yaffe and Mawdsley (17).

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 11.  Graph shows attenuation characteristics of three elements used in a composite material, demonstrating that the three elements provide more uniform attenuation than lead alone. (Adapted from reference 17.)

	RADIATION SAFETY REGULATIONS

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

Occupational safety in general comes under the jurisdiction of the U.S. Occupational Safety and Health Administration. However, inspection for, compliance with, and enforcement of, both occupational safety and radiation control regulations are typically performed by state regulatory agencies.

The U.S. Center for Devices and Radiological Health under the Food and Drug Administration sets standards for new equipment that produces radiation. However, this agency does not set occupational dose limits for workers who use the equipment.

The NRC regulates the production of radioactive materials that come from nuclear reactors as well as the safety of people who are exposed to the radiations from these materials. X-ray personnel who are also occupationally exposed to these radioactive materials are regulated by NRC rules.

States have authority to regulate radiation safety for all sources, including x-ray tubes, and all persons within their boundaries.

At the local level, institutions can create their own policies that are more strict than federal and state regulations for the purpose of keeping radiation exposures ALARA.

The interplay among the regulatory authorities is intricate and is made somewhat uniform for different radiation sources and users through the Conference of Radiation Control Program Directors (CRCPD), a nonregulatory organization. The CRCPD, whose members are mostly the heads of the state-controlled radiation control programs, has created the Suggested State Regulations for Control of Radiation (SSRCR) (20). The SSRCR are model regulations that combine all the federal radiation standards into one document, and the states can adopt any, all, or none of them. Most states use at least some of the SSRCR, and they are required to adopt NRC rules for safety associated with nuclear by-product radiation (ie, with radiation from reactors and from radioactive materials created in the reactors).

Dose Limits for X-ray Workers
The 1995 revisions to NRC rules 10 CFR Part 20 include some, but not all, aspects of the newer quantity effective dose equivalent (6). In these revisions, NRC used effective dose equivalent for evaluating the doses to individuals exposed to internal sources; however, the agency kept its previous policy that doses received from external sources were to be evaluated by the older quantity deep-dose equivalent. Such externally received doses are considered to have been received from whole-body irradiation when certain parts are exposed (such as the head or upper arm), without consideration of the scientific basis underlying the concept of effective dose equivalent. Because of a need to combine values to set limits for individuals exposed to both external and internal sources, the NRC defined new quantities, the total effective dose equivalent (TEDE) and the total organ dose equivalent (TODE) for specific organs.

The TEDE and TODE consist of the sum of the deep-dose equivalent from external radiation sources and a quantity related to effective dose equivalents from internally deposited radioactive sources (5). Only annual limits are set, and there is no allowance for a lifetime limit. For x-ray workers, the annual TEDE limit is 0.05 Sv (5 rem), which is equal to the previous whole-body limit; this is the limit against which the value from the required whole-body dosimeter is assessed. Some states have quarterly limits in addition to the annual limit.

The lens of the eye is limited to 0.15 Sv (15 rem), and any other organ or tissue is limited to 0.50 Sv (50 rem). Because of the high dose limit to the eye, it might be argued that protective eyeglasses are unnecessary. Certainly they are not warranted for routine fluoroscopy or when structural shielding is employed. However, wearing glasses is justified and certainly follows the ALARA philosophy for fluoroscopists who perform many fluoroscopically guided interventional procedures without upper-body shielding from interposed transparent barriers.

In NRC rules, the deep-dose equivalent for an individual does not allow consideration of partial-body shielding. That is, the NRC requires that a radiation worker wear a dosimeter, called an individual monitoring device, at the location at which the individual receives the highest deep-dose equivalent. This value (ie, the highest deep-dose equivalent) is considered to have been received uniformly by the whole body, without consideration of protective shielding that covers most of the body (eg, an apron worn by a fluoroscopist). However, one must keep in mind that NRC rules apply to users of nuclear by-product materials and do not necessarily apply to x-ray workers who do not use radioactive sources unless the individual state regulations incorporate the NRC rules and apply them to the latter individuals as well.

The SSRCR are required to be identical to NRC rules for users of nuclear by-product materials. Because of concerns that personnel in interventional radiology suites were not being offered consideration for wearing protective apparatus (eg, aprons, thyroid shields, eyeglasses) and that NRC rules were too restrictive in this situation (for which the rules were not intended), the CRCPD accepted optional special considerations for fluoroscopists. That is, the SSRCR have combined the NRC rules with a special addition for fluoroscopists, which allows consideration for determining the effective dose equivalent based on a fluoroscopist wearing an apron (21). However, each state must determine whether this feature (or another substitute) is adopted, and it is up to the affected x-ray workers in each state to provide input to the state to ensure that its rules are reasonable so that they can be followed.

