Principles of Patient Radiation Protection & ALARA
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One of the most important topics in the public domain is the radiation dose patients received during medical x-rays. This article discusses fundamental principles of patient radiation protection. X-ray technologists practice a principle called as-low-as-reasonably-achievable (ALARA) dose for each radiographic image made. This article takes a fresh approach to how radiographers use scientific principles of radiation to protect their patients while achieving diagnostic medical images.
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Author: Nicholas Joseph Jr., RT(R)
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OutlinePrinciples of Patient Radiation Protection and ALARA
4.1 Beam Filtration
4.2 Achieving ALARA through technique selection.
4.3 Positive Beam Limiting (PBL) and Automatic Exposure Control (AEC)
Protecting the patient from unnecessary radiation exposure include not only the Cardinal Principles of radiation protection, but also the behaviors of imaging personnel towards each patient. It is often asked, “Why do we need to discuss patient protection when it is the patient who is being imaged and therefore being exposed to radiation?” “And is not all radiation exposure made within safe limits so that patient protection is not really possible?” The answer centers on the concept of ALARA which was briefly discussed in volume five, issue three of this series of modules. Again, the operator of any medical imaging radiographic equipment is mandated to use as low as is reasonably achievable radiation dose to provide the highest quality radiographic image. Any unsafe exposure beyond the ALARA principle is considered unnecessary excess exposure. Let me give a simple and sadly true example that illustrates this point. In one hospital setting a surgery technologist was trained to operate a mini C-arm with the intent that a radiologic technologist would not be needed for simple procedures in the operating room. This person being uneducated in radiation exposure safety also functioned outside the auspice of the radiation safety officer (RSO), imaging department management, and safety committee. As a result, that person’s training measured short in performance in many ways. First, the use of intermittent beam on-off-on-off imaging did not occur during imaging procedures in which they were the fluoroscopist. Secondly, the O.R. technologist used long exposures while the surgeon viewed images when image hold on the monitor would have been more appropriate. Thirdly, the same person routinely operated the fluoroscopic tube at distances of 7-10 inches from the patient unaware of imaging distance regulations. The person’s rationale for using a short tube distance was that the surgeon never complained and seemed to like the magnified images. Can you see how serious these violations of the ALARA mandate are?
Not just anyone can be a satisfactory substitute to advocate patient radiation protection through equipment operation in the place of an educated imaging professional. Though radiographers are not the exclusive gatekeepers to thousands of rads of radiation dosed upon a trusting public annually, they are the educated professionals who practice daily safe exposure principles. Practicing ALARA as a form of patient radiation safety while providing diagnostic information for radiological interpretation is the focus of this issue. We will look at how radiographers make the most of the Cardinal rules of time, distance, and shielding as part of the ALARA mandate to protect patients from unnecessary radiation exposure. These principles are part of the continuum of competent imaging and positioning skills to bring about radiation safety for patients and personnel. This module discusses what measures are employed to assure ALARA while providing diagnostic images for radiological interpretation, and discusses how the radiographer controls various aspects of technique selection to augment the ALARA principle.
The foundation of the radiography profession is the production of quality radiographic images, and its pillars are personal and patient radiation safety practices. Understanding the principles of source and electromagnetic radiation, and exposure techniques are fundamental to the radiographers’ profession. Suffice it to say, radiographers manage radiation so that patient exposure is considered exceptionally safe and occupational risk is the same as for most safe professions.
In this age of technology many things are rapidly changing in the imaging discipline; however, the physical concepts of radiation exposure to produce diagnostic quality images remain a fundamental part of the imaging discipline. In addition, the x-ray tube apparatus design is engineered with patient protection as one of its highest standards. Likewise, the radiographer’s selection of radiographic techniques encompasses the highest radiation protection standards possible. Principles of patient radiation exposure that minimize unnecessary exposure and accent imaging benefits are cornerstone principles of our profession. In this module we will discuss the benefits of beam filtration, mAs and kVp selection, positive beam limitation, automatic exposure control, beam restriction, and shielding as means to apply the cardinal rules of radiation protection to the patient and to attain ALARA.
Section 4.1: Beam Filtration
The Food and Drug Administration (FDA) regulates the manufacturing standards for x-ray equipment. It is their mandate that radiation from the anode of an x-ray tube must pass through a beam attenuator to filter soft x-rays before reaching the patient. One of the most important but almost forgotten principles of patient protection is the utilization of inherent and added filtration of the primary beam as it exits the tube housing assembly. As radiographers we do not routinely manipulate beam filtration, and therefore its role in patient protection is often forgotten. But it should be remembered that the main purpose for the FDA requiring beam filtration is that it reduces skin entrance exposure dose. Studies show that without beam filtration low energy photons will be absorbed by the patient contributing nothing to the radiographic image. Low energy photons can be attenuated from the useful beam prior to it reaching the patient. In fact, a beam that is filtered will reduce the entrance skin exposure significantly. In a study by Trout and colleagues, it was calculated that using an 85 kVp beam with an intensity of 1225 mR without filtration was reduced to 287 mR with 3 mm aluminum filtration without compromising image quality. This is about a 77% reduction in the exposure by simply applying a filter to the primary beam. After filtration, the x-ray beam is said to be “hardened” meaning its average energy is increased giving the beam a higher quality.
Chart from Trout, E.D., Kelly, J.P., and Cathey, G.A., “The use of filters to control radiation exposure to the patient in diagnostic roentgenology,” American Journal Roentgen., 67,942,1952.
Trout and colleagues demonstrated that beam filtration significantly reduces patient skin dose, and also demonstrated that filtration above 3.0 mm aluminum can significantly reduce the intensity of the primary beam requiring an increase in the exposure technique to compensate for lost photons. Therefore, filtration greater than 3.0 mm aluminum equivalency is not recommended by the NCRP. Filtration removes mainly those photons of less than 40 keV effectively hardening the beam. The quality of the x-ray beam is therefore improved because it increases the half value layer (HVL) of the beam. NCRP regulations specifying filtration are in HVL units because this is the biomedical engineering standard for measuring and certifying beam filtration as required by the FDA. The halve-value-layer (HVL) is defined as that thickness of a given material that will reduce the intensity of a radiation beam to one half its original value(s). For energies used in diagnostic imaging up to three half-value-layers may be required by the FDA. When testing the quality of beam filtration, measuring the HVL is the testing standard.
