Equipment Design for Radiation Protection

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RADIATION SAFETY FEATURES OF RADIOGRAPHIC EQUIPMENT, DEVICES, AND ACCESSORIES

 

Diagnostic-Type Protective Tube Housing

Requirements

A lead-lined metal diagnostic-type protective tube housing) is required to protect the patient and imaging personnel from off-focus, or leakage, radiation by restricting the emission of x-rays to the area of the useful, or primary, beam (those x-rays emitted through the x-ray port tube window or port).

X-Ray Tube Housing Construction

The housing enclosing the x-ray tube must be constructed so that the leakage radiation measured at a distance of 1 m (3.3 feet) from the x-ray source does not exceed 100 mR/hr (2.58 × 10−5 C/kg per hour) when the tube is operated at its highest voltage at the highest current that allows continuous operation.

Control Panel

The control panel. where technical exposure factors such as milliamperes (mA) and peak kilovoltage (kVp) are selected and seen on indicators by the operator, must be located behind a suitable protective barrier that has a radiation-absorbent window that permits observation of the patient during any procedure. This panel, which is electrically connected to the x-ray equipment, must “indicate the conditions of exposure and must positively indicate when the x-ray tube is energized.” The visible mA and kVp meters permit the operator to assess exposure conditions. Generally a tone is emitted when an x-ray exposure begins. The sound stops when the exposure terminates. This audible sound clearly indicates to the operator that the x-ray tube is energized and ionizing radiation is being emitted.

Radiographic Examination Table

The radiographic examination table must be strong and must adequately support the patient. Frequently this piece of equipment has a floating tabletop that makes it easier to maneuver the patient during an imaging procedure. The thickness of the tabletop must be uniform, and the tabletop also must be as radiolucent as possible so that it will absorb only a minimal amount of radiation, thereby reducing the patient’s radiation dose. A carbon fiber material is commonly used in the tabletop to meet this requirement.

Source-to–Image Receptor Distance Indicator

When radiographing a patient, radiographers must have a way to measure the distance from the anode focal spot to the image receptor to ensure that the correct source-to–image receptor distance (SID) is maintained. To meet this need, radiographic equipment must have an indicator that will perform this function. Frequently a simple device such as a tape measure is attached to the collimator or tube housing so that the radiographer can manually measure the SID. Lasers are also sometimes used to accomplish the same task. SID accuracy is essential. “The indicator must be accurate to within 2% of the indicated SID.”

X-Ray Beam Limitation Devices

The primary x-ray beam must be adequately collimated so that it is no larger than the size of the image receptor being used for the examination. With modern equipment this is accomplished by providing the unit with a light-localizing variable-aperture rectangular collimator to adjust the size and shape of the x-ray beam either automatically or manually). The collimator is currently the most popular x-ray beam limitation device in use at the time of this publication.

Types of X-Ray Beam Limitation Devices

In addition to the light-localizing variable-aperture rectangular collimator, earlier xray beam limitation devices include aperture diaphragms, cones, and cylinders. All of these devices confine the useful, or primary beam, before it enters the area of clinical interest, thereby limiting the quantity of body tissue irradiated. This also reduces the amount of scattered radiation in the tissue, preventing unnecessary exposure to tissues not under examination. Scattered radiation is all the radiation that arises from the interaction of an x-ray beam with the atoms of a patient or any other object in the path of the beam. When the size of the x-ray field is restricted to include only the anatomic structures of clinical interest, patient dose is significantly reduced because a smaller field size produces less scatter radiation. This improves the overall quality of the radiographic image.

 

Light-Localizing Variable-Aperture Rectangular Collimators

Construction

As stated earlier, the collimator is the most versatile device for defining the size and shape of the radiographic beam. The light-localizing variable-aperture rectangular collimator is the type of collimator most often used with multipurpose x-ray units. It is box shaped and contains the radiographic beam-defining system. This system consists of two sets of adjustable lead shutters mounted within the device at different levels, a light source to illuminate the x-ray field and permit it to be centered over the area of clinical interest, and a mirror to deflect the light beam toward the patient to be radiographed.

The first set of shutters, the upper shutters, are mounted as close as possible to the tube window to reduce the amount of off-focus, or stem, radiation (x-rays emitted from parts of the tube other than the focal spot) coming from the primary beam and exiting at various angles from the x-ray tube window. This radiation can never be completely eliminated because the metal shutters cannot be placed immediately beneath the actual focal spot of the x-ray tube, but placing the first set, or upper shutters, as close as possible to the tube window can reduce it significantly. This practice reduces patient exposure resulting from off-focus radiation.

The second set of collimator shutters, the lower shutters, are mounted below the level of the light source and mirror and function to confine further the radiographic beam to the area of clinical interest (Figures 10-3 and 10-4). This set of shutters consists of two pairs of lead plates oriented at right angles to each other. Each set may be adjusted independently so that a variety of rectangular shapes can be selected. In this way the field is not limited to the circular or square shapes that sometimes irradiate areas of the patient not requiring imaging.

Skin Sparing

To minimize skin exposure to electrons produced by photon interaction with the collimator, the patient’s skin surface should be at least 15 cm below the collimator. Some collimator housings contain “spacer bars,” which project down from the housing to prevent the collimators from being closer than 15 cm to the patient.

Luminance

Luminance is a scientific term referring to the brightness of a surface. Specifically, luminance quantifies the intensity of a light source (i.e., the amount of light per unit area coming from its surface). Luminance is deter mined by measuring the concentration of light over a particular field of view. This may be understood by examining the units used to describe luminance. The primary unit is the candela per square meter, known more simply as the nit. One candela corresponds to 3.8 million billion photons per second being emitted from a light source through a conelike field of view. A good analogy is the sound intensity emerging from a drill sergeant with a megaphone held to his lips. With appropriate dimensions, the megaphone’s larger opening corresponds to the conelike field of view associated with the candela. Luminance of the collimator light source must be adequate to permit the localizing light beam to outline the margins of the radiographic beam adequately on the patient’s anatomy. If the light field were not sufficiently bright, a radiographer could improperly position the x-ray field on a patient or at the very least have great difficulty accurately centering the x-ray beam, especially in the case of a dark-skinned patient. With insufficient brightness, the x-ray unit may fail a state inspection. The luminance must be high enough so that a calibrated light meter reading taken at a distance of 40 inches will be at least 15 foot-candles when averaged over the four quadrants of a 10-inch × 10-inch field size. A foot-candle is approximately equivalent to 10.76 nit (the unit of luminance). Therefore a reading of 15 foot-candles corresponds to a collimator light source with a luminance of about 161 nit or 161 candela per square meter. In summary, if the luminance of the collimator light source is adequate, the localizing light beam will adequately outline the margins of the radiographic beam on the area of interest on all patients.

Coincidence between the Radiographic Beam and the Localizing Light Beam

When a light-localizing variable aperture rectangular collimator is used, good coincidence (i.e., both physical size and alignment) between the radiographic beam and the localizing light beam is essential to eliminate collimator cutoff of the body structures being irradiated. Both the sum of the cross-table and along–the-table alignment differences and the sum of the length and width dimensions of the radiographic and light beams must correspond to within 2% of the SID (x-ray source-to–image receptor distance). These requirements are respectively known as alignment and congruence. As an example, 40 inches, which is equal to 101.6 cm, is the most commonly used SID in radiography. At this SID the maximal allowable total difference in length and width alignments of the projected light field with the radiographic beam at the level of the image receptor must be no more than 2% of 40 inches (0.8 inch) or 2% of 101.6 cm, or approximately 100 cm (2 cm). For acceptable congruence at 40-inch SID, the sum of the dimensions of the x-ray field should also differ from the length and width span of the light field by no more than 0.8 inch or 2 cm.

The SID used in radiography actually depends on the individual radiographic projection. For example, a 72-inch SID (182.9 cm) is normally used for routine chest x-ray examinations performed on ambulatory patients. In some imaging departments the use of 48-inch SID (121.9 cm) has become standard because increasing the SID significantly improves recorded detail of the radiographic image.

Positive Beam Limitation

In some earlier collimation systems the radiographer could inadvertently use an image receptor size much smaller than the size of the radiation field. Thus areas of the patient would be irradiated that would not be recorded on the image receptor. Either the radiation field size should be smaller (if the additional anatomy is not of diagnostic interest) or the image receptor should be larger (if the anatomy is indeed of diagnostic interest). To prevent such a mismatch, radiographic collimators that are part of fixed radiographic equipment manufactured in the United States generally include a feature called positive beam limitation (PBL). The PBL feature consists of electronic sensors in a cassette holder that sends signals to the collimator housing. When PBL is activated, the collimators are automatically adjusted so that the radiation field matches the size of the image receptor. If special conditions require the radiographer to have complete control of the system, with the turn of a key the PBL feature may be deactivated. However, in such a circumstance a warning light is automatically lit to indicate that the PBL system has been deactivated.

