Cardinal Principles of Radiation Protection

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This article discusses a traditional topic in radiation protection-Cardinal Principles of Radiation Protection. The three Cardinal Principles of radiation safety are: Time, Distance, and Shielding. This article takes a practical approach to radiation protection for the occupational worker and the patient. Radiographers have low occupational risk of late adverse effects of working with ionizing radiation when the Cardinal Principles are adhered too. This article discusses practical approaches to applying the fundamental rules that afford x-ray technologists a lifetime of safe occupation employment.
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Author: Nicholas Joseph Jr., RT(R)(CT)
Credits: 2.5

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Section 3.1: Cardinal Rule of limiting time of exposure

Section 3.2: Cardinal Rule of distance

Section 3.3: Cardinal Rule of applying shielding

Section 3.4: Work Area Definitions

Summary and References



Upon completion the reader will be able to:

  • List and discuss the three Cardinal Principles of radiation protection.
  • State how the primary fluoroscopist can limit exposure time to patients and others during dynamic imaging procedures.
  • Discuss FDA regulations for fluoroscopy equipment that help the technologist in regulating the cumulative time of radiation exposure.
  • Discuss why distance is the best method of limiting radiation exposure.
  • State the inverse square law and discuss its meaning.
  • State what types of ionizing radiation the inverse square law applies to and which types do not follow the inverse square law.
  • Discuss why scatter radiation is an occupational risk for radiographers.
  • Discuss why 90 degree scatter is particularly harmful to a radiographer positioned at the head end of the radiographic table vs. being behind the console.
  • State at least three rules for remaining at a safe distance using distance as a means of radiation safety and protection for personnel.
  • Discuss the reason for using the shielding formula to shield radiographic imaging rooms and the application of the linear attenuation coefficient.
  • Differentiate between a primary and a second barrier, stating how shields such as lead aprons, lead gloves, and mobile barriers are classified.
  • Define the terms HVL and TVL and solve radiation intensity problems.
  • Discuss the use and shielding requirements of the bucky slot cover and the protective lead that hangs from the image intensifier.
  • Be able to solve intensity problems using the inverse square law.
  • Be able to explain the concepts of linear attenuation coefficient applied to shielding.
  • Given the linear attenuation coefficient be able to solve a problem involving the relative quantity of photons absorbed by the absorber.
  • Clearly list and state the Pb equivalence required by NCRP for various types of protective apparel commonly used in radiology.
  • State the location of the fluoroscopic tube and relative tube current these tubes output.
  • State why technologist often accumulate greater radiation exposure during fluoroscopic procedures than during single exposure radiographic imaging.
  • State the NCRP and NCR requirements for posting of the universal radiation symbol in radiology departments.
  • Differentiate between a radiation area and a high radiation area.
  • State the purpose for having a light source indicator of an energized x-ray tube on the outside of the x-ray room door.
  • Discuss the mandate of ALARA as a means of radiation protection.

  • Outline


    Section 3.1 Cardinal Rule Limiting Time of Exposure
    1. Four dynamic practices during fluoroscopy to reduce the time of patient and personnel exposure.
    2. FDA regulations for timer control on fluoroscopic equipment.
    3. FDA regulations for tabletop exposure rate by fluoroscopic equipment with and without automatic brightness stabilization controls (ABS).
    4. Types of ABS control mechanisms found in fluoroscopic equipment.

    Section 3.2 Cardinal Rule of Distance
    1. The Inverse square law (ISL) and distance.
    2. Types of ionizing radiation to which the inverse square law is applied.
    3. Relationships between radiation intensity and distance, and intensity to area of beam coverage according to the inverse square law.
    4. Differentiating the behavior of ionizing radiation from a point source vs. an extended source such as from the patient.
    5. Rules governing the intensity of scatter radiation.
    6. Understanding the resultant energy of a photon that scatters based on the angle at which it scatters.
    7. Calculating photon energy based on single and multiple scatter events.

    Section 3.3 Cardinal Rule of Shielding
    1. Commonly used materials to shield diagnostic x-ray rooms.
    2. Assessing shielding using linear attenuation coefficient and half-value-layer.
    3. Shielding formula
    4. Defining a primary barrier vs. a secondary barrier, and NCRP guidelines on location and barrier thickness.
    5. Assessing shielding by HVL and TVL formulas.
    6. Classification and lead equivalency requirements of common secondary and primary barriers:
      1. Lead aprons
      2. Lead gloves
      3. Bucky slot cover
      4. Image intensifier
      5. Protective curtain
      6. Mobile shields, etc.
      7. Thyroid shield
    7. Effectiveness of different lead equivalency aprons at various kVp.
    8. Additional NCR requirements for shielding in nuclear medicine.

    Section 3.4 Work Area Definitions
    1. Defined:
      1. Radiation area
      2. High radiation area
      3. Very high radiation area
      4. Uncontrolled radiation area
    2. NCRP and NCR required postings of radiation symbol and warning.
    3. Additional radiation warnings at room entrance such as light indicator of an energized x-ray tube.


    There testing are three principles of radiation protection commonly practiced in nuclear medicine for dealing with a live source of radiation that are applied to other areas of radiology as well. These three principles are called the Cardinal Rules of radiation protection; they are: time, distance, and shielding from ionizing radiation. A fourth principle that applies directly to diagnostic imaging technologists who make static and fluoroscopic images, stay out of the primary beam unless you are the subject under study. The fifth and perhaps the most important principle is that of ALARA, an acronym for as low as is reasonably achievable radiation dose to the patient. These principles also assume that direct radiation exposure to the technologist should not occur routinely as an occupational risk. Scatter radiation should be the only radiation diagnostic imaging professionals are potentially exposed to; however, nuclear medicine technologists who handle source radiation only have three principle protections: time, distance, and shielding since they cannot completely refrain from source radiation which is an occupational risk for them. This module will explore the various aspects of the cardinal rules and ALARA to effect patient and personnel protection.