The x-ray tube is enclosed in a glass envelope and submerged in oil to promote dissipation of heat generated in the production of x-rays. During the production of x-rays 99% of the energy is converted to heat and <1% is converted to producing x-rays. Both the Pyrex glass envelop and the oil surround are inherent filters, meaning part of the x-ray tube design, are together equivalent to approximately 1.0 mm aluminum filtration as the beam pass through to exit the tube. The tube is further encased in lead, called the housing, which restricts radiation emanating in all directions from the tube target. The housing protects the patient and personnel from randomly directed x-rays, permitting exposure only from a small control opening in the tube housing, called a port. The housing is thick enough to act as several half value layers of protection; The NCRP requires that leakage radiation from the housing must not exceed 100 mR/hr at a distance of 1 meter.
Three-phase imaging equipment is today’s industrial standard for x-ray machines because they are designed to reduce radiation exposure and increase the quality of the useful beam. A collimator is attached to the tube housing to which filtration is added to further reduce entrance skin exposure (ESE) to the patient. For example, an exposure of 10 mAs@ 70 kVp without filtration would have an intensity of 87 mR; the same exposure would be reduced to 54 mR with a total filtration of 2.5 mm Al equivalency. Primary beam filtration is not an option; it is required by the FDA based on NCRP recommendations for all x-ray equipment including mammography equipment. Any medically used x-ray machine that operates at greater than 70 kVp must have a total filtration of 2.5 mm Al equivalent which means additional filtration must be added to the inherent filtration provided by the glass envelop and oil. The technologist may add or change filtration at the collimator; however, above 3.0 mm Al equivalency the benefit of filtration diminishes greatly because the exposure technique must increase to maintain image quality.
The path newly made x-rays must pass to exit the assembly is through the Pyrex glass x-ray tube, insulating oil surround, and bakelite window in the housing. These structures provide an inherent filtering of the beam to an equivalency of between 0.5-1.0 mm Al. Furthermore, if the beam passes through a mirror in the collimator, a design that allows for true alignment of the light field, then the mirror will give an additional filtration of 1 mm aluminum because of the silver in the mirror backing. The sum of components that attenuate low energy photons from the primary beam are included in the recommended minimum total filtration standards. The collimator, light source/mirror, and aluminum or copper inserts are all classified as added filtration. Added and inherent filtration is included in the total filtration standards for x-ray beam attenuation set by the NCRP and FDA.
Anything that the radiographer can do to reduce additional exposure should be done; use of the anode heel effect and compensation filters can reduce repeat and additional images. Whenever possible the radiographer should use the anode heel effect to produce uniform film density across varying anatomical thicknesses. This is important because uneven density on a radiograph may require several exposures to complete a study, which gives additional exposure to the patient.
Another tool, use of a compound filter, is even more effective in producing even radiographic densities while reducing patient exposure. Several types of filter applications effectively reduce repeat and additional patient exposure due to substandard image quality. Using a compound filter is not considered a direct application of radiation protection; however, it can reduce repeat exposure because of extreme under and overexposed areas on a radiograph. An example of this would be a scoliosis film in which the upper cervical spine is extremely overexposed and the lumbosacral region optimally. This can be corrected by using a compound filter over the upper spine to reduce overexposure of the film.
Some common types of filters used in radiography are: compound filters, wedge filters, trough filter, and bow-tie filter used in CT. A common type of compound filter worth mentioning is the K-edge filter. It is sometimes used with studies requiring iodinated contrast agents because it attenuates low and high energy photons effectively improving subject contrast. A K-edge filter is constructed of high atomic number material towards the anode side, and low atomic number material, aluminum, is towards the patient. Another example of a compound filter is the Thoraeus filter used in radiation therapy for photon energies between 250-400 kV. It consists of three metals: tin, copper, and aluminum in that order from the tube side to the patient side. The primary beam is first filtered by tin; however, tin produces undesirable characteristic radiation that is then absorbed by the copper; subsequently the copper produces characteristic radiation that is absorbed by the aluminum, and finally low energy characteristic rays produced by aluminum are absorbed in air before reaching the patient. The key point here is that filtration is matched to the energy of the photon beam; for lower energies aluminum is sufficient whereas for higher energies copper or tin plus aluminum are indispensable.
To correct for uneven film densities due to overexposure of thin parts of the anatomy and under exposure of thick parts, by the heterogeneous x-ray beam, the technologist may, in addition to applying the anode heel effect, use a wedge filter. A wedge filter is an uneven filter that absorbs radiation according to the thickness of the filter on one edge versus an opposite edge. The radiation dose to thinner parts of the anatomy is reduced and the area that would otherwise appear relatively dark on the finished radiograph will have more even tone. By compensating for uneven densities, especially when using analog film-screen imaging, the technologist is able to acquire images in one film rather than in two or more.
Section 4.2: Achieve ALARA through technique selection
Technique selection is another way in which the radiographer can reduce patient exposure. In order for the technologist to take full advantage of technical selections that reduce patient dose, one must understand the relationship between mAs and kVp to each other and to patient dose. Truly understanding how reducing patient dose factors affect the overall image quality must also be considered. The radiographer’s role is to administer a prescribed dose of ionizing radiation to provide the radiologist with a quality diagnostic image, therefore noise or graininess of the image that is outside an acceptable viewing criterion is unacceptable.