The PBL system illustrates an important principle of patient protection during radiographic procedures. The radiographer must ensure that collimation is adequate by collimating the radiographic beam so that it is no larger than the image receptor). In most states regulatory standards require accuracy of 2% of the SID with PBL. However, there are some states in which regulatory standards may require an accuracy of 3% of the SID with PBL.

Alignment of the X-Ray Beam

It is imperative that the x-ray beam and the image receptor be correctly aligned with each other. Every radio graphic tube must have a device in place to ensure accurate beam alignment.

Aperture Diaphragm

An aperture diaphragm is the simplest of all beam limitation devices. It consists of a flat piece of lead with a hole of designated size and shape cut in its center. The dimensions of the hole determine the size and shape of the radiographic beam. Different image receptor sizes and different SIDs require aperture diaphragms of various sizes to accommodate them. Diaphragm openings are rectangular, square, or round, with the rectangular shape being most common. Aperture diaphragms are used in trauma radiographic imaging systems, x-ray units designed specifically for chest radiography, and dental radiographic units.

Placed directly below the window of the x-ray tube, the aperture diaphragm confines the primary radiographic beam to dimensions suitable for covering a given size image receptor at a specified SID Because an aperture diaphragm limits field size and thus the area of the body irradiated, the amount of scattered radiation produced decreases.

Cones

Light-localizing variable-aperture rectangular collimators have replaced cones for most radiographic examinations. However, cones are still sometimes used for radiographic examinations of specific areas such as the head (e.g., coned-down lateral projection of the sella turcica] or projections of the paranasal sinuses vertebral column, and chest.

Flared Metal Tubes and Straight Cylinders

Radiographic cones are circular metal tubes that attach to the x-ray tube housing or variable rectangular collimator to limit the x-ray beam to a predetermined size and shape. The design of this collimating device is simple, consisting of either a flared metal tube with the diameter of the upper end smaller than the diameter of the lower end, or a straight cylinder with the diameter the same at both the upper and lower ends  Although the length and diameter of the cones vary, it is primarily the lower rim of the cone that governs beam limitation. Sharper size restriction is achieved when the cone or cylinder is longer. Field size at selected SIDs should be indicated on the cone.

Beam-Defining Cones Used in Dental Radiography

Beam-defining cones are widely used in dental radiography. Because dental x-ray equipment is usually less bulky than general-purpose equipment, a one-piece beam limitation device, such as a cone made of plastic, is convenient. Some dental cones are lined with lead. By using lead-lined cones instead of the conventional plastic cones, dentists reduce patient exposure by eliminating the source of secondary radiation (the plastic cone itself).

Filtration

Purpose of Radiographic Beam Filtration

Filtration of the radiographic beam reduces exposure to the patient’s skin and superficial tissue by absorbing most of the lower-energy photons (long-wavelength or soft x-rays) from the heterogeneous beam This increases the mean energy, or “quality,” of the x-ray beam. This change is also referred to as “hardening” the beam.

 

Effect of Filtration on the Absorbed Dose to the Patient

Because filtration absorbs some of the photons in a radiographic beam, it decreases the overall intensity (quantity or amount) of radiation. The remaining photons, however, are as a whole more penetrating and therefore less likely to be absorbed in body tissue. Hence the absorbed dose to the patient decreases when the correct amount and type of filtration are placed in the path of the radiographic beam. If adequate filtration were not present, very-low-energy photons (20 keV or lower) would enter the patient and be almost totally absorbed in the body, increasing the patient’s radiation dose but contributing nothing to the image process. They should be removed from the radiographic beam through the process of filtration. Filter material used for this purpose includes elements that are built in or added to the x-ray tube.

Types of Filtration

The following two types of filtration are available:

•        Inherent filtration

•        Added filtration

Inherent filtration includes the glass envelope encasing the x-ray tube, the insulating oil surrounding the tube, and the glass window in the tube housing. This inherent material amounts to approximately 0.5 mm aluminum equivalent, meaning that the built-in material provides the same amount of filtration as a 0.5-mm thickness of aluminum. The light-localizing variable-aperture rectangular collimator provides an additional 1 mm aluminum equivalent. The reflective surface of the collimator mirror provides most of this aluminum equivalent.

Added filtration usually consists of sheets of aluminum (or the equivalent) of appropriate thickness. This additional filtration is located outside the glass window of the tube housing above the collimator shutters. It is readily accessible to service personnel and may be changed as the x-ray tube ages. The inherent and added filtration combine to equal the required amount of total filtration necessary to filter the useful beam adequately

BOX 10-1 Total Filtration
Total   filtration  =  Inherent filtration plus added filtration

Requirement for Total Filtration

The peak kilovoltage of a given x-ray unit determines the total amount of filtration required. Total filtration of 2.5 mm aluminum equivalent for fixed x-ray units operating above 70 kVp is the regulatory standard (). Because such fixed x-ray equipment comes from the manufacturer with inherent filtration of 0.5 mm aluminum equivalent, and because 1 mm aluminum equivalent is attributed to the collimator components, the manufacturer needs only to place an additional 1–mm aluminum equivalent filter between the tube housing and collimator to meet the requirement for minimum total filtration.

Stationary (fixed) radiographic equipment requires total filtration of 1.5 mm aluminum equivalent for x-ray units operating at 50 to 70 kVp, whereas fixed units operating at below 50 kVp require only 0.5 mm aluminum equivalent. Mobile diagnostic units and fluoroscopic equipment require a minimum of 2.5 mm aluminum equivalent of total permanent filtration. A summary of required minimum total filtration may be found in

Filtration for Mammographic Equipment

Appropriate filtration also is necessary for mammographic equipment, which produces photons with an energy range of 17 to 20 keV. Metallic elements such as molybdenum (Z = 42) and rhodium (Z = 45) are commonly employed as filters. When the x-ray tube target is made of molybdenum, either a 0.03-mm molybdenum filter or a 0.025-mm rhodium filter may be selected. For rhodium x-ray tube targets, rhodium filters are used. These filtration materials facilitate adequate contrast in the radiographic image over the clinical extent of compressed breast thickness by preferentially selecting a particular range or window of energies from the x-ray spectrum emerging from the x-ray tube target. Molybdenum filters allow a lower energy window (17 to 20 keV) than rhodium filters (20 to 23 keV) (). Molybdenum filters are therefore suitable for small and average breast thickness, whereas rhodium filters used with a molybdenum anode are better for larger or dense breasts (i.e., compression thickness of 6 cm and greater) because they will produce an x-ray beam with higher energy. Systemic use of such materials has the effect of reducing the mean glandular dose in firm breast tissue. Maintaining and enhancing subject contrast is important in mammography. Beryllium (Z = 4) takes the place of glass in the window of the low-kVp–producing mammographic x-ray tube to accommodate this need. This light, strong metal permits the relatively soft characteristic radiation important for enhancing contrast to exit the tube without undergoing any significant attenuation.

Filtration for General Diagnostic Radiology

In general diagnostic radiology, aluminum (Z = 13) is the metal most widely selected as a filter material because it effectively removes low-energy (soft) x-rays from a polyenergetic (heterogeneous) x-ray beam without severely decreasing the x-ray beam intensity. Also, aluminum is lightweight, sturdy, relatively inexpensive, and readily available. In compliance with the Radiation Control for Health and Safety Act of 1968, a diagnostic x-ray beam must always be adequately filtered. This means that a sufficient quantity of low-energy photons has been removed from a beam produced at a given peak kilovoltage. The half-value layer (HVL) of the beam must be measured to verify this. HVL may be defined as the thickness of a designated absorber (customarily a metal such as aluminum) required to decrease the intensity (quantity or amount) of the primary beam by 50% of its initial value. A radiologic physicist should obtain this measurement at least once a year and also after an x-ray tube is replaced or repairs have been made on the diagnostic x-ray tube housing or collimation system. For diagnostic x-ray beams, the HVL is expressed in millimeters of aluminum. Because HVL is a measure of beam quality, or effective energy of the x-ray beam, a certain minimal HVL is required at a given peak kilovoltage. Examples of required HVLs for selected peak kilovoltages are listed in

Compensating Filters

Dose reduction and uniform radiographic imaging of body parts that vary considerably in thickness or tissue composition may be accomplished by use of compensating filters constructed of aluminum, lead-acrylic, or other suitable materials. These devices partially attenuate x-rays that are directed toward the thinner, or less dense, area while permitting more x-radiation to strike the thicker, or more dense, area. For example, the wedge filter () may be used to provide uniform density when the foot is undergoing radiography in the dorsoplantar projection. For this examination, the wedge is attached to the lower rim of the collimator and positioned with its thickest part toward the toes and its thinnest toward the heel. The trough, or bilateral wedge, filter, which is used in some dedicated chest radiographic units, is another example of a compensating filter. This filter is thin in the center to permit adequate x-ray penetration of the mediastinum and thick laterally to reduce exposure of the aerated lungs. With this device a radiographic image with uniform average density is obtained.