    Section 3.1: Cardinal Rule of limiting time of exposure

    It stands to reason that the longer a person is exposed to a field of radiation the greater that exposure is and its consequences may be. From a diagnostic imaging point of view occupational exposure risk can be minimized by limiting unnecessary patient dose which is beneficial to the patient and the radiographer. The principle of limiting time of exposure to ionizing radiation applies to both the patient and the radiographer. Imaging professionals must practice diligent exposure principles during fluoroscopy and all x-ray procedures. There are four dynamic ways to reduce time of exposure to ionizing radiation during fluoroscopy: intermittent beam on-off imagings, use of last image hold techniques, avoiding long static fluoroscopic imaging, and keeping occupational dose below the regulated equivalency limits.

    For any fluoroscopic imaging procedures it is important that the fluoroscopist practice intermittent on-off beam imaging technique. The total patient exposure time to radiation during most fluoroscopic studies is out of the radiographer's direct control unless they are the fluoroscopist. When the technologist is the primary fluoroscopist, such as when using a mobile c-arm in surgery, the use of the last image hold function along with beam on-off imaging should be the standard of radiation exposure. Most modern stationary digital fluoroscopic rooms have last image hold capability so that the physician can study motionless frames as well as dynamic images. Utilizing the monitor to study displayed images when appropriate can significantly reduce patient exposure.

    The picture above demonstrates how image hold techniques can be useful in decreasing radiation exposure to the patient during dynamic fluoroscopic imaging procedures. The high resolution of modern monitors allows the physician to make observations not easily seen with archaic imaging equipment. It can be seen in this picture that an ERCP procedure is being done; this imaging department uses two monitors in their fluoro room, one for dynamic imaging the other for last image hold.

    The Food and Drug Administration (FDA) requires that fluoroscopic imaging equipment record and warn when exposure accrual times reach 5 minutes for fluoroscopic procedures. The FDA requires that a 5 minute cumulative timer with audible tone remind the physician when a significant dose to the patient due to beam on time has accumulated. It should be noted that local radiation burns have been reported with fluoroscopic exposures of greater than 15 minutes. Some of these reported burns were so significantly injurious that long term treatment and management was necessary. Fluoroscopy, C-arm and interventional radiographic imaging providers must respect successive fluoroscopic timer alarms as well as keep static imaging exposure to a minimum.

    The FDA also mandated that the maximum exposure rate (intensity) of the x-ray beam at tabletop should not exceed 10 R/minute for units equipped with automatic brightness stabilization (ABS), and 5 R/min for those units without ABS operating systems. Automatic brightness control or automatic brightness stabilization is an electronically controlled system that allows the fluoroscopic unit to automatically maintain the brightness level of the image with variations in thickness and attenuation of photons by tissues. There are two main types of controls that adjust brightness: 1) variable kVp, 2) variable tube current (mA), or variable pulse width. Regardless of the method, brightness maintenance is near constant during dynamic fluoroscopic imaging so that static imaging for prolonged periods to acquire smooth presentation of diagnostic imaging on the monitor is exceptionally reduced.

    A very important area of radiology is that of Nuclear Medicine. Nuclear Medicine Radiation technologists practice strict radiation safety guidelines, and because they handle source radiation, must apply the cardinal principle of limiting their duration of exposure to reduce their occupational dose. There are three groups that time of exposure affects mostly: health care personnel who provide direct care to radiology patients, visitors, and patients in beds adjacent to those patients being exposed during mobile imaging or following an ingested or injected radiation dose. The Nuclear regulatory commission regulates exposure time under 10CFR20.1301. Here are a few regulations which are related to the safety of non radiation workers:


    Section 3.2: Cardinal Rule of distance

    In spite of the advances in radiation protection, such as collimators, cones, and positive beam limiting devices, distance is still the best tool for radiation protection and remains the most common method of protecting personnel, visitors, and adjacent patients from ionizing radiation use. But few persons in the health care environment understand why distance effectively protects them and therefore continuously question, “At what distance am I considered safe? The answer lies in understanding the relationship of ones distance from a source to exposure intensity. The type(s) of radiation one is exposed to as well as its energy content are also factors that affect personal dose. A safe distance can be accurately estimated from the vector of radiation exposure and its initial intensity using the inverse square law. The radiographer should note that this law applies only to a point source of radiation such as the primary beam or a radionucleotide source. Additionally, the inverse square law applies only to electromagnetic radiation (x-rays and gamma rays), and does not apply to particulate ionizing radiation, or scatter radiation which is the major type of occupational radiation exposure personnel should encounter. The inverse square law states:


    Almost any radiographer can quote the inverse square law as it is applied to radiation exposure, especially from an x-ray tube or from a radiopharmaceutical isotope. But in order for the radiographer, layperson or other health care personnel to understand what distance from the radiation source means to radiation safety, we must understand what the inverse square law is actually stating. Only then is the radiographer's teaching moment, which all radiographers do have, meaningful because we will all be teaching the same concept accurately. Only in this way can we together end the fleeing of nursing and other health personnel from what they perceive as radiation exposure rather than from radiation exposure itself.

    A basic characteristic of radiation emitted from a small point source, such as an x-ray tube, is that the beam diverges from that source to cover an increasingly larger area. The inverse square law states that the intensity (concentration) of x-ray photons or gamma radiation) decreases inversely as the area the beam covers increases. This can be seen in the example of a flash light aimed at a wall; the area the light covers on the wall will be greater than the area at the light source (bulb) but its intensity or brightness is less. Another way of looking at the inverse square law using our flashlight example is to imagine holding a flashlight one inch from your eyes, the intensity or brightness would be too great to take a look at because the concentration of light photons would be too great. However, the intensity or brightness of the light as it is shown on a wall six feet away would not be as intense. Not only could you look at the light source from this distance, but you would notice that the area the light covers on the wall would be great compared to the area represented by the size of the bulb. These two observations represent the two main components of radiation intensity and the inverse square law. Let’s now discuss how the intensity (concentration) of radiation varies as the area it covers remotely from its source increases.

    The picture above demonstrates the typical x-ray tube used to produce a point source of x-rays. Then as radiation exits the tube it diverges to cover an increasingly larger area as the distance from the source increases. Notice that area "A" is smaller and the radiation is more concentrated than in an equal area "A1" which is some distance from "A." Each square A1 is the same size as "A" but only 1/4 the number of photons occupies it because of the divergence of the radiation with increasing distance.