A key relationship is that between mAs and film density, and mAs and radiation intensity. The mAs and its intensity is directly proportional, that is, if the mAs is doubled, the density of the radiograph will double and the radiation intensity to the patient is also doubled. Here a distinction must be made between the mAs selected and intensity of that radiation. We have stated earlier that the intensity of radiation refers to the concentration of photons per unit area, whereas mAs is a working unit of radiation exposure referring to the selection of exposure factors. The exposure factors the radiographer select termed mAs, is an acronym for the milliamperage (mA) and time (second) combination. The term mAs is the radiographers working clich¨¦ for selecting a quantity of radiation needed to produce a radiographic image. When we talk about radiation intensity we use the term milliroentgen (mR) rather than mAs; the roentgen (R) is the unit of radiation exposure reflecting a change in mAs with change in distance from the source. The concept here is that the mAs is a quantity of radiation produced at the anode of the x-ray tube; however, it is not the quantity of radiation that exposes the patient 40 inches from the anode. A dose remote from the source is calculated using the Inverse Square Law. To understand why mAs changes as a function with increasing distance from the source, one must understand that a concomitant increase in the area covered by the diverging beam also occurs. To express the change we must reference another term that expresses this change, radiation intensity. The formula below quantifies the relationship of the mAs to changing intensity of a diverging beam of ionizing radiation.
Consider the following example using the intensity formula: if 40 mAs used to make a KUB radiograph has an intensity of 116 mR, what is the intensity of the radiation beam if the mAs is adjusted to 20 mAs and all other factors remain the same?
Understanding what the mAs-intensity formula means will allow us to quantify the reduction in radiation intensity or exposure to the patient. We know that a minimum quantity of photons is needed to reduce noise or quantum mottling. In our effort to reduce patient exposure we must not diminish the quantity of photons to a point that the image becomes grainy due to too few photons. Consider the analogy of having 10 dots with which to draw a picture of a face. With only ten dots very little detail would be demonstrated. But if one were given 10 million dots it would be easy to sketch details about the eyes, nose, mouth, hair, and facial expression because a sufficient number of dots are used. For the radiographer with enough photons (mAs) detail can be demonstrated on the radiograph. What happens in clinical reality is that techniques radiographer’s routinely use are already reduced to ALARA standards; however, if for example a patient is pregnant, and the benefit of radiography outweighs the risk to the fetus, the technologist may further adjust the beam¡¯s intensity using the mAs-intensity formula so that the image is adequate but not necessarily ideal.
To maintain an equivalent density on the film when mAs is decreased, there must be a concomitant increase in kVp. Likewise, if the mAs is increased, a decrease in the kVp must also be made to maintain an equivalent image density. For example, if the original suggested technique is 40 mAs at 74 kVp, then 20 mAs at 84 kVp will reduce the gonadal dose by about 30% and produce equivalent radiographic density. When mAs and kVp adjustments, along with shielding and collimation are used, the technologist is significantly able reduce fetal dose. One may ask why this isn’t done for all patients. The answer is that some studies require high contrast images that are not produced by a low intensity high energy beam; and in many cases too little mAs may also be the cause of repeating images which increases the patient’s total dose. A kVp change accompanies any change in mAs for which the radiographer wishes to maintain the same film density.
In order to maintain an equivalent radiographic density when mAs is adjusted, the kVp must also be adjusted according using the 50/15 rule. This rule states that a 50% decrease in the mAs is equivalent to a 15% increase in the kVp, and that a doubling of the mAs is equivalent to a 15% reduction in the kVp, to maintain a comparable radiographic image. What is important here in terms of radiation protection is that kVp has a greater effect on film density than does the mAs; and is therefore and important factor that can be used to decrease patient exposure when a change in subject contrast is acceptable. For example, let’s say an exposure of 40 mAs @ 66 kVp provides inadequate film density. The radiographer could choose to double the mAs to 80 mAs which is a 100% increase in dose to the patient, or the technologist could keep the patient dose the same and increase the kVp by 15%. By choosing to increase the kVp the radiographer is actually decreasing the patient dose because less radiation will be absorbed in the process of optimizing the exposure technique.
This formula tells us two important ways technique selection can be used in radiation protection. One is that radiation intensity is related to both the mAs and the kVp the radiographer selects. The second point is that kVp has a greater effect on radiographic density than does the mAs; however, if the radiographer changes the kVp purposely to decrease patient dose, the contrast will also change. This is why beginning technologists usually choose to use mAs to control density because the relationship is direct, easy to calculate, and the contrast does not change. By comparison, a veteran technologist may choose to adjust kVp and mAs, which effects density and contrast but together can effectively reduce patient dose.
The 50/15 rule inadequately describes the relationship between kVp and radiation intensity. The relationship between kVp and ionizing radiation intensity is best described by the kVp/intensity formula below:
According to formula above, the intensity of radiation changes by the square of kVp change. A simple radiography rule of thumb is that increasing the kVp by 10 at 80 kVp, with compensatory decrease in the mAs, will decrease the intensity by about 25%. A compensatory decrease in the mAs at 60 kVp with a 10 kVp increase results in a 15% decrease in the intensity of radiation. We see that if the initial energy of the x-ray beam selected is 75 kVp having an intensity of 100 mR, then the intensity of the beam at 90 kVp is, assuming all other factors remain the same will be 144 mR. Thus the kVp intensity formula tells us that increasing the kVp with all other factors remaining the same will also increase the intensity of radiation. The mechanism by which kVp has its effect on radiation intensity is related to the effect of kilovoltage on tube current. When the kilovoltage across the tube (from cathode to anode) is increased, the accelerated electrons will have greater energy upon bombardment of the anode. Consequently, there will be more bremsstrahlung interactions required to bring the electron to rest in the anode. This will inherently increase the number of photons contributing to the primary beam. Therefore, increasing the kVp increases the quantity of photons and their energy. Because more bremsstrahlung photons are produced by raising the kVp, the mAs (or number of electrons needed) can be reduced as a compensatory technique change. This is a real benefit to the patient in terms of reducing radiation exposure. Even if no other changes in technique are made, a kVp change alone will affect photon absorption in the patient, hence patient dose. If a change in kVp necessitates optimizing to produce an equivalent radiographic density, a 50% decrease in the mAs will be required for each 15% increase in kVp.