Exposure Reproducibility

In some important provisions included in the code of standards for diagnostic x-ray equipment that went into effect on August 1, 1974 are described. Exposure reproducibility is defined in that chapter as consistency in output in radiation intensity for identical generator settings from one individual exposure to subsequent exposures. This means that the x-ray unit must have the ability to duplicate certain radiographic exposures for any given combination of kilovolts at peak (kVp), milliamperes (mA), and time. A variance of 5% or less is acceptable. Reproducibility may be verified by using the same technical exposure factors to make a series of repeated radiation exposures and then observing how radiation intensity typically varies.

Exposure Linearity

Exposure linearity is defined in as consistency in output radiation intensity at selected kVp settings when settings are changed from one milliamperage and time combination (mAs = mA × exposure time) to another. Linearity has been defined as the ratio of the difference in mR/mAs values between two successive generator stations to the sum of those mR/mAs values. It must be less than 0.1. When settings are changed from one mA to a neighboring mA station, the most that linearity can vary is 10%.

Screen-Film Combinations

With advances in technology, many health care facilities that perform radiographic procedures are now using image receptors other than radiographic film. These newer imaging technologies such as digital radiography (DR) and computed radiography (CR) are discussed later in this chapter. Because some health care providers have continued to use film as an image receptor, the following discussion concerning screen-film combinations continues to be of significant value.

Value of Intensifying Screens in Patient Dose Reduction

X-ray film, most of which is double-emulsion x-ray film (i.e., emulsion coated on both sides of the film), responds strongly to the light emitted by intensifying screens. Intensifying screens enhance the action of x-rays on film by converting x-ray energy into visible light. About 95% of the radiographic density of the recorded image results from the visible light photons emitted by the intensifying screens. Because a single x-ray photon can produce 80 to 95 light photons, this conversion drastically enhances the film exposure process and permits radiographic exposure time to be substantially reduced. This leads to a sizable reduction in patient dose. At the time of this writing, when screen-film image receptors are still used in some health care facilities, intensifying screens used in conjunction with matching radiographic film are predominantly rare-earth screens. These screens are made with rare-earth phosphors, namely gadolinium, lanthanum, and yttrium. The rare-earth elements used in these screens have high atomic numbers ranging from 57 to 71. Consequently, because the elements have high atomic numbers, the screens facilitate higher x-ray absorption of the incident x-ray beam. Therefore they can convert the x-ray energy to light more efficiently (by 15% to 20%). Rare-earth screens are noticeably faster than the calcium tungstate screens that were used until the 1970s. Rare-earth screens also place less thermal stress on the x-ray tube, increasing its life span. In addition, when these screens are used, radiation shielding requirements for the x-ray room are decreased because a general reduction of x-radiation in the environment occurs.

Effect of Faster Screen-Film Systems on Patient Dose

To reiterate, film speed and the use of intensifying screens significantly influence radiographic exposure time. Although rare-earth screens and matching film combinations with relative speeds from 200 to 1200 are available, 400-speed systems are considered standard for general radiography at the time of this writing. When the speed of screen-film systems doubles—for example, when a change is made from a 200-speed system to a 400-speed system—patient radiation exposure is reduced by approximately 50%. When the amount of silver halide crystals (approximately 95% of which are silver bromide) contained in radiographic film emulsion is increased, the speed of the film is increased. This means that less radiation is required to obtain an image. As radiographic exposure decreases, patient dose decreases. One manufacturer has developed an 800-speed medical film that can be used with regular intensifying screens. The use of such a film can reduce the patient’s radiation exposure by as much as 50%.

It is essential that screen-film systems be matched correctly. If they are not correctly matched or compatible, patient dose can increase.

Effect of Kilovoltage on Screen Speed and Patient Dose

Kilovoltage also affects screen speed. As kilovoltage increases, effective screen speed increases for rare earth screens, which reduces the patient dose. The higher atomic numbers that the materials used in rare earth screens have increase the probability for photoelectric interaction between the incident x-rays and the rare earth atoms in the screens. The selection of peak kilovoltage and the screen-film combination are two of the most important technical considerations in the amount of patient dose.

Selection of Film-Based Image Receptor Systems

Although the high-speed film-screen image receptor systems with calcium tungstate intensifying screens that were used until the 1970s significantly reduced patient dose, a loss of radiographic quality was also possible because the recorded image may have had poorer resolution. As a result, the use of these image receptor systems was not practical for all radiography. When compared with slower rare-earth film-screen image receptor systems, faster rare-earth film-screen image receptor systems can demonstrate an effect referred to as quantum mottle. These faint blotches (image noise) can degrade the radiographic image and be annoying to the radiologist interpreting the image. Therefore higher-speed rare-earth systems may not be suitable for all radiography either. To be able to select the appropriate film-based image receptor system for a given radiographic examination, the radiographer must be aware of the capabilities and limitations of the different systems available. Manufacturers or distributors supply product information about the various film-based image receptor systems, which can be obtained by contacting the appropriate source.

Summary of Benefits of Rare-Earth Intensifying Screens

As mentioned previously, rare-earth intensifying screens are more efficient than their predecessors, calcium tungstate intensifying screens, in converting x-ray energy into light photons. Made of gadolinium, lanthanum, or yttrium, these screens absorb approximately five times more x-ray energy than the previously used calcium tungstate screens; hence they emit considerably more light. This significantly reduces the radiographic exposure required to obtain an image of acceptable quality. An additional benefit of rare-earth screens is that high resolution (the ability of a system to make two adjacent objects visually distinguishable) of the recorded image remains constant. This ensures radiographic quality. Higher-speed rare-earth systems do, however, produce quantum mottle (faint blotches) in the recorded image, causing some degradation of the image. Therefore they may not be suitable for all radiography.

X-ray tube life span is increased because rare-earth screens place less thermal stress on the tube. Also, because required x-radiation intensity is reduced when rare-earth screens are used, radiation shielding requirements for the room decrease.

Use of Carbon Fiber as a Front Material in a Radiographic Cassette

The use of carbon fiber as a front material in a cassette that holds radiographic film and intensifying screens is a technologic advancement over previously used cassette front materials. When compared with the traditional cassette front materials such as aluminum or cardboard, the cassette front containing the carbon fiber absorbs approximately half as much radiation. This lowers the patient dose, because lower radiographic techniques are required to produce the recorded image. In addition, because lower radiographic techniques are employed when cassettes with carbon fiber front material are used, the life of the x-ray tube may also be prolonged.

Use of Asymmetric Film Emulsion and Intensifying Screen Combinations

Another technologic advancement that is still in use is the use of asymmetric film emulsion and intensifying screen combinations. With this system the front screen and film emulsion (the side that faces the x-ray tube) is slower than the back screen and film emulsion, which contains a faster system. This screen-film combination results in a recorded image with greater uniformity and a decrease in patient exposure.

Radiographic Grids

Construction, Purpose, Technical Value, and Impact of a Radiographic Grid on Patient Dose

A radiographic grid (Figure 10-14) is a device made of parallel radiopaque strips alternated with low-attenuation strips of aluminum, plastic, or wood. It is placed between the patient and the radiographic image receptor to remove scattered x-ray photons that emerge from the patient before they reach the film or other image receptor. This significantly improves radiographic contrast and visibility of detail. Generally this device is used when the thickness of the body part to be radiographed is greater than 10 cm. Although the use of a grid increases patient dose, the benefit obtained in terms of the improved quality of the recorded image, making available a greater quantity of diagnostic information, is a fair compromise. Because several different types of grids are available, care must be taken to ensure that the correct type is used for a particular examination, or else a repeat may be necessary, which would negate the benefit of increased image quality.

Summarizing the Function of a Radiographic Grid

To summarize, when x-rays pass through an object, some of the photons are scattered away from their original path as a result of coherent and Compton scattering processes. Radiographic quality is highest when these scattered photons are not recorded on the image. If scattered photons are recorded, a general darkening of the image occurs, which detracts from the viewer’s ability to distinguish between the different structures of the object being radiographed. Only those photons that have passed through matter with no deviation from their original path should be recorded. To minimize the influence of scattered photons, a grid is inserted between the patient and the image receptor. It is designed to act as a sieve to block the passage of photons that have been scattered at some angle from their original path (Figure 10-15).