    It should be remembered that the inverse square law only applies to electromagnetic point source radiation such as gamma or x-rays, and not to particulate radiations like alpha and beta particles. Particulate radiation does not follow the physical principles of the inverse square law because their distance of travel is limited to only a few millimeters for alpha particles, and a few centimeters for beta particles, then their kinetic energy is reduced to zero delimiting their ionization potential. This is because particulate radiation has mass and charge, which are properties that electromagnetic radiation does not possess. Just moving a couple of feet or so away from the source of particulate radiation is usually enough protection. Electromagnetic radiation by comparison has an unlimited range in space and therefore its intensity is reduced only by interactions with matter. In order for the intensity of x-rays or gamma rays to be reduced they must undergo absorption by P.E. interaction.

    Notice that the intensity of radiation at any distance plane through an area of a radiation field is inversely proportional to the square of that distance. We see that the total amount of radiation in the area covered remains the same as the area of coverage increases; however, it becomes less concentrated. In fact, the inverse square law tells us that at any given distance from the source, the intensity (concentration) is inversely proportional to the square of the area covered by the beam, and the area covered is the square of that distance. Then, if the distance from the source is 1 meter the area of coverage is one squared or one square meter. If the distance is 2 meters from the source, then the area of coverage is two square meters, and intensity in any one of the four 1 x 1 meter squares is one-fourth the original intensity. Then, the sum of the intensities of the four 1x1 squares will equal the total initial radiation intensity. Likewise, if the distance is 3 meters the area of coverage is three squared or nine meters of total coverage and an intensity of one-ninth the original intensity in any one of the nine square meters. So the more one increases the distance between them and a source, the more dilute the radiation will be that reaches them.

    Using the inverse square law we see that if the distance from the source is doubled, then the intensity of the radiation in the same unit area would be reduced to one-fourth the original concentration, and that the total radiation would be spread out over an area that is the square of the distance. In practical terms, distance as a radiation protection technique is even more pronounced when the radiation worker is positioned in a location opposite the direction of the primary beam, so that the only potential for exposure is from scatter radiation. Unless the primary beam overlaps the occupational worker, or one is in a location that makes the patient an extended point source, the inverse square law is not an indicator of the effectiveness of radiation protection. This is because scatter radiation generated within the patient seems to follow the rules of radiation from an extended source, and not a point source like the x-ray tube target. However, in some instances if the distance from the source exceeds five times the source diameter it too may be considered a point source. Scatter from a point source is more damaging than that emitted from an extended source. The two sources cannot be distinguished during dynamic imaging procedures; therefore, the technologist must always assume their exposure to scatter follows the inverse square law unless they are positioned behind a barrier as this will overestimate their exposure risk.


    The inverse square law only applies to primary beam electromagnetic radiations of x-rays or gamma rays. It does not hold true for particulate radiation or scatter radiation. Although distance is the best method of decreasing the intensity of radiation it is more pronounced with less penetrating forms of radiation such as alpha and beta particles. Nuclear medicine technologists apply this formula only to gamma radiation.

    Another issue of importance to us is that of exposure to scatter radiation when it behaves as if emitted from a point source. Scatter radiation as we stated earlier are either photons or particles from a point source (e.g., primary beam) that has undergone a change from its initial direction because of interactions with matter. Because scatter radiation is a primary photon that has undergone a change in direction when it interacts with atoms in the patient’s body we must be cautious of its ability to cause biological damage. Because of the summation of commutative doses to the technologist from scatter, radiation risk increases. Because the direction a scattering photon will take is purely a matter of chance we must rely on known physical observations that guide the practice of radiation safety. Backscatter refers to those photons that return in the near direction from which they came, that is backwards 180 degrees towards the tube. Forward scatter continues in the direction of the original photon with a few degrees of directional change; these photons are generally projected towards the image receptor and are the cause of image fog. It is that scatter which is propagated at angles between zero and 180 degrees that is particularly harmful to the technologist. At low kVp there tends to be more backscatter, whereas greater forward scatter occurs at high kVp. A significant amount of "90 degree" scatter, that is scatter that occurs at right angles to the primary beam, occurs during real-time fluoroscopy imaging. This occurs in about one-third to one-half as often as forward scatter at fluoroscopic kVp energies, and there is also multidirectional back and side scatter.

    Photons suffer a change in wavelength and a change in energy as a consequence of a scattering event. For example, 110 keV photons that scatter at 90 degrees to the primary beam, will loose merely 17% of their energy retaining 91 keV should they strikes a naive technologist positioned at the head or foot end of the table during fluoroscopy. So in practical terms if the technologist fails to practice the cardinal rule of modifying their distance from the patient, exposure could be comparable to being in the primary beam itself, in spite of wearing a lead apron. Instead, the radiographer should position themselves behind a protective barrier such as the curtain that hangs from the image intensifier, or a mobile barrier that allows them to remain close enough to the patient to assist the radiologist. Another effective technique is to move to an out-of-the-way position that requires radiation to scatter twice before reaching them.

    The picture above demonstrates how mobile shields can be used to reduce radiation exposure from the patient. During dynamic imaging some scatter behaves as if it is from a point source and some behaves as if from an extended source.

    Whenever possible the technologist should locate themselves behind the stationary barrier in the area where the control panel is located. In addition, the radiologist should use intermittent beam on-off and last image hold techniques to protect all persons in the fluoroscopy room as well as the patient. Having the technologist hold patients such as is commonly done in pediatric and geriatric imagings to save time are practices that breach radiation safety principles and should be abandoned. Instead the technologist must take the time to immobilize the patient using standard apparatus found in any radiology department. In this manner the technologist may safely provide support to the radiologist and the patient yet maintain a save distance that reduces their radiation exposure.

    The head or foot ends of the radiographic table during fluoroscopy are not good positions for the technologist to be at because of the lack of protective shielding at these locations. Likewise, removing the lead apron for procedures involving sterile draping of the patient, for instance to provide physician access to the patient during a myelogram procedure, will create a situation that facilitates exposure to right angle scatter. In most cases this scatter is reduced very little, and the technologist and radiologist alike should be cognizant of this increased occupational exposure.

    A technologist standing at the location of the blue line would receive about 500 mR/hr. But by moving to the position where the white line is the technologist would receive about 5 mR/hr. High badge readings of technologist doing lots of fluoroscopy are usually due to their location being at the head of the table during beam on imaging. They are therefore in the path of 90 degree scatter radiation that unnecessarily adds to their dose.