There are three reasons why increasing the average energy of the photons cause a reduction in intrinsic patient dose: 1) some of the scatter radiation that is absorbed by the patient at a low kVp will now have sufficient energy to exit, 2) because the average photon energy is increased there is a decrease in the relative number of primary photons that undergo the photoelectric effect, instead they will become exit radiation, 3) more exit radiation is made available to effect radiographic image density and contrast so that the mAs can be decreased, which is a reduction in patient exposure.
We stated earlier that scatter radiation is a photon that has undergone a change in direction following interaction with an atom’s valance shell electron. If its resultant energy is amply decreased before it exits the patient, it will undergo a photoelectric interaction and be absorbed by the patient. Thus, an increase in the initial energy of the photon results in increased energy of the scattered photon. For example, a photon of 75 keV that scatters at an angle of 90¡ã will have a resulting energy of 66 keV. This photon will not contribute to the diagnostic image, and therefore, if it does not exit the patient will only contribute to the patient’s radiation dose. If a compensatory kVp change of 100 kVp is made, the same photon could have energy of 84 keV; then at 90¡ã scattering that photon will possess sufficient energy to escape the patient’s body reducing patient dose. This point illustrates the concept of continuity of radiation protection, that is, radiation protection begins first with the patient, then those in the near vicinity of the exposure. For the radiographer using kVp to reduce patient exposure there are two main concerns: 1) as more scatter exits the patient, the more at risk the occupational worker is, and 2) forward scatter causes low contrast images due to fog which may not be desirable for all imaging procedures. How the radiographer deals with the risk of occupational exposure is covered in Volume V of this series of modules and will not be further discussed here. Scatter radiation which presents a problem on the radiographic image is generally removed with a device called a radiographic grid. A grid is not a radiation protection device; it is used specifically to clean up scatter before it reaches the image receptor. Understanding that the amount of scatter radiation produced is directly related to the volume of tissue exposed; then it stands to reason that the least amount of tissue necessary for a diagnosis should be exposed (ALARA).
In order for the radiographer to produce high quality diagnostic images, they must achieve a delicate balance between exit radiation, photoelectric effect, and Compton scatter. In the low kVp range, the physics of ionizing radiation favors the photoelectric effect; at high kVp the balance is shifted to favor exit radiation and Compton scatter. The radiographer tries to keep the exposure balance tilted towards a greater proportion of exit radiation and scatter in compliance with the thesis called ALARA. This means as high a kVp as is reasonable to achieve the high quality radiographic detail to render clinical diagnostic differentials. In this manner the photoelectric effect and patient dose photon is kept to a minimum.
Our third premise is that increasing the kVp will make more exit radiation available to effect radiographic density so that the mAs can be decreased. Unfortunately increasing exit radiation by raising the kVp results in more transmitted photons and scatter radiation. Scatter radiation will also affect the radiographic image just as will transmitted radiation. Radiographic grids are designed to absorb scatter radiation minimizing scatters effect on the radiographic image; however, photons passing through a grid and reach the image receptor will contribute fog and distortion to the image. Consider the two pictures below that illustrate the four possible outcomes for a photon based on its keV: A) the photon may be absorbed by the PE effect if its energy is too low to exit the patient, B) a photon with sufficient energy scatters and exits the patient, but is absorbed by the grid because of its angle of scatter, C) the photon may possess sufficient energy to undergo scattering, then being reduced in energy be absorbed by the PE effect, D) the photon may pass through the patient without any interaction with the patient¡¯s atoms, E) a scattered photon may have sufficient energy to exit the patient and its angle may allow it to pass through the grid and reach the image receptor.
A photon that causes image fog also adds a measure of distortion to the image because it will create a density where there should not be one. Let’s consider the fate of our five photons: photon “A” contributes only to the patient’s dose because it is absorbed; the fate of photon “B” does not contribute a dose to the patient or technologist, nor does it contribute unwanted density to the radiographic image because it is absorbed by the grid; photon “C” will suffer the same fate as photon “A” and be absorbed in the patient’s body; photon “D” will contribute the most accurate information to the formation of the radiographic image; Photon “E” will have the same fate as transmitted radiation, but its role in image formation is undesirable.
We see that patient dose is related to the photoelectric effect and to the amount of scatter that does not exit. Increasing the kVp raises the average energy of photons so that there is a significant decrease in the photoelectric effect making these additional photons available to contribute to the film’s density. In addition, scatter exiting the patient increases as the kVp does. At higher kVp the patient dose decreases because of the reduction in the P.E. effect and increase in scatter leaving the patient’s body. It is this exit scatter that is harmful to the radiographer and other health care personnel and is the reason for wearing protective shielding when close to the patient. Adding higher kVp to the patient protection menu is in keeping with ALARA and makes sense because the radiographer is educated to effectively protect themselves and others from scatter radiation. The chart below shows the effectiveness of kVp on scatter, PE effect, and percent of ionizing radiation transmitted through the patient:
Section 4.3: Positive Beam Limiting (PBL) and Automatic Exposure (AEC)
The use of ionizing radiation to produce diagnostic images is an art and a science. Non imaging professionals who are allowed to use x-rays for medical imaging purpose are simply unprepared to use of all of the tool that control how ionizing radiation is administered to the patient. A thorough knowledge of human anatomy is required in order to practice the art of radiography; otherwise the operator must oversize the beam in order to be assured they will include the required anatomy. Skilled radiographers limit the exposure to the area of interest exposing any adjacent anatomy according to strict diagnostic imaging criteria. There are at least four controls that the radiographer routinely uses to limit patient exposure. These are: positive beam limitation (PBL), automatic exposure controls (a.k.a. phototimers), beam restriction, and shielding.