Grid Ratio and Patient Dose

As previously noted, grids are made of parallel radiopaque lead strips alternated with low-attenuation strips of aluminum, plastic, or wood. Because some fraction of the image receptor is covered with lead, mAs must be increased to compensate for the use of the grid. Hence, patient dose increases whenever a grid is used, and because more lead is contained in higher-ratio grids (e.g., 16:1), patient dose increases as grid ratio increases.

Minimal Source-Skin Distance for Mobile Radiography

Requirement

Mobile radiographic units require special precautions to ensure patient safety. When operating the unit, the radiographer must use a source-skin distance (SSD) of at least 12 inches (30 cm) (Figure 10-16). The 12-inch distance limits the effects of the inverse square falloff of radiation intensity with distance. This falloff is more pronounced the shorter the SSD. In practice, longer distances (e.g., 40 inches from x-ray source to image receptor) are generally used.

Effect of Source-Skin Distance on Patient Entrance Exposure

When the SSD is small, patient entrance exposure is significantly greater than exit exposure. By increasing SSD, the radiographer maintains a more uniform distribution of exposure throughout the patient.

Use of Mobile Units

Mobile (portable) units should be used to perform radiographic procedures only on patients who cannot be transported to a fixed radiographic installation (an x-ray room). Mobile units are not designed to replace specially designated rooms.

RADIATION SAFETY FEATURES OF DIGITAL IMAGING EQUIPMENT, DEVICES, AND ACCESSORIES

Digital Imaging

Use of the Computer

The computer is a device that electronically processes independent groups of information. Since the 1970s the use of computers has virtually revolutionized the medical industry. In particular, computers have had a major impact on imaging. They are now used extensively in almost all imaging modalities. The primary examples of this are computed tomography (CT), CR, DR, digital fluoroscopy (DF), nuclear medicine (NM) imaging, magnetic resonance imaging (MRI), and ultrasound.

Conventional Radiography: Analog Image

In conventional radiography, after x-rays pass through an anatomic area of clinical interest, they form an invisible, or latent, image of that area on radiographic film. This temporary image produced conventionally by ionizing radiation must then be chemically processed to make the unseen image visible. The finished radiograph that results from this process is an “analog image.” Conventional radiography permits the production of optimal-quality images that make possible adequate visualization and demonstration of various anatomic structures. However, there are some disadvantages to the use of this technology in addition to the waiting time it takes for the chemical processing of these film-based images. Radiographic film must be physically handled by authorized personnel and then stored in a centralized file. Manually retrieving radiographs is often time-consuming and requires adequate personnel power. As a consequence of human error, film jackets containing patient radiographs may be misfiled or misplaced, making these records unavailable at a time when a physician may need them for patient care.

 

Digital Radiography

The information contained in a conventional radiograph consists of various shades of gray that represent the amount of x-ray penetration through various biologic tissues. As stated earlier, the latent imaged produced by conventional means must be chemically processed to make the unseen image visible. With digital radiography (DR). the latent image formed by x-ray photons on a radiation detector is actually an electronic latent image. Because this image is produced by a computer representation of anatomic information, it is called a digital image The numeric values of the digital image are aligned in a fixed number of rows and columns that form individual miniature square boxes, each of which corresponds to a particular place in the image. These individual boxes collectively constitute the image matrix. Each miniature square box in this matrix is called a picture element, or pixel. The pixels collectively produce a two-dimensional representation of the information contained in a volume of tissue. The size of the pixels determines the sharpness of the image. Resolution (detail) is sharper when pixels are smaller.

The numeric value in each miniature square box that makes up the image matrix can be converted into a visual brightness, or density level, that can be seen on a video display monitor. Matrix size also affects detail of the image. Detail is better when the size of the matrix is larger because a larger matrix contains smaller pixels. When compared with the resolution of an optimal-quality image produced on radiographic film, the resolution of the digital image is actually somewhat lower. However, the digital image is diagnostic and permits adequate visualization of anatomic structures because it has better image contrast, and the radiographic density and contrast in the image can be manipulated to improve its overall quality. The image receptors used in DR convert the energy of x-rays into electrical signals. The image receptor is divided into small detector elements that make up the picture elements, or pixels, of the digital image. There are various types of DR image receptors. Some use a scintillator, such as amorphous silicon,* to convert the x-ray energy into visible light. The visible light is then converted into electrical signals by an array of transistors or an array of charge-coupled devices (CCDs), such as those found in video cameras. Other systems use a photoconductor, such as amorphous selenium, to convert the x-ray energy directly into electrical signals that are then read by an array of transistors. In these systems, the number and size of small transistors or CCDs determine the number and size of pixels in the digital image. Advances in materials technology have resulted in pixel sizes as small as 50 micrometers, which approaches the resolution of film-screen imaging systems ().

DR images can be accessed at several workstations at the same time, making image viewing very convenient for physicians providing patient care. Patient information and reports can be included in the patient’s DR imaging file along with records from other imaging modalities.

Retake Rates in Digital Radiography

Because the image contrast and overall brightness may be manipulated after image acquisition, DR eliminates the need for almost all retakes required because of improper technique selection (). However, retake rates for reasons of mispositioning will not be affected. Because the image receptor is part of the imaging equipment and does not need to be removed for processing, the technologist can simply view the image on a monitor in the room. This raises a concern about retakes required because of mispositioning. There is no “penalty” of a quality control technologist viewing the image and monitoring retakes required because of mispositioning, so the examination might be repeated without knowledge of supervisors. Therefore either each image should be monitored by an independent quality control technologist at a separate monitor, or a quality control system should be used, whereby the number of images per examination is compared with the number ordered for each technologist.

Computed Radiography

Process

When the invisible, or latent, image generated in conventional radiography is produced in a digital format using computer technology, the process is called computed radiography (CR). CR involves the use of conventional radiographic equipment, traditional patient positioning performed by a radiographer, and the selection and use of standard technical exposure factors. The unseen radiographic image is actually produced in a rectangular, closed cassette containing a photostimulable phosphor (europium-activated barium fluorohalide is the most commonly employed phosphor) imaging plate as the image receptor. This reusable imaging plate is used instead of radiographic film in a light-tight, closed cassette that resembles that found in conventional radiography. The CR cassette may be referred to as a filmless cassette. When the enclosed phosphor is exposed to x-rays, it becomes energized. An image reading unit is used to scan the photostimulable phosphor imaging plate with a helium-neon laser beam. This results in the emission of violet light that is changed into an electronic signal by a device called a photomultiplier tube. A computer then converts the electronic signal into a digitized image of the anatomic area or part and stores the digital image for visual display on a monitor. If desired, the image can be printed on a laser film when hard copy is needed. While the digital image is displayed on a monitor, the radiographer can adjust it to the correct size, radiographic density, and contrast by manipulating the computer mouse (). After adjustments have been completed, the image can be electronically sent for reading.

Computed Radiography Phosphor Sensitivity

The sensitivity of the phosphor used in CR has been described as approximately equal to a 200-speed screen-film combination. Phosphor sensitivity, however, can be significantly greater under certain conditions. What this implies is that CR technique factors will generally be somewhat greater than those used in conventional radiography, where 400-speed film-screen combinations are typically employed.

Kilovoltage

As in conventional radiography, kilovoltage controls radiographic contrast. However, CR imaging has greater kilovoltage flexibility than does conventional screen-film radiography. Therefore a radiographer can select an appropriate kVp setting from a broader range of settings than are suitable for a particular radiographic projection. An acceptable range of kVp that is adequate for penetration of the anatomy of interest should always be used. Kilovoltage above or below this acceptable range should not be used. Technique charts indicating optimal kVp for all CR projections should be available in the x-ray room near the operating console for the radiographer.

Use of Radiographic Grids

When compared with conventional screen-film systems, the photostimulable phosphor in the CR imaging plate is much more sensitive to scatter radiation before and after it is sensitized through exposure to a radiographic beam. Because of this increased sensitivity, a radiographic grid may be used more frequently during CR imaging. For chest radiography, Carlton and Adler advocate the use of a grid for optimum images when chest measurements exceed 24 to 26 cm. Some CR imaging manufacturers recommend the use of a grid for certain radiographic projections that require the use of relatively high kVp settings. Grid selection depends on several factors—for example, the size of the anatomy to be radiographed, kVp selected, amount of scatter removal preferred, and grid frequency (lines per centimeter or inch).

With film-screen systems (FSSs), it is customary and, for the best image quality, necessary to use a grid for anatomy sections over 10 cm in thickness or for techniques that exceed 70 kVp. This need remains true with both CR and DR. The problem one faces with CR is that the mAs required, and consequently the patient dose received, is significantly higher than is delivered with film-screen imaging. The addition of a grid will only further increase that dose. Many quality assurance teams, however, are now realizing that CR, because of its higher exposure latitude, makes grid use on the pediatric population less necessary than was previously believed. As a result, satisfactory nongrid pediatric protocols have been developed and used.