    Theoretically speaking, isoexposure line patterns during fluoroscopic imaging flow in such a way that the cardinal principles of shielding and distance can be advantageous to the radiologist and the technologist. Standing behind the lead protective curtain or even backing away from the table 3 meters or more when not absolutely needed, will reduce the technologists' exposure to an acceptable occupational level. The goal is to maximize distance from the patient to reduce the dose to the technologist and assure that only scatter radiation as an extended source can expose the technologist.

    The best position in which to be located during any x-ray exposure is one in which the radiation must scatter twice in order to reach the radiographer. In this scenario the intensity of radiation is reduced by a factor of 1000 for each scatter event. So two scatter events would reduce the exposure value of a 100 mR dose down to 0.0001 mR, or 1,000,000th its original intensity, which is an acceptable level of occupational exposure.

    The picture above demonstrates how isoexposure lines can affect the dose the technologist receives during fluoroscopy procedures. When the protective curtain is removed during aseptic procedures, for instance, a myelography or hip injection for pain management, there is additional exposure to the radiation worker that was restricted by the lead curtain. When in place, the curtain will protect the technologist and radiologist from a significant amount of scatter radiation.



    Section 3.3: Cardinal Rule of applying shielding

    The use of shielding as a cardinal principle of radiation protection is required by both the National Council on Radiation Protection and Measurement (NCRP), the Nuclear Regulatory Commission (NRC), and various federal and state regulations. Specific shielding designs are related to the shielding formula used to protect personnel, patients, and visitors from ionizing radiation. Shielding is a required practical method of radiation protection. Commonly used materials for shielding are lead, concrete, or bricks to prevent or reduce the transmission of radiation. The main primary radiation types encountered in diagnostic imaging for which shielding is designed are x-rays, gamma rays, alpha and beta particles, and recently-gamma photons from positron annihilation reaction. Alpha particles are the most damaging of these but can be stopped with a sheet of ordinary paper. Beta particles can be stopped completely with 1/4 inch of plastic, but gamma and x-rays are never completely stopped by shielding materials used in walls or found in protective apparel. A syringe containing radioactive phosphorus 32 placed next to a scintillation detector would register many counts of radiation; however, it is not the alpha or beta radiation being detected because of their limited range. It would mostly be bremsstrahlung radiation produced by particulate radiation interacting with the shielding material itself. In the case of highly penetrating radiation the shielding formula is used to compute the reduction in radiation intensity using half-value-layer and tenth-value-layering schemes.

    Institutions are required to restrict radiation exposure in unrestricted areas to less than 2 mR/hr. The reason a zero level is not required is that as radiation passes through shielding material its intensity and energy is reduced, but not all photons are completely absorbed. In order for radiation to be completely absorbed it must loose all of its kinetic energy through a series of ionization events in the absorber (shielding material). When wearing a radiation barrier, such as a lead apron, there is a reduction in exposure to the wearer although the apron does not afford a complete stoppage of radiation. The density and thickness of an absorber are effective means of determining the degree of shielding; however, the relationship between density of the absorber and its atomic number is not a reliable indicator, but is commonly referred to in many text books. The reason density of the material is an unreliable indicator is seen in the example of water, which exists in different states: liquid, solid (ice) and vapor (steam), its density changes but its atomic number does not. The same phenomenon can be applied to different types of materials that would be unsuitable for shielding on basis of atomic number. Two values that are useful in determining shielding in a medical radiology setting are the linear attenuation coefficient and the mass attenuation coefficient. These are the most common methods used to measure the ability of an absorber to absorb radiation.

    The linear attenuation coefficient (symbol, ́) is defined as the fraction of the number of photons removed from the radiation field per centimeter of absorber through which it passes. It is generally expressed as a percentage constant such as 10%, with a function like the decay constant. The linear attenuation coefficient can be adjusted to whatever percent desired. However, it is commonly expressed as 10% or 10-1 meaning that 10 percent attenuation is provided by an invariable thickness of the stated material. A linear coefficient of 10% would be expressed as (́) = 0.10 cm-1. The example below represents how this formula is applied.


    The linear attenuation coefficient can be adjusted to whatever percent desired. In the example above it is set at 10% meaning that 10% percent of attenuation provided by a constant one centimeter thickness of material. A beam of 100 photons passing through a 1 cm thickness of material with a ́ of 10% will absorb approximately 10 of them, and if another thickness is added it will absorb 9 more, and so on until the beam is completely reduced. For shielding purposes, a high linear attenuation coefficient material is selected.

    The mass attenuation coefficient is not useful in medical shielding but it is covered here because it represents a measurement that does not change based on the physical state of the absorber material. When calculating shielding requirements for medical imaging purposes, a changing physical state of the absorber is not relevant, and therefore the shielding formula is more useful.


    The shielding formula contains attenuation data for shielding any type of radiation using any type and thickness of an absorber. There are other shielding calculations necessary to comply with an x-ray room design. These formulas can become quite complex but the main point is that shielding is a practical way to protect from ionizing radiation. During mobile C-arm fluoroscopy and portable static imaging there is no specific shielding for adjacent patients unless a mobile shield is used. In the absence of a mobile shield, the use of distance and individualized shielding is used to protect adjacent patients and personnel in portable imaging situations. Physicians, nurses, and health care workers should realize that shielding from ionizing radiation is an integral part of the design of a radiology department. Imaging rooms are properly shielded to keep in ionizing radiation; this is a major reason why patients should have radiographs done there whenever possible. The practice of ordering routine portable imaging for patient convenience rather than because of the patient's condition must be abandoned. Not only does portable imaging produce inferior image quality but also the quality of the diagnostic information it contains is also compromised.