Positive beam limitation (PBL) is an added control in which the collimator is electronically linked to the bucky tray through sensors that adjust the collimator to the size of the cassette. Although PBL is not required by the FDA, if one is used it must be accurate to within 2% of the source-to-image distance (SID). The reason the FDA does not require PBL for all exposures is that it would require the technologist to use the entire field for any cassette selected. Because of the education and skills of radiographers (ARRT) certified, the FDA agrees that collimating to the area of interest by the technologist far outweighs any need to regulate by field size.
Automatic exposure control (AEC) is a type of imaging system that involves radiation detection apparatus that measures the quantity of radiation at the image receptor. There are two parts to the AEC device, the sensor(s) and the comparator that work together so that when a predetermined exposure that correlates to a preset optical density (O.D.) is reached, the exposure is automatically terminated at the generator. What this means to the patient is that with proper positioning the minimum dose is delivered to record an acceptable radiographic image. Comparably, a manual exposure setting is calculated by the radiographer using milliamperage (mA) and time (s) settings (mAs) to achieve optical densities that are in the useful range for diagnostic viewing. The useful ranges of optical densities throughout a radiographic image are between 0.25-2.5 O.D.
Automatic exposure control devices have preset optical density so that when the determined amount of ionizing radiation sensed reaches the preset optical density, the exposure is terminated.
Generally the radiographer has three sensors available that can be activated to select the anatomy through which density should be measured.
The type of sensor in the radiographic unit is indicated by its location. There are three main types: Photodetector, ion chamber, or solid-state detectors. The photodetector is the original type of AEC control and was marketed under the name “phototimer.” These types are somewhat obsolete because it relies on crystals to emit light entering a photomultiplier tube system. Through a process called photoemission electrons are produced generating a current equal to the quantity of the photons. Phototimer sensors are not radiolucent and must be placed behind the cassette. This means special cassettes are required that have limited lead backing (cassette modification that reduces backscatter) in order to accurately detect the exposure. Another type of AEC relies on the ion chamber, which is the most common design type in current use. They have a design advantage of being radiolucent so it can be placed between the grid and the cassette for more accurate readings, and is compatible with any cassette type. Also in use are solid-state detectors that use silicon or germanium crystals which are the most sensitive types of sensors and are also designed to be radiolucent.
The second part of the AEC apparatus is the comparator. A comparator is basically a device that receives the signal from the sensor(s). It contains a capacitor that stores voltage during the exposure so that when the voltage reaches the preset, the generator terminates the exposure. What is important to the radiographer about the comparator is that its voltage can be adjusted at the console. For example, suppose a client is obese and will require a large exposure to achieve the desired radiographic density. Increasing the preset voltage at the comparator, which is the reference voltage for terminating the exposure, will result in an increase in the film density. On the console, the comparator reference voltage is in units of -2, -1, 0, +1, +2, with zero setting being the programmed preset. Each step will adjust the reference voltage 25 to 30%, which is also the amount of density change, as long as adequate back-up time is allowed. The density setting can be adjusted down when thin or pediatric patients are imaged using -1 or -2 density settings. AEC is an effective method of reducing patient exposure because it delivers the minimum exposure to produce consistent radiographic images.
In pediatric imaging, small patients such as premature birth with low birth weight, present a special problem when using AEC. There is a minimum response time associated with automatic exposure control. The minimum response time (MRT) can be defined as the time of the shortest possible exposure with a particular AEC device. If the minimum response time is greater than the required exposure to achieve an optimal radiograph, the patient will be overexposed and the radiographic image will have excessive density. In such cases the radiographer should use a manual exposure or increase SID when using AEC.
Automatic exposure control is regulated by the FDA and accordingly, it must be able to adjust the exposure for any selected mA on the control panel to within 10% of the preset value. In addition, because AEC relies on a sensor and comparator to terminate exposure, the FDA requires these units be equipped with a back-up timer in case of system failure or excessive heat units that may damage the tube. The FDA requires the back-up timer to terminate the exposure at 600 mAs, or 6 seconds of exposure, whichever comes first.
Section 4.4: Beam restriction and types of beam limiting devices
Perhaps the most important factor in controlling patient radiation dose is limiting the field size. The radiographer employs beam restriction to limit the x-ray beam to the area of interest. The patient benefits because only that tissue that is important to the diagnosis is irradiated. The technologist benefits because less scatter is produced when the amount of tissue irradiated decreases. There are three basic types of beam restrictors that are of interest to the radiographer: 1) the collimator, 2) cylinder cones, and 3) the aperture diaphragm. A collimator is a beam restrictor that may be used alone or in combination with other beam restrictors such as a cone or cylinder to limit the patient¡¯s dose.
The most commonly used and perhaps the most efficient beam restrictor is the collimator. All medically used x-ray tubes have a collimator attached to the tube housing to further delimit the primary beam. A light source will demonstrate the area of exposure emanating from the collimator. How well the collimator regulates the field size, that area visualized by the light from the collimator and the actual area exposed by x-rays, is called the light field-radiation field congruency. NCRP regulations require that the light field-radiation field congruency measurements are within 2% of the source-to-image-distance (SID). The SID indicator must be accurate to within 2% of the measured source-to ¨Cimage-distance.
As we have already stated, the first rule of patient protection is to limit the beam to the area of diagnostic interest. Now this does not mean that if a KUB is ordered then only the kidneys, ureters, and bladder should be demonstrated radiographically. Radiographers are professional imagers who work within practical diagnostic criteria to provide images that meet broad standards in the patient’s interest. For example, a requested order for a “KUB & UPRIGHT” abdomen radiograph will be broadly interpreted by the radiographer based on the patient’s medical history. If for example the patient is over 40 years of age and is a male, the technologist may include the entire pelvis extended through the prostate gland on the flat plate image. When the upright or decubitus film is exposed, the technologist will include the diaphragm above and the entire rectum below. When the technologist follows the standard criteria for a procedure it is for the benefit of the radiologist who interprets the images broadly.