Digital radiographic systems offer a number of advantages over both CR and conventional FSSs. Some of these include lower dose, ease of use, and immediate imaging results. One potential disadvantage, however, is that most fixed DR systems either do not allow the user to change the grid to accommodate the imaging task or have a preinstalled grid that is not easily accessible to the user. These conditions result in grids being used for pediatric imaging whereby pediatric patients unnecessarily receive a higher dose of radiation. Facilities will now need to work more with their radiation safety officer (RSO) and physics group than ever before, to ensure the highest-quality imaging for the smallest patients. As a technologist, one must find out whether grids are truly not removable from the digital imaging equipment or if they are being used merely because it is the manufacturer’s recommendation.

 

RADIATION SAFETY FEATURES OF FLUOROSCOPIC EQUIPMENT, DEVICES, AND ACCESSORIES

Fluoroscopic Procedures

Patient Radiation Exposure Rate

Fluoroscopy is the process in which an x-ray examination is performed that demonstrates dynamic, or active, motion of selected anatomic structures (e.g., a stomach filled with barium sulfate and air during an upper gastrointestinal series) by producing a temporary image of those structures on a television monitor working in conjunction with an image intensification system under low-light conditions. Fluoroscopic procedures (Figure 10-20) produce the greatest patient radiation exposure rate in diagnostic radiology. In view of this fact, the physician should carefully evaluate the need for a fluoroscopic examination to ascertain whether the potential benefit to the patient in terms of information gained outweighs the potentially adverse somatic or genetic effects of the examination. If the fluoroscopic procedure is necessary, every precaution must be taken to minimize patient exposure time.

Fluoroscopic Imaging Systems

Traditionally, fluoroscopic imaging systems have the x-ray tube positioned under the x-ray examination table and the image intensifier and spot film system (or in significantly older fluoroscopic equipment, an image receptor called a fluoroscopic screen) mounted on a C-arm and centered and suspended over the x-ray examination table. The C-arm design keeps the x-ray tube and the image receptor in constant alignment. Other equipment configurations are possible; for example, the unit can be arranged so that the x-ray tube can be placed over the x-ray examination table while the image receptor lies beneath the x-ray examination table. A fluoroscopic imaging system can also be set up as a remote control facility, permitting the equipment operator to remain outside of the fluoroscopic room. In the interest of patient and personnel safety, radiologists and assisting radiologic technologists have a responsibility to become fully knowledgeable regarding the safe operation of the equipment they use.

 

Image Intensification Fluoroscopy

Benefits

Image intensification fluoroscopy (involves the use of an image intensifier to increase the brightness of the real-time image produced on a fluorescent screen during fluoroscopy. It is used in virtually all state-of-the-art fluoroscopic equipment. Image intensification fluoroscopy has three significant benefits. These benefits are listed in

Brightness of the Fluoroscopic Image

The x-ray image intensification system converts the x-ray image pattern into a corresponding amplified visible light pattern. Overall brightness of the fluoroscopic image increases to roughly 10,000 times the brightness of the image on the discontinued non–image intensifier fluoroscopic systems operating under the same conditions. This dramatic increase in image brightness greatly improved the radiologist’s perception of the fluoroscopic image.

BOX 10-3 Benefits of Image Intensification Fluoroscopy
1.      Increased image brightness2.      Saving of time for the radiologist

3.      Patient dose reduction

Use of Photopic or Cone Vision to View Fluoroscopic Image

Because an image intensification system permits viewing of the fluoroscopic image at ordinary brightness levels (regular white light), the radiologist makes use of photopic, or cone, vision (daytime vision) when viewing the image through this system. With cone vision, the radiologist does not need to adapt to the darkness (by wearing of red goggles for up to 30 minutes), which was required before a non–image intensification fluoroscopic examination in order for radiologists to use scotopic, or rod, vision (night vision) to view the dim fluoroscopic image. This saves considerable time. Cone vision also significantly improves visual acuity, thereby permitting the radiologist to better discriminate between small structures.

Milliamperage Required and Effect on Patient Dose

Because an image intensification system greatly increases brightness, image intensification fluoroscopy requires less milliamperage than does old-fashioned fluoroscopy (about 1.5 to 2 mA is used for many procedures with image intensification systems, whereas 3 to 5 mA was usually required for pre–image intensification fluoroscopy). The consequent decrease in exposure rate can result in a sizable dose reduction for the patient.

Multifield, or Magnification, Image Intensifier Tubes

An image intensifier tube is basically an “electronic device that receives the image-forming x-ray beam and converts it into a visible-light image of high intensity.” A simple diagram of this tube with components labeled may be found in. Multifield, or magnification, image intensifier tubes are found in the majority of image intensifiers. They are also found in digital fluoroscopic units (see the discussion on DF presented later in this chapter). Depending on their manufacturer and geographic location, multifield image intensification tubes vary in size, but the 25/17/12 cm (10/6.8/4.8 inch) diameter trifield model may be the most common commercial tube used in general-purpose image intensification fluoroscopic units. However, other sizes and magnification modes are available.

When the normal viewing mode (25 cm) is used, photoelectrons from the entire surface of a cesium iodide (CsI) input phosphor (i.e., when the x-ray photons passing through the patient first strike the image intensifier assembly) are accelerated to a zinc–cadmium sulfide output phosphor. However, when magnification in the fluoroscopic image is needed and the viewing mode is changed to the 17-cm mode (6.8 inches) or even less—for example, 4.8 inches or 12 cm in many new systems—the voltage on the electrostatic focusing lenses increases, thereby causing the focal point of the electrons to move to a greater distance away from the output phosphor. As a result, only electrons from the central 17-cm diameter portion of the input phosphor actually reach the output phosphor of the image intensifier. The change in the focal point of the electrons decreases the field of view, with a corresponding increase in magnification of the image (. The quality of the magnified image, as viewed on a television monitor, is somewhat degraded. This decrease in image clarity occurs because of a decrease in minification gain (i.e., increase in brightness resulting from minification of the image) caused by a lesser number of photoelectrons being available to strike the output phosphor on the image intensifier. Therefore the resultant image is dimmer. Because it is necessary and desirable to maintain a constant level of brightness on the television monitor, fluoroscopic mA increases automatically. However, this increase in tube mA increases the dose to the patient. Although the use of smaller-diameter modes results in increased patient dose, the overall quality of the image is improved when compared with the use of larger-diameter modes because in smaller-diameter modes a greater number of x-ray photons is needed to form the image. This image will have a more even appearance (less noise), and it will be possible to more easily distinguish between similar tissues, thereby improving contrast.

Intermittent, or Pulsed, Fluoroscopy

Effect on Patient Dose

Intermittent, or pulsed, fluoroscopy involves manual or automatic periodic activation of the fluoroscopic tube by the fluoroscopist rather than lengthy continuous activation. This practice significantly decreases patient dose, especially in long procedures, and helps extend the life of the tube. Many systems include a lastimagehold feature that allows the fluoroscopist to see the most recent image without exposing the patient to another pulse of radiation. This feature also reduces patient dose.

Technical Exposure Factors

Selection of Technical Exposure Factors for Adult Patients

The fluoroscopist must select technical exposure factors that will minimize patient dose during manual fluoroscopic procedures. Increases in peak kilovoltage and filtration reduce the patient radiation exposure rate. Most fluoroscopic examinations performed with image intensification systems employ a range of 75 to 110 kVp for adult patients, depending on the area of the body being examined. This peak kilovoltage range produces the correct level of fluoroscopic image brightness. Lower peak kilovoltage, when used for greater-diameter regions, increases patient dose because use of a lesser penetrating x-ray beam necessitates the use of a higher milliamperage (a larger quantity of x-ray photons in the beam) to obtain adequate image brightness. Besides using the correct kilovoltage, the operator can further limit excessive entrance exposure of the patient by ensuring that the x-ray source-to-skin distance (SSD) is not less than 15 inches (38 cm) for stationary (fixed) fluoroscopes and not less than 12 inches (30 cm) for mobile fluoroscopes. A 12-inch (30-cm) minimal distance is required, but a 15-inch (38-cm) minimal distance is preferred for all image intensification systems. On the other hand, the position of the input phosphor surface of the image intensifier should be maintained as close as is practical to the patient to also reduce the patient’s entrance exposure rate.

Selection of Technical Exposure Factors for Children

Technical exposure factors for fluoroscopic procedures for children necessitate a decrease in peak kilovoltage by as much as 25%. The peak kilovoltage chosen should depend on part thickness, just as it does in radiography. In addition to lowering technical exposure factors, maintaining SSD and minimizing the height of the image intensifier entrance surface above the patient, as described earlier, further limit excessive entrance exposure of the pediatric patient.