    The radiology imaging rooms have two types of barrier designs that reduce radiation exposure beyond the patient. These are classified as primary and secondary barriers designed to absorb primary, secondary, and scatter radiation. A primary barrier is intended to shield any area that can be directly hit by the primary source of radiation (e.g. a beam coming from the x-ray tube, or source of radioisotope). A secondary barrier is designed to absorb radiation exiting the patient. Both primary and secondary barriers are regulated under NCRP and NRC regulations that specify the category, location, and minimal quantity of shielding material. Specific instructions on shielding include workload formulas to aid in the design of the radiographic room. Structures that are designated as primary barriers are the floor and walls up to 7 feet; the ceiling and wall protecting the console is specified as a secondary barrier. The FDA mandates that for a single exposure by the technologist the exposure switch must be located in a place behind a fixed secondary barrier so that the technologist may not make an exposure and be exposed while doing so. Furthermore, any modification that allows for single exposure while in the x-ray room is a violation of federal standards. This is a safety design implemented to protect the technologist from scatter radiation when not wearing a lead apron.

    Secondary barriers are designed for protection from scatter radiation. They are positioned in such a way that a scattering photon must twice scatter before reaching the person behind such a barrier. The thickness of a secondary barrier must be at least 1/32 inch lead equivalent. A primary barrier must be at least 1/16 inch lead equivalency. In this picture the CT technologist sits behind a wall which is a secondary barrier designed to protect them from scatter radiation.

    A more practical way of understanding shielding is to look at the concept of the half value layer (HVL). The HVL is defined as that quantity of absorber material that will reduce the incident radiation field to half its initial value. For example, the HVL of lead for cobalt 60 is 1.25 cm. This means that a given quantity of radiation from a cobalt 60 field that strikes an absorber with a HVL of 1.25 cm lead or equivalent would be reduced in intensity by half. A 10 mR exposure would be reduced to 5 mR by a one HVL equivalency. The HVL is also an efficient way to describe the effectiveness of secondary shields such as lead gloves, lead aprons, thyroid shields, breast shields, gonadal shields, and mobile barriers.

    The HVL is independent of the amount of radiation that passes through it, it simply reduces the intensity of a radiation field by half; but does not stop it completely no matter how many half value layers are interposed. The adding of half value layers reduces radiation intensity with each gradation, so that the amount of radiation exposure to the person behind successive half value layers is minimized. The take home point here is that when wearing a lead apron or lead gloves the protection mechanism responsible for exposure reduction does not completely stop all radiation. Therefore, to place oneself in the path of the primary beam because one is wearing leaded gloves, or a lead apron, is not truly practicing radiation safety. Consider the chart below for HVL beam attenuation:


    The myth that a lead apron is worn to stop radiation is false with the truth being that it can only reduce the amount of radiation exposure to an acceptable level. All lead shields should be thought of as barriers having the quality of a half value layer allowing a small quantity of ionizing radiation to pass through. Radiation that passes through an attenuator having reduced energy and intensity will most likely be absorbed in the tissues of the wearer. This is why an occupational worker must never place themselves in the primary x-ray beam or in the field of a source of particulate radiation with a shield on. The Cardinal principles imply that time, distance, and shielding is practiced together in all imaging situations. Shielding is the most over used and over rated protection method of them all because of the false sense of total protection the wearer sometimes assumes.

    Now let's consider some of the ways personnel protect themselves using lead barriers and how they use these same principles to protect the patient. Shields used in personnel and patient protection are lead equivalent aprons, gloves, gonadal shields, breast shields, and thyroid shields. One should not confuse these shields with filters that are used to harden the radiation beam improving its quality while decreasing skin dose. Lead apparel such as aprons and gloves are not designed to be used in the primary beam. For example, the radiologist wearing a lead glove to palpate anatomical structures such as when spotting the ileocecal valve is mindful to limit their time of exposure. To function with a lead apron as if it is a complete shield against radiation exposure is a technical misgiving. This is not to say that this practice should be abandoned, instead, if fluoroscopy is the technologists primary area of employment and their hands are commonly exposed to the primary beam, then they should wear a ring or wrist badge to monitor the dose to their extremities. The NCRP standard for extremity monitoring is to just ask the radiation safety officer (RSO) for it.

    To help the radiographer understand the role of lead gloves in clinical practice there are two important observations that should extend into clinical practice: 1) the minimum required shielding in a lead glove is mandated by the NCRP No. 102 to be no less than 0.25 mm lead equivalency although most gloves in a radiology department will be about 0.5 mm lead equivalency for extra protection, 2) the occupational worker is allowed 50 rem of exposure to the hands each year which should not be exceeded under normal work practice. Extremity dose is only monitored upon special request or when the RSO chooses to monitor it because the person's radiation exposure level has reached the investigational level.

    Left. Lead gloves are used to protect the worker when a potential for exposure to the hands may occur. A common practice is to wear lead gloves when exposure from the primary beam may occur which is not their purpose. They are considered secondary barriers which means they are for protection from scatter radiation not primary radiation.

    Right. Devices such as the film holder shown are commonly available to the technologist to use during portable and departmental imaging. These devices allow the technologist to maximize their distance from the patient thereby reducing their occupational dose. Holding patients to save time is an unnecessary practice and those who continue to do so are in violation of NCRP regulations.

    A properly worn lead apron can protect up to 80% of the active blood forming organs in the body. A well chosen apron should cover from the manubrium of the sternum down to include the symphysis pubis and mid thigh. But for it to be an effective protector the wearer must always face the source of scatter and not utilize the apron as protection from the primary beam. Aprons are delicate devices that are filled with powered lead or an equivalent such as tin, copper, barium, tungsten, or combination of like materials. In many radiology and surgery departments there may be lead aprons containing only 0.25 mm Pb equivalence because they were purchased under older NCRP regulations. The current NCRP report No. 102 standard recommends a minimum equivalency of 0.50 mm Pb for aprons. Most radiology departments also have special pregnancy aprons which have 0.5 mm Pb throughout and 1 mm Pb eq. at fetal level. In handling aprons they should not be folded or thrown carelessly about, and they must be inspected annually to assure that there are no cracked areas in the apron covering vital areas. By inspection the NCRP requires fluoroscopic examination or radiographic examination to check for cracks.

    Left. Thyroid shields used to help reduce exposure to the thyroid gland during fluoroscopy exams. Lead aprons used to protect from scatter radiation can protect up to 80% of the active blood forming organs in the body.