Technologists apply beam limiting as both a protective measure and as a means to improve the image quality. Beam limiting devices and body part shielding is used whenever possible; however, it requires positioning skill so not to clip required study anatomy by over restricting the beam. An incorrect interpretation of the patient’s history could contribute to an incorrect imaging protocol and repeat radiograph(s) being needed. The truth of the matter is that beam limiting devices are used primarily to increase image quality because they give a higher image contrast. The chronology of image quality using a beam restrictor is that limiting the beam restricts the area of patient exposure, which decreases scatter production, which diminishes scatter reaching the image receptor that translates into higher image contrast. There are several types of beam limiting devices: aperture diaphragm(s), cones, and extension cylinder, that attach to the collimator to extend the port of exit of the primary beam.
The aperture diaphragm is the least effective type of beam restrictor and is primarily used to control the shape of the primary beam. The collimator is equipped with lead shutters that open a rectangular area of coverage. If the corners of the beam could be rounded to reduce patient exposure such as when radiographing the paranasal sinuses, a round face aperture can be added. But because the diaphragm is a flat beam restrictor the divergence of the primary beam beyond the diaphragm is limited only to the shape of the aperture.
The extension cylinder is the most commonly used beam restrictor used in conjunction with a collimator. It is by far the most effective beam limiting device because the start point for beam divergence can be brought closer to the area of interest without decreasing the distance of the source to patient distance. To best illustrate this consider the picture below of a horizontal beam lateral of the lumbar spine. Notice that the lower vertebrae, L5/S1 are not well penetrated. The technologist could repeat the film adjusting technical factors to compensate for the area under penetrated; however, there would be unnecessary exposure to the entire lumbar spine unless beam limitation such as tighter collimation of the use of a cone or cylinder is applied. Notice in the picture below that the area of exposure to the patient using a collimator only does not always produce the highest quality because scatter radiation is related to the area of exposure. Even though collimation limits the patient dose, scatter is still great with collimation only, whereas with the use of a cone or cylinder close to the point of entrance of the beam for imaging, less scatter is produced and image quality increases.
It is difficult to determine that all of the vertebrae are demonstrated or if there is pathology in the under penetrated area of the film. Beam restrictors in addition to affording patient protection also reduces scatter which is the cause of fog and poor image quality. Consider the picture below in which an extension cylinder is used to limit patient exposure:
The application of an extension cylinder is very useful during trauma imaging where detail is needed. For example when imaging C7/T1 for evaluation, often it is difficult to provide adequate detail within a narrow contrast range that is not too low due to high kVp and scatter radiation. By extending the source of beam divergence the technologist effectively practices patient protection that translates into less scatter produced, higher contrast, and more detail in the final image. The use of a cone is especially useful for digital imaging because photostimulable plates require higher kVp and mAs than analog film imaging. These plates respond better to tightly collimated or cone images than to total plate coverage. Those institutions with digital computerized radiography will see great improvement in image quality when a cone is used as a contrast enhancer.
The two pictures above show an x-ray tube with the adjustable extension cylinder attached. This type of beam restrictor is used to bring the divergence of the useful beam close to the patient so that less tissue is exposed thereby improving radiographic contrast and detail.
Section 4.5: Patient Shielding
As professional radiographers, we must do all that we can to minimize general exposure, especially to the gonads of patients of child bearing age, because of the risk of inducing genetic effects. Unnecessary exposure to body parts outside the interest of the study should be avoided because of the risk of somatic effects. The first rule of medical radiation exposure is that the benefit must outweigh the risk. Shielding radiosensitive organs during imaging examinations should be a routine part of the imaging protocol so long as diagnostically important anatomy is not obscured, which is a cause of repeat imaging and a higher patient dose. In addition, radiographers are required to protect the gonads even if they are external to the anatomy of interest but situated within 10 cm of a well collimated beam. Unfortunately, females because their gonads are within the abdominal-pelvic cavity, shielding is less likely during many radiologic studies such as the KUB.
Other shielding practices include using a flat piece of lead, called a contact shield, to protect the patient. A contact shield is a piece of lead that is placed over the patient to protect them from irradiation. Shielding of the breast and gonads with special shield should be used for radiographic examinations such as scoliosis series, or a hip series. Remember, good radiation protection technique means that the primary beam should not pass through the shield because it only has the effectiveness of a HVL effect and does not stop radiation. Instead, the shield should be seen on the radiograph as a result of scatter radiation forming its silhouette. Remember, the best form of radiation protection is to limit the area of exposure and not to apply a shield to the gonads and use a wide beam for imaging.
Knowing how to effectively use lead shielding for patient protection is very important, especially during fluoroscopy, angiography, and surgical procedures involving C-arm use. Radiographers know that the x-ray tube in stationary fluoroscopy equipment is located at a fixed distance under the fluoroscopic table.
This arrangement of the x-ray tube in stationary fluoroscopy is confusing to the patient and to non imaging health care providers not familiar with x-ray equipment design and use. In order to protect the patient from unnecessary x-rays during stationary fluoroscopy lead shielding must be placed on top of the table beneath the patient. Unfortunately most patients and health care providers that are not radiographers are so conditioned to having lead placed on top of them for x-ray procedures that they may think they are not being given protection unless it is explained to them. Communicating radiation shielding to the patient is an important part of radiation protection. The radiographer should take the time to explain how the placement of a lead shield is important especially to the parents of pediatric patients who might otherwise think their child was not afforded radiation protection. Let me give another example of why it is important for the radiographer to teach radiation protection to the patient and other health care personnel who can have an effect on patient protection. A female patient undergoing surgical repair of her femur requested the orthopedic surgeon see too it that she is given lead shield during the surgical procedure. The surgery team placed a small flat contact shield over the patient¡¯s abdomen. Fortunately the radiographer was consulted by the surgeon just before draping at which time it was decided that a wrap around shield would be more appropriate to the patient’s request. Another not so surprising scenario is the one in which operating room personnel change the shielding placed on the patient by the technologist thinking they are doing the patient a courtesy; instead the carefully placed lead shield based on the direction of the tube during the procedure is breached. Most laypersons think that in all cases the lead shield should be placed over the top of their gonads and are not cognizant of the beam¡¯s direction.