 

Filtration

Purpose and Requirements

The function of a filter in fluoroscopy, as in radiographic procedures, is to reduce the patient’s skin dose. Adequate layers of aluminum equivalent material placed in the path of the useful beam remove the more harmful lower energy photons from the beam by absorbing them. A minimum of 2.5 mm total aluminum equivalent filtration must be permanently installed in the path of the useful beam of the fluoroscopic unit. With image intensification systems, a total aluminum equivalent filtration of 3.0 mm or greater may be preferred. Patient dose decreases by one fourth during fluoroscopic procedures when aluminum filtration increases from 1 mm aluminum to 3 mm aluminum. Although this increase in filtration causes a slight loss of fluoroscopic image brightness, increasing peak kilovoltage somewhat may compensate.

Half-Value Layer

As in radiography, when filtration of the x-ray beam is questionable, the HVL of the beam must be measured. In standard image intensification fluoroscopy, an x-ray beam HVL of 3 to 4 mm aluminum is considered acceptable when peak kilovoltage ranges from 80 to 100.

 

Source-to-Skin Distance

Requirement

In accordance with National Council on Radiation Protection and Measurements (NCRP) regulations, the SSD must be no less than 15 inches (38 cm) for stationary (fixed) fluoroscopes and no less than 12 inches (30 cm) for mobile fluoroscopes. As discussed earlier, this standard ensures that the patient’s entrance surface is not excessively exposed. Maintaining an appropriate SSD reduces the radiographer’s exposure as well.

Cumulative Timing Device

A cumulative timer must be provided and used with each fluoroscopic unit. This resettable device times the x-ray beam-on time and sounds an audible alarm or temporarily interrupts the exposure after the fluoroscope has been activated for 5 minutes. It makes the radiologist aware of for how long the patient receives exposure for each fluoroscopic examination. When the fluoroscope is activated for shorter periods, the patient, radiologist, and radiographer receive less exposure. Total fluoroscopic beam-on time should be documented for every fluoroscopic procedure.

 

Exposure Rate Limitation

Current federal standards limit entrance skin exposure rates of general-purpose intensified fluoroscopic units to a maximum of 10 roentgens (R) per minute (10 × 2.58 × 10−4 coulomb [C]/kg per minute). Measured at tabletop with the image intensifier entrance surface at a prescribed 12 inches above, this standard has been imposed to give consideration to the cumulative small doses of radiation the patient receives over a lifetime. Fluoroscopic units equipped with high-level control (HLC) may permit a skin entrance exposure rate as great as 20 R/min (20 × 2.58 × 10−4 C/kg per minute). Because fluoroscopic procedures can result in the largest patient doses in diagnostic x-ray imaging, sometimes reaching the level of therapeutic doses, fluoroscopic exposure rates and fluoroscopic exposure times must be kept within established limits.

Primary Protective Barrier

A primary protective barrier of 2 mm lead equivalent is required for an image intensifier unit. The image intensifier assembly itself provides this barrier. The assembly must be joined with the x-ray tube, which is commonly located underneath the tabletop and interlocked so that the fluoroscopic x-ray tube cannot be activated when the image intensifier is in the parked position.

Fluoroscopic Exposure Control Switch

The fluoroscopic exposure control switch (e.g., the foot pedal) must be of the dead-man type (i.e., only continuous pressure applied by the operator [usually a radiologist] can keep the switch activated and the fluoroscopic tube emitting x-radiation). This means that the exposure automatically terminates if the person operating the switch becomes incapacitated (e.g., has a heart attack).

RADIATION SAFETY FEATURES OF MOBILE C-ARM FLUOROSCOPY EQUIPMENT, DEVICES AND ACCESSORIES

Mobile C-Arm Fluoroscopy

A mobile C-arm fluoroscopic unit is a portable x-ray unit that is C-shaped. It has an x-ray tube attached to one end of its arm and an image intensifier attached to the other end. C-arm fluoroscopes () are frequently used in the operating room for orthopedic procedures (e.g., pinning of a fractured hip). They are also used for cardiac imaging, interventional procedures, and other potentially lengthy tasks. The use of C-arm fluoroscopy in procedures such as these carries the potential for a relatively large patient radiation dose. C-arm fluoroscope operators, if standing close to the patient, could also receive a significant increase in occupational exposure during such cases (see 2 for discussion on C-arm operator protection). For this reason, equipment operators, including attending physicians, must have appropriate education and training to ensure that they will be able to follow guidelines for safe C-arm operation and also meet radiation safety protocols essential to patient and personnel safety.

Mobile fluoroscopic units are required to have a minimal source-to–end of collimator assembly distance of 12 inches (30 cm). Some type of spacer or collimator extension is usually installed to prevent any part of the patient from getting closer than 12 inches from the tube target. During C-arm fluoroscopic procedures, the patient–image intensifier distance should be as short as possible (). This reduces patient entrance dose. Also, for dose-reduction purposes it is preferable to position the C-arm so that the x-ray tube is under the patient. With the x-ray tube in this position, scatter radiation is less intense (When the x-ray tube is positioned over the patient, scatter radiation becomes more intense, and radiation exposure of personnel increases correspondingly.

 

RADIATION SAFETY FEATURES OF CINEFLUOROSCOPY EQUIPMENT, DEVICES, AND ACCESSORIES

Cinefluorography

Film Size

The techniques to reduce patient dose during fluoroscopy also apply to dose reduction during cinefluorography. In cinefluorography, or cine, a movie camera that uses either 16-mm or 35-mm film is used to record the image of the output phosphor of the image intensifier. Because patient exposure is greater when 16-mm film is used, 35-mm film is most frequently used in the United States.

High–Dose-Rate Procedures

Dose-reduction techniques are especially important in cine because cine procedures can result in the highest patient doses of all diagnostic procedures. The high dose resulting from cine is caused by a relatively high inherent dose rate and the length of the procedure, particularly in cardiology, which involves cardiac imaging procedures such as heart catheterization, and in neuroradiology. Therefore a percentage decrease in cine dose yields a greater actual dose reduction than the same percentage decrease in noncine procedures.

Filming Frame Rate

Cinematic, or cine, cameras have filming frame rates of 7.5, 15, 30, and 60 frames per second. Filming frame rate affects patient radiation dose. When the frame rate is higher, so is the radiation dose. Swallow function studies and cardiac imaging procedures require higher frame rates because they are dynamic function studies. When compared with patients undergoing procedures that use lower frame rates, patients undergoing more rapid dynamic function studies such as heart catheterization receive higher radiation doses.

Inference of Patient Dose from Tabletop Exposure Levels

Patient dose may be inferred from tabletop exposure levels. The amount of exposure varies with a number of operator-adjustable parameters. Typical cine tabletop exposure is approximately 25 mR per frame for 6- to 7-inch image intensification mode and 15 frames per second. This translates to 45 R/min if a frame rate of 30 frames per second is used. Therefore any limitation of beam-on time is significant as long as the efficacy of the procedure is not compromised. Patient exposure increases when a smaller viewing mode (6 inches, compared with 9 inches) or a lower-speed cine film is used. For example, switching from a 9-inch to a 6-inch field of view approximately doubles the tabletop exposure rate. Increasing the frame rate from 30 frames per second to 60 frames per second doubles the exposure rate as well. If both adjustments are made at the same time, the resulting exposure rate goes up by a factor of 4. Other characteristics such as the image intensifier input phosphor exposure level set by the vendor, grid factor, and SSD play a role in determining the typical dose levels for a system.

Dose-Reduction Techniques

The radiologist or cardiologist can reduce exposure during cine procedures by shortening the time of the cine run and using fluoroscopy, when possible, to locate the catheter. When fluoroscopy is used, intermittent pulsed exposures to verify location and movement of the catheter between exposures can limit total fluoroscopy time as well. Some optional equipment features such as the last-frame-hold feature, in which the most recent fluoroscopic image remains in view as a guide to the radiologist when the x-ray beam is off, also promote lower patient dose by decreasing the total fluoroscopic beam-on time.

Patient Dose Determined by Procedure

The typical dose delivered to the patient depends on the procedure. In selective coronary arteriography, most radiation exposure is from the cine. In other procedures, although the dose rate is lower, the dose from fluoroscopy may exceed the dose from cine if the total fluoroscopy time is longer. This is often the case in percutaneous transluminal angioplasty.