The optional SSRCR rules for handling partial-body exposures from external radiation sources are written specifically for fluoroscopists. These rules were written as a way of estimating effective dose equivalent for fluoroscopy personnel who are likely to exceed the NRC-defined whole-body deep-dose equivalent limit of 0.05 Sv (5 rem) in a year in the normal performance of their work. The individuals involved are typically those who perform interventional procedures on a regular basis and, in particular, those who do not use transparent barriers to shield their upper bodies during procedures. The rules require a dosimeter to be worn at the neck as an estimate of the dose received by the unshielded portion of the body, and they allow an additional dosimeter to be worn under an apron of 0.5 mm lead equivalence to give a more realistic estimate of the dose received by the major portion of the body. When the two dosimeters are worn in this way, the effective dose equivalent for fluoroscopists may be estimated from the relationship H_E = 0.04(N) + 1.5(W), where H_E is the effective dose equivalent, and N and W are the unshielded neck and shielded waist deep-dose equivalent values, respectively (22). This equation was derived by Webster (23) from data reported by Faulkner and Harrison (24), who evaluated the relationships among effective dose equivalent, peak tube potential, and apron thickness for a fluoroscopic geometry.

Because an individual might not wear two dosimeters, the SSRCR also allow an estimation of effective dose equivalent from a single unshielded neck dosimeter from the relationship H_E = 0.3(N). This 0.3 factor overestimates the value determined by weighting the values for two dosimeters; however, it provides a better estimate of the dose than the value reported for the unshielded dosimeter worn at the neck. This neck dosimeter can also be used to estimate eye dose when protective eyeglasses are not worn. A ring dosimeter is worn to estimate the extremity dose, with the dosimeter facing the beam (ie, toward the palm of the hand) on the hand that is likely to be exposed (25).

Considerations for Pregnant Workers
Regulations.—Because of the relationship between the NRC and the CRCPD in creating the SSRCR, NRC rules regarding pregnancy have found their way into state regulations for x-ray workers. Foremost in consideration is the right to privacy for the individual: She is not required to make known that she is pregnant to her employer, even if it is obvious that she is. Special rules about dose limits for the conceptus apply only after a written, voluntary, official declaration that includes the estimated date of conception.

These rules stipulate that (a) the occupational dose to an embryo or fetus of an occupationally exposed individual must not exceed 5 mSv (0.5 rem) during the entire gestation period; (b) any dose must be received relatively uniformly over time, so that typical doses are not high in any one particular phase of gestational development; and (c) the deep-dose equivalent of the worker must be used as the dose to the embryo or fetus (ie, the value reported from an unshielded dosimeter worn on the trunk of the worker) (26). For x-ray workers who wear protective aprons and one dosimeter outside the shielding, use of the deep-dose equivalent clearly results in a gross overestimate of the dose to the conceptus. NCRP Report No. 116 recommends "a monthly equivalent dose limit of 0.5 mSv [equivalent to 50 mrem] to the embryo-fetus (excluding medical and natural background radiation) once the pregnancy is known" (8).

For personnel wearing protective clothing, the dose to the embryo or fetus is estimated much more appropriately from a dosimeter worn beneath the shielding. Some states have separate rules for pregnant x-ray workers. In Florida, for example, all radiation workers are required to wear one dosimeter, and a radiation worker with a declared pregnancy is required to wear an additional monitor underneath protective clothing (27). This rule should be followed by all pregnant radiation workers who sometimes or routinely wear protective aprons.

Supporting Data.—Table 3 shows data correlated between unshielded neck and shielded waist dosimeters for individuals working in special procedures (ie, angiography and interventional radiology) at Shands Hospital at the University of Florida for a total of 343 person-months. It can be seen that most shielded waist dosimeter values were less than 0.05 mSv (50 mrem) in 1 month, even for some individuals with very high unshielded neck values (0.41–23 mSv [410–2,300 mrem]). Five person-months exhibited waist values of 0.6 mSv (60 mrem), and only one exceeded 0.6 mSv (60 mrem). These high values were received by radiology fellows who were performing angiographic and interventional procedures continuously for more than 40 hours a week. It can be seen that typical shielded waist doses are well within limits for pregnant personnel.

Maternity Aprons.—Commercially available maternity aprons are wide and have a double thickness of lead in a location that is appropriate for many pregnant individuals. Other workers may want to add additional shielding to an apron that they prefer wearing. In either case, the wearer needs to ensure that her unshielded back is not facing the x-ray source: Not only is there no shielding, but the dosimeter measures the radiation field after it has been attenuated by the conceptus.