In some circumstances the use of a shield may be unnecessary and may promote radiation absorption by the patient. For example, during CT examination of the thoracic spine a patient requested a lead shield over her lower abdomen. The technologist advised against this shielding since the shield itself would become a source of scatter radiation back towards the patient by photons that would have continued away from the patient reducing their exposure. However, since the patient insisted, lead shielding was provided. Because CT is a special application of ionizing radiation using high kVp for all procedures, and the field is tightly collimated, shielding may promote patient absorbed dose.
The two pictures above demonstrate that patient shielding is based on the patient¡¯s position and the direction of the x-ray beam. The picture to the left is a single C-arm unit designed for interventional radiography procedures; the right picture is of a bi-plane system that allows for bidirectional imaging. When the technologist shields a patient for these procedures, they must consider beam direction and not simply use a flat contact shield over the gonads. The same scenario is true of imaging in the operating room using a C-arm. The technologist should be consulted when it is desired to provide shielding during aseptic procedures using a C-arm.
When shielding is to be used during any aseptic procedure, the technologist should be consulted prior to draping the patient so that the proper shielding can be applied.
In the pictures above we see how the use of a mobile C-arm can require special shielding especially during aseptic procedures. The lead shielding must be placed prior to surgical draping of the patient.
A common practice that is growing across the United States is for non radiographic personnel to operate a mini C-arm without radiographer supervision of their training. In such cases the technologist should provide training to these personnel or they should undergo radiation safety training. In no case should this practice be occurring without the approval of the radiation safety officer and radiation safety committee, although it does.
The cardinal rule of practicing a safe distance from the radiation source applies to the patient as well as to the technologist. How close to the patient the x-ray tube can be is another concern of patient protection and safety and is regulated under NCRP report #102. It mandates that the minimum source-to-skin distance, which is the distance from the radiation source to the patient’s skin, for a stationary fluoroscopy tube can be no less than 38 cm (15 inches), and for a mobile C-arm not less than 30 cm (12 inches). When operating a C-arm the technologist is aware of these limitations and is careful not to breach the required limits. Maintaining the 12 inch maximum closeness to the patient is particularly difficult to manage with a mini C-arm because the diameter of the arch is undersized. The distance from the source to the image receptor is a limiting factor in mine C-arm operation. The operator should therefore position the tube at an acceptable distance from the part and lock it into place providing the surgeon have sufficient clearance for surgical instruments. The magnification buttons should be used to magnify structures rather than using object to receptor distance (OID) to control magnification. To manage image quality during C-arm use keep the part close to the image intensifier and use the magnification buttons to manipulate image size and detail.
NCRP regulations do not permit any x-ray exposure to the patient of less than 12″ from the source; this includes any single exposure or fluoroscopy exposures.
Radiation exposure from fluoroscopic imaging must be recorded in a document such as the radiologist report or the patient’s chart. It is the responsibility of the fluoroscopist to administer patient dose in compliance with ALARA during imaging. Intermittent beam on-off-on off exposure is one of the best ways to reduce patient exposure. The FDA mandates that controls on the console remind the fluoroscopist when a significant dose to the patient has been reached. Specifically, a 5 minute cumulative timer with audible sound must be operational when using fluoroscopy. The audible tone reminds the physician that a reasonable exposure dose has been administered. It also serves as a warning that the exposure time if to a local area cannot be taken for granted as literature shows several examples that local radiation burns are possible from fluoroscopic imaging procedures.
Exposure time can be further reduced by those factors that reduce repeat radiographs such as immobilization of the patient, correct positioning, of the subject, and using a high mA with short exposure time when selecting mAs. To reduce patient exposure the radiographer may need to use immobilization. Each repeat radiograph is a repeat exposure and dose to the patient; therefore, the radiographer is obligated not to make an exposure unless they are reasonably sure the parameters of the procedure are in order. Imaging professionals should not routinely hold patients for imaging. Instead, a family member may be needed to assist in immobilization. The NCRP states that no person shall be routinely used to hold patients. It is also important in pediatric and trauma imaging that the radiographic equipment is capable of fast exposure times by providing equipment capable of high mA stations. Some x-ray generators are capable of mA in excess of 2000 mA and exposure times of 0.0001 ms.
The FDA requires that mA stations are routinely checked for linear accuracy of the mA station for various timer combinations. A technique of 400 mA @ 50 ms gives an exposure of 20 mAs. If the technologist were to change the technique to 2000 mA @ 10 ms an obvious decrease in the exposure time, there should be a consistency in the mAs for any given technique. The FDA mandates the radiation intensity must be accurate to within a variance of 10% between adjacent mA stations. To test for linearity the time is held constant and each change in mA station equal to a doubling of the mAs is measured, for example, going from 100 mA to 200 mA should double the mAs, and likewise at 400 mA and 800 mA; each adjacent mA change should be within 10% of calculated mAs. Linearity is expressed in units of mR/mAs. The last component is the timer control during radiation exposure. Timer accuracy is regulated by the FDA. Accordingly, the timer linearity must be accurate to within 5% for exposures greater than 10 ms and 20% for exposures less than 10 ms. Testing for three-phase generators require a synchronous timer test tool with a rotation speed of 1 revolution per second (rps). A single phase equipment timer can be measured with a spinning top test tool or a synchronous timer test tool. All of these measures are implemented by the FDA to assure patient safety by those who use ionizing radiation for diagnostic purposes.
Measuring Patient Dose
The concept of patient protection is not new, in fact, within a year of the discovery of ionizing x-rays its potential harmful effects were known. Even so the respect for its potentially harmful biological effects did not deter its use by laypersons such as shoe fitters and carnival thrillers who mesmerized the public with its awesome revelations. Today the circus of non imaging professionals using ionizing radiation with limited knowledge of its overall relationship to public safety is growing at an alarming rate. Those specifically educated in the medical use of ionizing radiation are aware of the biological and hereditary implication of safe and unsafe exposures to the population. Statistical concerns about patient dose are commonly reported in three ways: entrance skin exposure (ESE), mean marrow dose (MMD), and gonadal dose (GD).