    The second highest occupational dose area is to the thyroid. One study concluded that the dose to the thyroid gland area averaged 6 mrad for each fluoroscopic examination in which the radiographer was positioned at the head end of the radiographic table. Statistically thyroid tumors are known to be more prevalent in females than males. This is thought to be due to fluctuation in hormones related to thyroid function, although this theory is not concretely proven. It is therefore important for all who are exposed to ionizing radiation, especially during fluoroscopy studies, wear a thyroid shield and use distance to decrease their radiation exposure.

    Two other very important protective shields worthy of mention are the protective curtain that hangs from the image intensifier, and the bucky slot cover that covers the slit made when the bucky tray is moved to either end of the radiographic table to remove it from the field of view during fluoroscopic imaging. Radiographers know that for stationary fluoroscopy the tube is permanently located under the radiographic table. Being mindful of the tube location in all fluoroscopic situations is what makes a difference in how one protects themselves and patients from the radiation beam. During C-arm operation the tube changes positions so that a wrap around shield may be the only way to protect the patient’s gonadal tissues.

    In this picture the x-ray table is raised to the upright position to demonstrate the location of the x-ray tube under the table. This tube is very different from the overhead tube used to make exposures tabletop or through the bucky tray. It operates at low milliamperage in the range of 5 mA or less. Overhead x-ray tubes operate in the hundreds of mA (300-2,000 mA). However, the fluoro tube operates for several minutes during the average procedure; therefore, radiation exposure is greater with fluoroscopy than with static imaging.

    Left. The blue arrow in the picture points to the image intensifier which is classified by the FDA as a primary shield from radiation emitted by the tube which is below the table (white arrow). The FDA regulates the design of x-ray equipment. All image intensifiers must be constructed to be a primary barrier with a minimum protection equivalence of 2.0 mm lead.

    The image intensifier is a primary barrier against radiation coming from the tube located beneath the x-ray table. When the table is in the vertical position the image intensifier can be an effective means to protect the technologist from radiation. When the table is in the horizontal position and the technologist is required to be in the room for a procedure, occupational dose can be lessened by locating to a strategic location behind the image intensifier. As the primary beam exiting the patient strikes the image intensifier to produce images on the monitor, the image intensifier serves as a primary barrier. Fluoroscopy equipment and operations are regulated under federal guidelines in Title 21 of the Code of Federal Regulations Part 1020 (21 CFR 1020) subchapter J6. According to these regulations, the image intensifier must have a minimum absorption equivalency of 2.0 mm lead. Furthermore, the entire cross section of the useful beam should be intercepted by a primary protective barrier at all source-to-image distances (SID)6. Standing in front of the patient and therefore the image intensifier when the table is upright, along with wearing a lead apron will offer the technologist outstanding radiation protection.

    The protective curtain and bucky slot cover are also required protective shielding under NCRP and CFR regulations. In order to provide protection the lead protective curtain must be in place on the image intensifier for fluoroscopic procedures. The protective curtain must have the minimum equivalency of 0.25 mm lead. The protective curtain should not be removed for routine fluoroscopic studies; however, sometimes to protect a sterile field during aseptic procedure, the curtain may be removed. It is especially during these procedures that the technologist should remain at a reasonably far distance from the exposure whenever possible, and limit their time of exposure. Whenever possible the radiologist should train the technologist in the use of vertical image intensifier lock for patient protection, and to manipulate the fluoroscopic controls during surgical aseptic procedures so that the curtain may remain in place for radiation protection purposes. There are times however when for patient safety and asepsis the curtain must be removed.

    Left. This picture demonstrates the protective curtain required by NCRP report #102 to be at least 0.25 mm Pb equivalency. It should not be routinely removed for fluoroscopic procedures (white arrow).

    The bucky slot cover is a metal cover device that moves into place when the bucky tray is moved to either end of the radiographic table. When fluoroscopy is being used, the bucky tray will obscure anatomy during fluoroscopic imaging procedures. It is required that the bucky slot cover be operational having an attenuation equivalency of 0.5 mm lead which is one tenth-value-layer (TVL). The bucky slot cover should be inspected semi-annually and should be determined that it is working properly as designed. Its purpose is to add protection for the gonads which are near bucky level during fluoroscopy.

    Left. The bucky (A) and bucky slot cover (B) are located at gonad level during fluoroscopy. When the table bucky is moved a slit-like opening in the table must be closed by the bucky slot cover. The bucky slot cover must be 0.5 mm Pb equivalency.

    We are concerned about the thickness of a shielding material because it ultimately determines how much radiation will be attenuated. Most shields are made of 0.25, 0.5, or 1.0 mm lead equivalency. But it is rare to find a 1.0 mm Pb equivalent apron because the added weight is too heavy to wear routinely and may cause back trouble. Lead aprons of 0.5 mm lead are commonly found in imaging and surgery departments. These 0.5 mm lead aprons will attenuate approximately 75% of a 100 kVp beam. Notwithstanding, a person wearing one of these aprons during fluoroscopy studies day by day will potentially accumulate high occupational doses over time, especially if one expose themselves to 90 degree scatter. Now having looked at the attenuation capabilities of lead equivalent shields one should be made aware of a few observations: 1) most radiology, surgery, and orthopedic departments purchase 0.25 and 0.5 mm lead aprons, meaning that the exit radiation reaching the wearer can approach 50 to 25%;


    2) fluoroscopy technologists (including angiographers), and CT technologists are exposed to radiation of up to 140 kVp energies; therefore, their exposure could be greater than the chart above predicts; 3) a female who declares pregnancy should be sure to wear an apron designed for pregnancy because it will have an extra lead patch over the vital area of the fetus. A wrap-around type with skirt and vest is also recommended for better back support; 4) when doing radiographic procedures in which the health care worker must be in the exposure area, wear lead and maintain a safe distance; 5) technologists should never hold patients for procedures in order to get a better view or to expedite the study by exposing them self to unnecessary radiation; 6) using the three Cardinal Principles together they are more effective than any one practiced alone. As one can see from the attenuation data in the table above, shielding alone does not completely stop radiation exposure. Therefore, health care workers must be educated in the use of distance and time as well as shielding to reduce radiation exposure. At best the wearing of a lead apron may reduce exposure 75-85 % with the remainder being absorbed by the wearer. This is an important reason why shielding must be coupled to the other two cardinal principles of radiation protection, distance and limited time of exposure to ionizing radiation, to have a wholly effective protection standard.