RADIATION SAFETY OF DIGITAL FLUOROSCOPIC EQUIPMENT, DEVICES, AND ACCESSORIES

Digital Fluoroscopy

Use of Pulsed Progressive Systems for Dose Reduction

Various methods are used to obtain digital images in some fluoroscopic equipment. The electrical signal from the video camera attached to the output phosphor may be digitized. Alternatively, the TV camera may be replaced by a digital device such as a CCD camera or other digital detector. However the digital image is acquired, the use of digital technology offers the possibility of some methods of dose reduction. One method makes use of the fact that a brief high-intensity pulse of radiation may create an entire image of the output phosphor. The lines composing the image are progressively scanned (i.e., the image on the camera, namely the TV lines, is scanned or painted in a natural sequence, left to right followed by right to left and so on from top to bottom) to provide the picture that appears on a monitor during a brief time period (one sixtieth of a second). The x-ray beam is turned off while the image is being scanned, thereby decreasing patient dose, and then pulsed back on for the next image. These systems are known as “pulsed progressive” systems and are commonly used to lower patient dose.

 

Use of Last-Image-Hold Feature for Dose Reduction

Another dose-reduction technique that is particularly effective in DF systems is “last image hold.” In last image hold, an image is stored from the last time that the foot switch was depressed. In a digital system, this image could be composed of several frames of information that have been added together to reduce the effect of quantum noise that would be particularly apparent in a single frame.

 

RADIATION SAFETY FOR HIGH-LEVEL-CONTROL INTERVENTIONAL PROCEDURES

High-Level-Control Interventional Procedures

Justification for Use of High-Level-Control Interventional Procedures

Interventional procedures are invasive procedures performed by a physician with the aid of fluoroscopic imaging. The interventional physician, usually a radiologist or cardiologist, inserts catheters into vessels or directly into patient tissues for the purpose of drainage, biopsy, or alteration of vascular occlusions or malformations. For these procedures, high-level-control fluoroscopy (HLCF) is often employed. HLCF is an operating mode for state-of-the-art fluoroscopic equipment in which exposure rates are substantially higher than those normally allowed in routine procedures. The higher exposure rate allows visualization of smaller and lower contrast objects that do not usually appear during standard fluoroscopy. HLCF, therefore, is used for interventional procedures in which visualization of fine catheters or not easily seen structures is crucial. An audible signal constantly reminds personnel that the HLC mode is engaged.

Public Health Advisory about the Dangers of Overexposure of Patients and Exposure Rate Limits

Some fluoroscopically guided therapeutic interventional procedures have the potential for substantial patient exposure. On September 30, 1974, the Food and Drug Administration (FDA) issued a public health advisory to alert health care workers to the dangers of overexposure of patients through the use of high-level fluoroscopy. The FDA x-ray equipment standards, issued in 1994, limited the tabletop exposure rate of fluoroscopic equipment for routine procedures to 10 R/min unless an HLC mode was present, in which case routine fluoroscopy was limited to 5 R/min when the system was not in HLC mode and unlimited when it was in HLC mode. The authors of the standards felt that the high-level capability was necessary for certain vital situations involving therapeutic interventional procedures in which the potential risks to the patient of increased radiation exposure would be subordinate to a successful medical outcome of an intervention. The HLC mode, although allowing unlimited exposure, required continuous, positive-pressure manual operation (e.g., continuously depressing a foot switch) and a continuous audible signal to remind personnel that the high-level fluoroscopic mode was in use. In this mode, patient exposure rates have been estimated to range from 20 to 120 R/min. When the rule was issued, total patient exposure was limited by the heat-loading capabilities of the x-ray tube. The thinking was that the tube would reach its heat limit before any detectable nonstochastic radiation injury could occur. By the early 1990s, advances in x-ray tube technology and the development of vascular interventional procedures that require long fluoroscopy times 4) had created a situation in which serious skin reactions had been reported in some patients. Radiogenic skin injuries such as erythema (diffuse reddening) or desquamation (sloughing off of skin cells) are deterministic effects in which the severity of the disorder increases with radiation dose. As the data in show, a half hour of total beam-on time at one location on a patient’s skin is sufficient to produce erythema. The effect does not appear for approximately 10 days. Because manifestation of skin injury is delayed, a radiologist would not usually be the first person to observe the onset of the symptoms. Therefore, patient monitoring, radiation dosimetry, and accurate record keeping are important for the future medical management of adverse reactions. The FDA has recommended that a notation be placed in the patient’s record if a skin dose in the range of 1 to 2 gray (Gy) (100 to 200 rad) is received. The location of the area of the patient’s skin that received the absorbed dose should also be noted using a diagram, annotated photograph, or narrative description. Within the past 10 years, however, alarmed state regulatory agencies have imposed a restriction on high-level radiation exposure rates, namely that with the image intensifier at a distance of 12 inches above the tabletop, the maximum continuous fluoroscopic entrance exposure rate permitted is 20 R/min.

BOX 10-4 Procedures Involving Extended Fluoroscopic   Time
Percutaneous   transluminal angioplastyRadiofrequency   cardiac catheter ablation

Vascular   embolization

Stent   and filter placement

Thrombolytic   and fibrinolytic procedures

Percutaneous   transhepatic cholangiography

Endoscopic   retrograde cholangiopancreatography

Transjugular   intrahepatic portosystemic shunt

Percutaneous   nephrostomy

Biliary   drainage

Urinary   or biliary stone removal

BOX 10-5 Strategies to Manage Radiation Dose to   Patients, Operators, and Staff during Interventional Fluoroscopy
Immediate Long-Term
Optimize Dose to Patient
Use proper radiologic technique:•        Maximize distance between x-ray tube   and patient

•        Minimize distance between patient and   image receptor

•        Limit use of electronic magnification

Control fluoroscopic time:

•        Limit use to necessary evaluation of   moving structures

•        Employ last-image-hold function to   review findings

Control images:

•        Limit acquisition to essential   diagnostic and documentation purposes

Reduce dose:

•        Reduce field size (collimate) and   minimize field overlap

•        Use pulsed fluoroscopy and low frame   rate

Include medical physicist in decisions:•        Machine selection and maintenance

Incorporate   dose-reduction technologies and dose-measurement devices in equipment

Establish   a facility quality improvement program that includes an appropriate x-ray   equipment quality assurance program, overseen by a medical physicist, which   includes equipment evaluation/ inspection at appropriate intervals

Minimize Dose to Operators and Staff
Keep   hands out of the beamUse   movable shields

Maintain   awareness of body position relative to the x-ray beam:

•        Horizontal x-ray beam—operator and   staff should stand on the side of the image receptor

•        Vertical x-ray beam—the image receptor   should be above the table

Wear   adequate protection:

•        Protective well-fitted lead apron

•        Leaded glasses

Improve   ergonomics of operations and staff:•        Train operators and staff in   ergonomically good positioning for use of fluoroscopy equipment; periodically   assess their practice

•        Identify and provide the ergonomically   best personal protective gear for operators and staff

•        Urge manufacturers to develop   ergonomically improved personal protective gear

•        Recommend research to improve   ergonomics for personal protective gear

 

SUMMARY

•        A diagnostic-type protective tube housing protects the patient and imaging personnel from off-focus, or leakage, radiation by restricting the emission of x-rays to the area of the useful, or primary, beam.

•        Leakage radiation from the tube housing measured at 1 m (3.3 feet) from the x-ray source must not exceed 100 mR/hr (2.58 × 10−5 C/kg per hour) when the tube is operated at its highest voltage at the highest current that allows continuous operation.

•        The control panel must be located behind a suitable protective barrier that has a radiation-absorbent window that permits observation of the patient during any procedure.

•        This panel must “indicate the conditions of exposure and must positively indicate when the x-ray tube is energized.”1

•        The radiographic examination tabletop must be of the same thickness and be as radiolucent as possible so that it will absorb only a minimal amount of radiation, thereby reducing the patient’s radiation dose.

•        The tabletop is frequently made of a carbon fiber material.

•        Radiographic equipment must have a source-to–image receptor distance (SID) indicator.

•        X-ray beam limitation devices must be used to confine the useful beam before it enters the anatomic area of clinical interest.

•        The light-localizing variable-aperture rectangular collimator, aperture diaphragms, cones, and extension cylinders are the beam limitation devices used.

•        The patient’s skin surface should always be at least 15 cm below the collimator to minimize exposure to the epidermis.

•        Good coincidence between the x-ray beam and the light-localizing beam of the collimator is necessary; both alignment and width dimensions of the two beams must correspond to within 2% of the SID.

•        According to most state regulatory standards presently in effect, 2% of the SID is required with positive beam limitation (PBL) devices. Some states may require 3% of the SID with PBL devices.

•        Exposure to the patient’s skin may be reduced through proper filtration of the radiographic beam.

•        Inherent filtration amounting to 0.5 mm aluminum equivalent is required.

•        Together, the inherent and added filtration comprise the total filtration. Stationary x-ray units operating at above 70 kVp are required to have a total filtration of 2.5 mm aluminum equivalent.