Aprons with additional protection may not be desirable for everyone because the extra weight may be burdensome. The data for Table 3 indicate that, even for individuals working in interventional radiology suites, shielded waist values exceeded 0.02 mSv (20 mrem) only for some individuals who performed the procedures full-time. In no case did the values for any nurse or technologist exceed this value, and no maternity aprons were worn during the period of data collection. These data can be compared with data for personnel who work in typical fluoroscopy suites and perform more conventional procedures: Monthly unshielded neck values were typically 0.3–0.6 mSv (30–60 mrem), and shielded waist values were less than 0.1 mSv (<10 mrem).

All workers wore aprons with 0.5 mm lead equivalence, in accordance with recommendations of NCRP Report No. 102 (1). Some individuals who work farther from the patient than the fluoroscopist might prefer less shielding and less weight from a thinner apron. However, storing aprons of different thicknesses in the same area might lead to a misunderstanding on the part of an individual in choosing one to wear. Therefore, aprons different from the norm should be clearly identified. In any case, state regulations regarding minimum apron thickness need to be followed.

Maternity Policies.—In addition to regulations and guidelines, each employer needs to have policies in place, not only about personnel pregnancy but also for family leave in general. A 1986 report from the American Association for Women Radiologists (AAWR) (now the American Association for Women in Radiology) on the impact of maternity on radiologists was published as an editorial (28). This report states that scientific findings do not support limiting occupational fluoroscopy during pregnancy, and expresses concern that women might be subject to job discrimination if they require special assignments because of pregnancy.

A 1992 article in the American College of Radiology Bulletin recommends that written policies are necessary for pregnant workers in radiology (29). The AAWR performed a survey in 1993 on maternity policies in radiology departments, and recommendations on policies were published in a 1995 report (30). Written maternity policies are recommended because workers need to know (a) what is expected of them and (b) that they will receive unbiased consideration.

Decisions about working in a radiation environment need to be made by both the employer in setting up facility policy and by the employee in making personal choices. Decisions are best made with an understanding of the facts, both scientific and regulatory. The following three resources can help with understanding the issues: (a) NCRP Report No. 116 (8), which presents risks and scientific guidance; (b) NRC draft regulatory guide DG-8014, proposed revision to Regulatory Guide 8.13 (31), which describes risks and rules; and (c) a 1982 review publication by Wagner and Hayman (32), which evaluates various radiologic work environments.

	RADIATION SAFETY POLICIES

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Personnel Dosimetry
Personnel dosimetry policies need to be in place for all occupationally exposed individuals, regardless of pregnancy. All employers are required to provide a dosimeter to all employees who have a likelihood of receiving doses as large as 10% of the applicable legal limit (eg, whole body, lens of the eye, extremity). It is assumed that all personnel who perform x-ray procedures can potentially receive doses sufficiently large to require at least one dosimeter for monitoring whole-body dose. Fluoroscopists may need additional dosimeters.

All employees are required to wear their dosimeters: It is up to the employer to see that these rules are followed. With regulations in place that allow determination of effective dose equivalent from weighting factors for partial-body irradiation, personnel are able to stay within regulatory limits without fudging (ie, not wearing dosimeters part of the time). No excuses are acceptable: It is reasonable for a facility not to allow an employee to spend any time (ie, work) in a radiation area unless a dosimeter is worn properly. To do otherwise puts both the employer and the employee at risk.

Occupationally exposed individuals need to understand that the dosimeter provides a number that has regulatory and legal implications and that the dosimetry report is a legal document. The data are reliable only when the dosimeters are properly worn, receive proper care, and are returned on time. Proper care includes not irradiating the dosimeter except during occupational exposure and ensuring proper environmental conditions.

Detectability Thresholds
Dosimetry reports are prepared from dosimeters that are evaluated in dosimetry laboratories. The National Institute of Standards and Technology has a voluntary accreditation program for dosimetry laboratories that is designed to ensure that certain standards are met in dose assessment and reporting. All types of dosimeters have potentially variable responses for dose assessment. They also have a lower limit of detectability (LLD), which is the lowest value that can be reported with a specified certainty. The LLD depends on the type of dosimeter and the type and energy of radiation, and some new technologies have improved assessment of the LLD.

One concern with the LLD relates to pregnant workers. If the lowest reportable value is 0.01 mSv (10 mrem) and if the dose received is just below that level, it will be reported as a minimal value (ie, the smallest reportable value). This minimal value should not be assumed to be equal to zero. When a more precise number is desired, a more sensitive dosimeter should be selected. This issue is typically not critical with current work environments and regulations; however, if dose limits were to be lowered, technical problems might develop.