    On the left the x-ray tube is a single directional system so that the radiographer must be aware of the direction of the beam in order to remain at a save exposure distance during its multiple positions of movement during procedures. The arrows indicate the location of the image intensifier.

    The picture on the right is of an angiographic suite in which there is biplane imaging. In this example the technologists, radiologist, and nursing staff must be aware of their position so that they are not exposed to large cumulative multi directional doses of radiation.

    As all radiographers know one should not turn their back to the radiation beam during live fluoroscopy. Other health care workers often in the line of performing their duties while wearing a lead apron fail to keep their front to the beam. Often they assume that their understanding of radiation protection sufficiently justifies their practices. It should be understood unequivocally that a protective apron is less than fully protective unless the wearer uses it properly. These practices confirm that a need for better universal training of medical personnel in radiation safety is evident.

    Consider the arrow in the picture to the left. It identifies the position of the x-ray tube. The purpose is to demonstrate how the x-ray tube changes positions during dynamic C-arm and surgical imaging procedures as in the picture below.
    Right picture. Whether the tube is oriented over the table or under the table its position must be known to the radiographer. Not standing in the direction of the primary beam is paramount to radiation protection.

    Left. Once the patient is draped for an aseptic procedure surgical personnel tend to forget the location of the tube and its relationship to exposure. Instead, all attention is on the imaging quality; therefore, the radiographer must be mindful of the tube and maintain a position away from the direction of the primary beam’s divergence.

    Interventional and surgical radiology are similar disciplines in how radiation protection is practiced. Occupational workers in these areas tend to have higher exposure doses than radiographers in other areas such as general diagnostic imaging and CT. To minimize occupational exposure distance, shielding and limited duration of exposure must be practiced to protect oneself. Both of these environments are dynamic and do require that all participants be educated in the principles of self and patient radiation safety because of the dynamic motion of the tube.

    Shielding considerations in nuclear medicine slant toward the protection of personnel since handling radiation sources requires special shielding that is not concerning in other radiology modalities. It is actually quite amazing how much radiation is safely handled in what is usually a small area by nuclear medicine technologists. This is because no other radiology discipline work with radiation in the manner in which these specialists do. Nuclear medicine technologists handle source radiation and cannot turn it off and on as do radiographers who image with electromagnetic x-rays generated by x-ray tubes. The principles they use to remain safe in a variety of source situations are models of safe radiation standards for all to practice.

    Let's look at the most recent challenge for nuclear medicine technologists, PET imaging isotopes, and see what other imaging and health care professionals should know about their use. We stated earlier that a positron is an annihilation particle that interacts with a negatively charged electron, annihilating both the positron and the electron. The products are two gamma photons emitted at 180 degrees to each other, each having energy of 511 keV. Normally in nuclear medicine the energy of photons emitted by radioisotopes is below 150 keV; however, with the development of new technologies in the field of nuclear medicine, PET and SPECT imaging, special imaging agents are selected that emit gamma photons with energies as high as 500 keV. Nuclear medicine technologists must have extra protection from the gamma rays these isotopes emit beyond what is practiced in traditional nuclear medicine imaging. An example that demonstrates additional shielding is the transport container for PET isotope tracer. Usually lead casing of the isotope canister during transport of these isotopes is sufficient; however with PET isotopes the transport canister must be made of tungsten to prevent the leakage of gamma radiation.

    This picture is of a canister used to transport radioisotopes for PET imaging, and the syringe holder for protection during dose administration. Notice the size and thickness of the canister relative to the size of those used to carry a single radioisotope for nuclear medicine bone scan. Lead is not a sufficient absorber for PET isotopes because they emit >500 keV photons; the canister pictured is made of tungsten which is a sufficient absorber of high keV photons.

    When a shipment of radioactive material is received in the nuclear medicine department it carries with it the requirements of the Department of Transportation (DOT) to monitor the contents on receipt. If one thinks of the container as a compressed version of a point source we can simplify how that source is handled in all situations. It must be packaged such that the exposure does not exceed 10 mrem/hr measured at one meter away. The inside container must be wipe-tested and determined free of contamination otherwise it is to be treated as a contaminated specimen. The practice of accountability is extended into the work spaces where the radiation dose is administered. In addition, all areas are wipe-tested weekly to determine contamination levels if any. The key to the whole process is safe handling of the source from the beginning to end of its use.

    Left. Picture shows a typical radiation control area for receiving shipment(s) of radioactive tracers used in nuclear medicine and PET imaging. Skillful handling of the source radiation carries over into the patient care component to insure patient safety before and after procedures. Within this confined area all of the safeguards of limited time, distance and shielding from the radiation source are practiced.

    Radiation safety practices for visitors and nursing personnel who may contact a patient following therapeutic ingestion of radioisotopes are covered under NCR regulations for patients receiving a dose equal to or greater than 30 mCi, or that measures 5 mrem/hr at 1 meter from that patient. These patients must be provided a private room marked "Caution: Radioactive Materials," and a private bathroom until their readings are below the exposure limit. These patients are also required to maintain safe distances from children and pregnant women, and access to them must remain below 100 mrem/year and 2 mrem/hr for visitors. Nursing and other personnel providing routine care to these patients who may receive 10% of the annual occupational dose limit must be monitor as occupational workers.

    Picture to the right is of a survey meter used to document the results of a wipe-test and to determine if there is contamination from a shipment of isotope. The survey meter is used in the general procedures rooms to test for contamination there also as required by law.

    Section 3.4: Work Area Definitions

    Radiation areas where exposure can occur are designated as one of three types which are collectively designated as restricted areas: Radiation Area, High Radiation Area, or Very High Radiation Area. A fourth type is the unrestricted area which is essentially any area where the radiation exposure is the same as or less than exposure from background radiation. Background radiation is a form of natural radiation that emanates from the earth’s crust, or from cosmic rays that breach our protective atmosphere. Such areas require no monitoring or special posting; an example would be the reception and waiting areas of the radiology department.

    All restricted radiation areas require posting whose universal meaning should be understood by all health care personnel.