•        The high-value layer (HVL) of the beam is measured to determine whether an x-ray beam is adequately filtered.

•        Compensating filters are used in radiography to provide uniform imaging of body parts when considerable variation in thickness or tissue composition exists.

•        Diagnostic x-ray units must have exposure reproducibility, or the ability to duplicate certain radiographic exposures for any given combination of kVp, mA, and time.

•        Exposure linearity is essential. When a change is made from one mA to a neighboring mA station, the most linearity can vary is 10%.

•        When screen-film image receptors are used, intensifying screens used in conjunction with matching radiographic film are predominantly rare-earth screens. Carbon fiber is frequently used as a front material in a radiographic cassette.

•        Radiographic grids increase patient dose in radiography. Their use for examination of thicker body parts is a fair compromise because they remove scattered radiation emanating from the patient that would otherwise degrade the recorded image.

•        Because of increased sensitivity of photostimulable phosphor to scatter radiation before and after exposure to a radiographic beam, a grid may be used more frequently during computed radiography (CR) imaging. The use of a grid does increase patient dose but significantly improves radiographic contrast and visibility of detail.

•        To limit the effects of inverse square falloff of radiation intensity with distance during a mobile radiographic examination, an x-ray source–to–skin distance (SSD) of at least 12 inches (30 cm) must be used.

•        With digital radiography, the latent image formed by x-ray photons on a radiation detector is actually an electronic latent image. It is called a digital image because it is produced by computer representation of anatomic information. The image receptor is divided into small detector elements that make up the two-dimensional picture elements, or pixels, of the digital image. The pixels collectively produce a 2 dimensional display of the information contained in a particular x-ray projection. Radiographers must select correct technical exposure factors the first time to avoid overexposing patients when digital images are obtained.

•        Computed radiography results when the invisible, or latent, image generated in conventional radiography is produced in a digital format using computer technology.

•        The digital image can be displayed on a monitor for viewing, and it can be printed on a laser film when hard copy is needed.

•        Fluoroscopic procedures produce the greatest patient radiation exposure rate in diagnostic radiology.

•        Minimize patient exposure time whenever possible.

•        Limit the size of the fluoroscopic field to include only the area of anatomy that is of clinical interest.

•        Employ the practice of intermittent, or pulsed, fluoroscopy to reduce the overall length of exposure.

•        Select the correct technical exposure factors to help minimize the amount of radiation received by a patient.

•        Ensure that the SSD is no less than 15 inches (38 cm) for stationary (fixed) fluoroscopes and no less than 12 inches (30 cm) for mobile fluoroscopes.

•        During C-arm fluoroscopic procedures, the patient-image intensifier distance should be as short as possible.

•        Cinefluorography can result in the highest patient doses of all diagnostic procedures.

•        Reduce patient dose by using intermittent activation of the fluoroscope to locate the catheter, limiting the time of the cine run, and using the last-image-hold feature to view the most recent image.

•        During digital fluoroscopy the use of pulsed progressive systems lowers patient dose.

•        Use of the last-image-hold feature is another dose-reduction technique.

•        High-level-control fluoroscopy (HLCF) is used for interventional procedures.

•        The operating mode uses exposure rates that are substantially higher than those allowed for routine fluoroscopic procedures.

•        If skin dose is received in the range of 1 to 2 Gy (100 to 200 rad), the U.S. Food and Drug Administration (FDA) requires that a notation be placed in the patient’s record.

•        The radiographer generally has the responsibility for monitoring and documenting procedural fluoroscopic time when fluoroscopic equipment is used by nonradiologist physicians.

GENERAL DISCUSSION QUESTIONS

1.      What are the x-ray tube housing construction requirements when a tube is operated at its highest voltage at the highest current that allows continuous operation?

2.      What must the control panel that is electrically connected to the x-ray equipment indicate?

3.      How do light-localizing variable-aperture rectangular collimators, aperture diaphragms, and cones and cylinders reduce the amount of scattered radiation being produced during a radiographic examination?

4.      How does filtration of the radiographic beam reduce exposure to the patient’s skin and superficial tissues?

5.      When should the half-value layer of a diagnostic x-ray tube be measured?

6.      What filters are recommended for use with a molybdenum anode when a mammographic examination is performed on a patient with larger or dense breasts?

7.      What is exposure linearity?

8.      Why does the use of carbon fiber in a radiographic film-cassette lower patient dose?

9.      Why is the use of a radiographic grid a fair compromise, if the use of it increases patient dose?

10.    What can a radiographer do to avoid overexposing the patient when a computed radiographic system is used?

11.    What effect does the use of intermittent, or pulsed, fluoroscopy have on patient dose?

12.    Why is there concern over the use of mobile C-arm fluoroscopes during surgical, vascular, interventional, and other potentially lengthy procedures?

13.    What dose-reduction techniques can radiologists or cardiologists implement to reduce exposure during cinefluorographic procedures?

14.    What strategies can physicians use during interventional fluoroscopic procedures to control patient radiation dose and minimize exposure of occupationally exposed personnel and any other assisting personnel?

15.    During digital fluoroscopy, how does the use of a pulsed progressive system lower patient dose?

REVIEW QUESTIONS

1.      The radiographic beam should be collimated so that it is which of the following?

A.      Slightly larger than the image receptor

B.      No larger than the image receptor

C.      Twice as large as the image receptor

D.      Four times as large as the image receptor

2.      Both alignment and length and width dimensions of the radiographic and light beams must correspond to within:

A.      1% of the SID

B.      2% of the SID

C.      5% of the SID

D.      10% of the SID

3.      What is the function of a filter in diagnostic radiology?

A.      To permit only alpha rays to reach the patient’s skin

B.      To permit only beta particles to interact with the atoms of the patient’s body

C.      To decrease the x-radiation dose to the patient’s skin and superficial tissue

D.      To remove gamma radiation from the useful beam

4.      HVL may be defined as the thickness of a designated absorber required to do which of the following?

A.      Increase the intensity of the primary beam by 50% of its initial value

B.      Increase the intensity of the primary beam by 25% of its initial value

C.      Decrease the intensity of the primary beam by 50% of its initial value

D.      Decrease the intensity of the primary beam by 25% of its initial value

5.      When compared with conventional screen-film systems, the photostimulable phosphor in the computed radiography imaging plate is much more sensitive to scatter radiation before and after it is sensitized through exposure to a radiographic beam. Because of this increased sensitivity, which of the following is true?

1.      5 mm of added aluminum equivalent filtration must always be used during routine CR imaging

2.      A radiographic grid may be used more frequently during CR imaging

3.      Any source-to–image receptor distance can be used during CR imaging without adjustment in technical exposure factors

A.      1 only

B.      2 only

C.      3 only

D.      1, 2, and 3

6.      To minimize skin exposure to electrons produced by photon interaction with the collimator, how far below the collimator should the patient’s skin surface be?

A.      At least 1 cm below

B.      At least 5 cm below

C.      At least 10 cm below

D.      At least 15 cm below

7.      Which of the following aluminum equivalents for total permanent filtration meets the minimum requirement for mobile diagnostic and fluoroscopic equipment?

A.      0.5 mm aluminum equivalent

B.      1.0 mm aluminum equivalent

C.      2.0 mm aluminum equivalent

D.      2.5 mm aluminum equivalent

8.      The trough, or bilateral wedge, filter, which is used in some dedicated chest radiographic units, is an example of which of the following?

A.      Compensating filter

B.      Filter used in all digital imaging systems

C.      Filter used in all dedicated mammographic units

D.      Filter used in all computed tomography systems

9.      To decrease patient exposure during fluoroscopic procedures, the fluoroscopist can:

1.      Limit the size of the fluoroscopic field to include only the area of anatomy that is of clinical interest

2.      Employ the practice of intermittent, or pulsed, fluoroscopy to reduce the overall length of exposure

3.      Choose to use a conventional fluoroscope instead of an image intensification fluoroscope

A.      1 and 2 only

B.      1 and 3 only

C.      2 and 3 only

D.      1, 2, and 3

10.    A diagnostic-type protective tube housing must be constructed so that leakage radiation measured at a distance of 1 m from the x-ray source does not exceed ______ when the tube is operated at its highest voltage at the highest current that allows continuous operation.

A.      500 mR/hr (2.58 × 10−5 C/kg per hour)

B.      300 mR/hr (2.58 × 10−5 C/kg per hour)

C.      100 mR/hr (2.58 × 10−5 C/kg per hour)

D.      50 mR/hr (2.58 × 10−5 C/kg per hour)

Answer 10

1.      B

2.      B

3.      C

4.      C

5.      B

6.      D

7.      D

8.      A

9.      A

10.C

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