In a potentially high-dose environment, a pregnant radiation worker might wear an electronic self-reading dosimeter for short-term assessment in addition to a monthly dosimeter. In this way, she can monitor her dose for a particular task and ensure that she terminates her work or leaves the environment when the instrument value reaches a particular threshold.

Practices of ALARA
Personnel Dosimetry.—Local policies are likely to uphold ALARA principles for occupational radiation doses. At many institutions, ALARA policies include two sets of criteria for evaluation: An upper level is set to the legal annual or, in some cases, quarterly limit, and a lower level, and perhaps a monthly level, is set as a local standard for assessing values against those that are typical and expected. Investigating causes for unexpected findings is part of an ALARA program, and investigations should include looking for missing data or values that are inappropriately low as well as for causes of high values.

Quality Control.—When an unexpectedly high dosimetry value is returned for an individual, ALARA implementation implies that an attempt is made to explain the cause: Sometimes the reading is faulty, sometimes it relates to a change in work habits, and sometimes it relates to equipment malfunction. Therefore, in addition to monitoring reported dosimetry values, which relate to activities of individual workers, ALARA implementation includes quality control assessment of equipment and policies on equipment use.

Equipment needs to meet certain standards for producing radiation, and regulations and guidelines apply. For the safety of fluoroscopists, the most important criteria are that the primary beam be limited in size to the primary barrier and that the exposure rate be limited to known levels with adequate image quality. Because fluoroscopic imaging systems degrade over time, ALARA implementation includes periodic assessment of tabletop air kerma (or exposure) rates and associated image quality. Minimizing the radiation used in each patient procedure with careful use of the fluoroscopic exposure switch (ie, beam-on time) and reducing repeated studies simultaneously minimize personnel doses.

Some radiology personnel (eg, technologists and nurses) are exposed to scattered radiation from fluoroscopic procedures that are performed by physicians who are not radiologists. This situation occurs frequently in operating rooms and hospital areas not within the radiology department. It is important for all physicians who operate fluoroscopes to be educated and trained in principles of radiation safety and equipment operation and about the risks to patients. Hospital credentialing of physicians to operate fluoroscopes can be an effective method of oversight (33).

Continuing Education.—Personnel need to be aware of techniques that they can employ to minimize their doses as well as those of personnel working nearby. Using available shielding, particularly with certain overhead x-ray tube configurations, can be important. Collimating to the patient anatomy of interest not only reduces scatter but also improves image contrast. As technology develops, new types of procedures offer possibilities of potential benefits as well as hazards that need to be evaluated. Continuing education is key.

	SUMMARY

Top Abstract INTRODUCTION RADIATION SAFETY TERMINOLOGY RADIATION SAFETY PHILOSOPHY AND... BASIC PRINCIPLES OF X-RAY... RADIATION SAFETY REGULATIONS RADIATION SAFETY POLICIES SUMMARY Appendix 1 References

 
In summary, ALARA policies can be effected through understanding and using the key parameters time, distance, and shielding. Typically, reducing patient dose also reduces dose to personnel. Therefore, performing optimized procedures is an important aspect of radiation protection in diagnostic radiology. Optimization of procedures includes performing only medically necessary studies and performing them sufficiently well that they do not need to be repeated. Optimization also means ensuring that the patient dose for each study is as low as is commensurate with the medical charge. Using the best geometric configuration for fluoroscopic procedures (good collimation and proper distances among the x-ray tube, patient, and image intensifier) optimizes image quality and reduces the hazard from scattered radiation. Using last-image hold and pulsed fluoroscopy reduces exposure times in procedures, and operating the fluoroscope only while viewing the monitor minimizes wasted radiation. Similarly, using frame rates and series durations that are as low as diagnostically acceptable in serial radiographic and cine studies reduces the overall time of patient irradiation. In these ways, both patients and personnel receive ALARA radiation doses.

In addition, personnel can concentrate on reducing personal doses. One means is through judicious use of barriers, that is, protective clothing and structural shielding such as control booths or mobile transparent shielding to interpose between oneself and the patient (who is the main source of the exposure). Another is through keeping the distance between oneself and the patient as large as is reasonable, while not interfering with work efficiency.

Valid assessment of personal dose can be made only by wearing personal dosimeters properly and by returning them on schedule. Rules are state-specific regarding dose limits for x-ray workers, and institutional policies may be stricter. Wearing aprons is a regulation; wearing additional protection may be an issue of local policy, or it may be a personal choice.

Special rules apply for pregnant w