The entrance skin dose is also known as the patient dose because it is an easy way to estimate dose from exposure techniques used in plain film and fluoroscopy imaging. What is important here is that it is required by the Joint Commission on Accreditation of Healthcare Organizations (JACHO) and The Center for Devices and Radiological Health (CDRH) that diagnostic radiology facilities demonstrate an awareness of the amount of radiation exposure received by the patient. The most common way to provide this documentation is to place a dosimeter in the field of exposure and calculations made for many of the procedures performed at a given institution. For example, a PA chest x-ray may have a skin dose of 10-20 mrad and a gonadal dose calculated at about 1 mrad. The data collected will be particular to the institution doing testing.
Mean Marrow Dose (MMD)
The mean marrow dose (MMD) is an estimate of the amount of radiation exposure to the active blood forming organs. It is also used in population risk statistics to estimate the average the risk of leukemia for any member of the population, one of the known late effects of ionizing radiation.
The mean marrow dose is a simple calculation using the radiation exposure and the amount of active bone marrow exposed. The blood forming bones for calculation purposes are the head, sternum, ribs, and vertebrae, upper and lower limbs. An example of how to calculate the MMD would be to determine the percentage of the patient’s body exposed to a dose of ionizing radiation. For example, a patient’s radiation exposure during an examination includes 25% of the active bone marrow, and that dose is known to be 50 mrad. Then, the mean marrow dose would be 25% of that or 12.5 mrad. The estimated mean marrow dose in the United States approaches 100 mrad/yr (1 mGy/yr), which is a dose that is not capable of producing acute radiation effects in members of the general public.
The Genetically Significant Dose (GSD)
The genetically significant dose is an estimated dose to the population’s gene pool. It tells us the genetic load on the population that can affect genetic effects in our offspring. It is related to age, number of expected children, gender, and type of radiographic examination. It is defined as: that dose which received by every member of the population, would produce the total genetic effect on the population as the sum of the individual doses actually received. It is an estimate taken from the actual doses to real people and that data interpolated as an estimation of the risk to the gene pool if that dose was spread out over the entire population. The GSD is currently estimated to be 20 mrad/year above background radiation exposure. This dose is well below a detectable level and is well within radiation exposure tolerance for the population.
Pregnant Patients and Dose
It is the responsibility of any person who orders x-rays of a female of childbearing years to screen them for potential pregnancy. This responsibility is also extended to the technologist who should inquire prior to any x-ray exposure. Furthermore, the institution is required to post warnings requesting female patients to declare their pregnancy. These warnings must be posted in areas such as the preparation and waiting areas of the department where the patient is highly likely to see them. In centers that service multiethnic groups the warning should appear in several of the most common languages that are spoken.
It is required that all institutions accredited by the Joint Commission of Accreditation of Healthcare Organizations (JCAHO) must monitor doses from diagnostic imaging procedures. The Center for Devices and Radiological Health (CDRH) recommends that a diagnostic imaging facility maintain records of radiation doses received by a patient. Although is not a common practice for radiographers to record the number of films taken and the number of attempted radiographs taken on each patient, most institutions estimate dose for each type of imaging study and average calculated repeat rates.
Radiation dose to the fetus of less than 1 rad/rem is considered negligible or to have no realistic effect; however at greater than 1 rad/rem the risk is greater and concerning. NCRP report No 54, Medical Exposure of Pregnant and Potentially Pregnant Women states that a risk of 5 rad is negligible when compared to other risk associated with pregnancy. The concern with irradiating a pregnant woman is “should” the pregnancy be terminated because of exposure. There are at least three considerations that interplay in the decision: 1) the exact gestational time at exposure must be known, 2) a reasonably accurate estimated calculation of fetal dose, 3) a general understanding of how the patient feels towards pregnancy termination based on their age and health. What is considered in terms of fetal dose is that the risk of malformations increases greatly above 15 rad. A woman that receives 25 rad within 4 weeks of conception should consider abortion, whereas 5 rad in the third trimester would rarely be a reason for termination of pregnancy.
Certainly the most vulnerable time of embryo/fetus irradiation is the first trimester. Research supports the theory that any harmful effect to the fetus during the first two weeks of gestation (a time which the female may not know she is pregnant) is an all-or-none effect. The effect produces prenatal death which is spontaneous abortion of the embryo. Animal research correlation points to approximately 10 rad are required in the first 2 weeks to cause the spontaneous abortion rate to increase 0.1%. The normal spontaneous abortion rate is greater than 25% of all pregnancies. Based on these statistics the 10 day rule which states that the best time to image females is in the 10 day period following the onset of menstruation; however, this rule has largely be abandoned. The ICRP uses the 28 day rule since the effect on the embryo is dose dependent and all-or-none not favoring abortion.
Section 4.6: Concept and federal regulation of ALARA
The concept of ALARA is not new to radiology. It began when the Nuclear Regulatory Commission in December 1977 began pushing for radiation standards that lowered the dose to patients and occupational workers. As a result, The Office of Standards of the Nuclear Regulatory Commission published NUREG-0267, a follow up document to their attempts to reduce radiation exposure. This document was called, Principles and Practices for keeping Occupational Radiation Exposures at Medical Institutions As Low As Reasonably Achievable. The acronym ALARA remained as the documents impact on the radiology community to include patient and occupational exposure mandate for minimum necessary exposure. In 1994 the ALARA document became a part of title 10f the Code of Federal Regulations (10CFR35.20) which is binding on all institutions as a NRC regulation. Therefore, it must be practiced as a matter of mandate of federal code. So when the radiographer stresses the practice of ALARA it should be understood by all that it is because it is required and respectful to the patient.
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