    • Radiation Area – designates an area in which more than 5 mrem but less than 100 mrem of exposure may occur within an hour. An area designated as a radiation area must have controlled access such that the general public is not at risk of accidental exposure. It is required that a sign of purple, magenta, or black on a yellow background stating "Caution, radiation area" with the three blade radiation symbol be posted.
    • High Radiation area – is one in which the radiation level could be in excess of 100 mrem/hr. These areas must have controlled access such as door interlocks to restrict unauthorized entrance.
    • Very High Radiation Area – is one in which the absorbed dose to an individual may exceed 500 mrem/hr. Such an area is not commonly found in a medical imaging department.
    The posting of the radiation symbol along with the words "Caution" to warn that radiation exposure is possible beyond the protected limited access door is required by state and federal laws.

    In addition, to avoid unnecessary exposure to the embryo or fetus of any female during pregnancy, it is required that appropriate postings of signs or posters seeking to obtain pregnancy information before the patient, visitors, or health personnel are admitted into the radiographic procedure room. It is generally recommended that they be numerously found in areas such as the reception area, garment changing rooms, and in the x-ray rooms. The technologist is partly responsible for checking for potential pregnancy of all females within the child bearing years prior to exposure to ionizing radiation.

    Most x-ray imaging rooms have indicators on the outside door that light up when the x–ray beam is on. Personnel should be taught the meaning of these lights so that they to not accidentally enter the room.

    The picture to the left shows the door to an x –ray room Notice that the general access door for receiving patients is closed during active imaging. Some radiology departments equip their exterior room door with a light indicator (right & left picture) to warn nursing, patient transport, and others, that the room is energized, and no one should enter the room at this time.


  • The cardinal rules of radiation protection are: limit duration of exposure (time), maintain a safe distance, and always use shielding when unwarranted exposure may occur.
  • The duration of exposure during fluoroscopy can be minimized by the fluoroscopist by using image hold technique, intermittent beam on-off imaging, and avoiding long static imaging.
  • FDA requires a 5 minute cumulative timer with audible tone be operational during all fluoroscopic procedures.
  • Maximum exposure intensity at tabletop is 10 R/min for units equipped with ABS, and 5R/min for units without ABS.
  • Nuclear Regulatory Commission (NRC) regulates exposure time and have implemented the following mandates:
    1. TEDE to general public must remain below 0.1 rem (1 mSv).
    2. Adjacent patients to those given a radioisotope may not exceed 100 rem/yr or 2 mrem/hr.
    3. Nurses and other health care personnel who routinely care for patients who have ingested or been injected with a radioisotope must not exceed 10% of the annual equivalent allowed for imaging personnel without being monitored.
  • Distance is the most effective means of radiation protection.
  • Inverse square law applies only to x-rays and gamma radiation; and only to primary radiation, not to scatter except when it is an extended source.
  • Inverse square law tells us that as the distance from the source increases, the area the beam covers also increases, and a sample area the size of the original will be less concentrated.
  • Intensity is inversely proportional to the square of the distance from the source.
  • Scatter radiation poses the greatest radiation risk to occupational workers.
  • A photon that scatters at 90 degrees may retain approximately 83% of its original energy.
  • The most at risk positions for the technologist during fluoroscopy are the head and foot ends of the table where there is no shielding to protect them from scatter radiation.
  • Paying attention to isoexposure lines during fluoroscopic procedures will allow the technologist to stand in a location that greatly reduces their exposure.
  • Alpha particles are the most damaging type of ionizing radiation encountered in medical radiology; however, it can be stopped with an ordinary sheet of paper.
  • Beta particles can be stopped with 1/4 inch of plastic, whereas x-rays and gamma radiation has unlimited penetrability.
  • NCRP – exposure to ionizing radiation in an unrestricted area may not exceed 2 mR/hr.
  • Linear attenuation coefficient (́) is defined as that fraction of photons removed from a radiation beam per cm absorber.
  • Primary barrier is designed to protect from the primary beam; it should be constructed of at least 1/16 Pb equivalency.
  • Secondary barrier is designed to protect from secondary and scatter radiation; it should be constructed to 1/32 mm Pb equivalency.
  • Shielding requirements according to NCRP recommendation are:
    1. Lead aprons - 0.5 mm lead equivalency
    2. Lead gloves, protective curtain, thyroid shield – 0.25 mm lead equivalency
    3. Thyroid shields – 0.25 mm lead equivalency
    4. Aprons for pregnancy – total of 1.0 mm Pb equivalency at fetal level.
    5. Protective curtain 0.25 mm lead equivalency
    6. Bucky slot cover 0.5 mm lead equivalency
  • A properly worn lead apron can protect up to 80% active blood forming organs.
  • During fluoroscopy, for patient protection, the lead apron should be placed under the patient between the tube and the patient.
  • Dynamic C-arm and angiographic C-arm imaging, the lead protective apron should be placed on tabletop between the patient and the tube; a wrap around apron is needed for C-arm multi-directional beam imaging.
  • Canisters used to transport PET radioisotopes are made of tungsten since lead is not a sufficient barrier for positron emission and associated gamma photons.
  • The inside of the carrying case for PET isotopes must be wipe tested to determine contamination status. At no time should the package exceed 10 mR/hr measured at a distance of 1 meter.
  • The x-ray room door should be closed when the tube is energized, and the radiation symbol along with the words “caution radiation area” must clearly identify the room.
  • There are three types of radiation areas which must be clearly marked: radiation area, high radiation area, and very high radiation area, which is based on the amount of radiation exposure permitted in the designated area.

  • References

    1. *Bushong, S.C., Radiologic Science for Technologist: Physics, Biology, and Protection, 7th ed., St. Louis, Missouri: Mosby, Inc. 2001
    2. .
    3. *Early, P.J., Principles and Practice of Nuclear Medicine, 2nd ed., St. Louis, Missouri, Mosby Co. 1995.
    4. *Seeram, E., Computed Tomography, Philadelphia, Pennsylvania, W.B. Saunders Co. **1994.
    5. *Selman, J., The Fundamentals of X-ray and Radium Physics, 6th ed. Springfield, Illinois, Thomas Books, 1980.
    6. *Sprawls, P., Physical Principles of Medical Imaging, Rockville, Maryland, 1987, Aspen Publishers, Inc.

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    Copyright image Copyright 2006 Nicholas Joseph Jr.

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