What is an intensifying screen what is its importance in medical radiography?
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Chapter 12 Outline Introduction Radiographic Film Film Construction Latent Image Formation Film Characteristics Intensifying Screen Characteristics Automatic Film Processing Components Systems Quality Control Silver Recovery Digital Receptors Detector Types Image Acquisition, Extraction and Processing, and Display Acquisition Extraction and Processing Display Using Digital Receptors Quality Assurance and Quality Control Daily Monthly or Quarterly Picture Archiving and Communication Systems Summary Objectives • Explain how the latent image is formed. • Describe film characteristics, including speed, contrast, latitude, and spectral sensitivity. • Describe the purpose and function of intensifying screens. • Explain how screens can be characterized based on the type of phosphor, spectral emission, and screen speed. • Describe factors that affect screen speed. • State the automatic film processing stages and their function. • Discuss the purpose of replenishment, recirculation, and temperature control during automatic film processing. • Identify important quality control measures to ensure good radiographic quality. • State the importance of and methods for silver recovery. • Describe the design of cassette-based detectors. • Describe the design of cassetteless detectors. • Explain the process of image acquisition using cassette-based detectors. • Explain the process of image acquisition using the three general types of cassetteless detectors. • Explain the process of image extraction and processing for cassette-based and cassetteless systems. • Describe digital image display and postprocessing functions. • Explain the use of exposure indicators for cassette-based systems and dose-area product for cassetteless systems. • Correctly identify the role of kVp, mAs, and geometric factors with digital systems. • Identify quality control tests and test patterns used with digital systems. • Describe the Picture Archiving and Communication System, including its role, principal systems, and challenges. Key Terms automatic film processor cassette cassette-based systems cassetteless systems charge-coupled device detective quantum efficiency (DQE) detector array developing agents diffusion Digital Imaging and Communications in Medicine (DICOM) dose-area product (DAP) double-emulsion film dynamic range emulsion exposure indicators exposure latitude feed tray film-screen contact fixing agent fluorescence histogram analysis hydroquinone immersion heater intensifying screen latent image latent image centers luminescence manifest image modulation transfer function (MTF) phenidone phosphor layer photoconductor photodetector photostimulable luminescence photostimulable phosphor (PSP) plate Picture Archiving and Communication System (PACS) plate reader rare earth elements recirculation system reducing agents relative speed replenishment screen film screen speed silver halide silver recovery single-emulsion screen film spectral emission spectral matching spectral sensitivity speed standby control teleradiology thin-film transistor (TFT) values of interest (VOI) x-ray scintillator IntroductionThis chapter covers both film and digital media as image receptors. Radiography is changing. The industry is transitioning from film-screen as the primary image receptor to digital forms. Radiography is the last of the medical imaging modalities to make this transition. Although the transition to digital is almost complete in the United States, we still have facilities that use film-screen and this material is still covered in the American Society of Radiologic Technologists (ASRT) curriculum and on the American Registry of Radiologic Technologists (ARRT) radiography examination. Certainly, the age of film-screen will soon enter the annals of medical imaging history, but for now it remains a part of practice. As digital radiography establishes its place, new and experienced radiographers alike must learn a few new concepts and practices. But it is equally important that they learn what remains the same. Digital receptors bring many benefits to medical imaging, but they also bring challenges as to how best to use them in the best interest of the patient and the profession. Radiographic FilmFilm ConstructionRadiographic film acquires the image and must then be chemically processed before it is visible. As a result, film serves as the medium for image acquisition, processing, and display. Several types of radiographic film are still used in medical imaging departments. Depending on the specific application, film manufacturers produce film in a variety of sizes ranging from 20 × 25 cm (8 × 10 inches) to 35 × 43 cm (14 × 17 inches). The composition of film can be described in layers (Box 12-1). The most important layer for creating the image is the emulsion layer. The emulsion layer is the radiation-sensitive and light-sensitive layer of the film. The emulsion of film consists of silver halide crystals suspended in gelatin. Silver halide is the material that is sensitive to radiation and light. The emulsion layer is fairly fragile and must have a layer composed of a polyester base so that the film can be handled and processed, yet remain physically strong after processing. Most film used in radiographic procedures has a blue dye or tint added to the base layer to decrease eye strain when viewed on a view (illuminator) box. BOX 12-1 Composition of Radiographic Film  Screen film is the most widely used radiographic film. As its name implies, it is intended to be used with one or two intensifying screens. Screen film is more sensitive to light and less sensitive to x-rays. Screen film can have either a single- or double-emulsion coating (sometimes referred to as duplitized). Double-emulsion film has an emulsion coating on both sides of the base. Film-screen imaging typically uses double-emulsion film with two intensifying screens. Single-emulsion screen film, with only one emulsion layer, is used with a single intensifying screen. It has many uses, including duplication, subtraction, computed tomography (CT), magnetic resonance imaging (MRI), sonography, nuclear medicine, mammography, and laser printing. Latent Image FormationThe term latent image refers to that image that exists on film after that film has been exposed but before it has been chemically processed. Film processing changes the latent image into a manifest image. The term manifest image refers to the image that exists on film after exposure and processing. The manifest image typically is called the radiographic image. The specific way in which the latent image is formed is not really known, but the Gurney-Mott theory of latent image formation is most widely believed to best explain the manner in which this process happens. To explain latent image formation, it is necessary to describe what happens at the molecular level in the emulsion layer of film, specifically what happens to silver halide crystals when exposed to x-rays and light. Chapter 3 X-rays and gamma rays have characteristics of both waves and particles, but because of their high energy, they exhibit more particulate characteristics than those at the other end of the electromagnetic spectrum. One additional particulate characteristic that is unique to the highest two members of the electromagnetic spectrum (x-rays and gamma rays) is the ability to ionize matter. When a photon possesses sufficient energy, it can remove electrons from the orbit of atoms during interactions. This removal of an electron from an atom is called ionization. The atom and the electron that was removed from it are called an ion pair. Physical imperfections in the silver halide crystals are the site of the latent image formation and are described in detail in Box 12-2. BOX 12-2 The Gurney-Mott Theory of Latent Image Formation Silver halide is made up of both silver bromide and silver iodide. However, because silver bromide (AgBr) is the primary constituent of the silver halide in the emulsion layer of film, only silver bromide is discussed. The process by which the latent image is formed is precisely the same for silver iodide as it is for silver bromide. Silver (Ag) and bromine (Br) are bound together as a molecule in such a way that they share an electron (1). This electron is shared through ionic bonding because silver is a transitional atom, having only one electron in its outer shell, and it tends to either lose it or share it. The silver in AgBr is in effect an ion because it shares only its outer-shell electron with bromine. Energy in the form of x-rays or light is absorbed by the emulsion layers of radiographic film. This energy absorption raises the conductivity level of the electrons in the AgBr molecules, and these electrons move faster as a result. If enough energy is absorbed by a particular AgBr molecule, it becomes a positive ion of silver, neutral bromine, and a free electron (2, 3). Physical imperfections in the lattice or architecture of the AgBr crystals occur during the film manufacturing process. These imperfections are called sensitivity specks. Each sensitivity speck serves as an electron trap, trapping the electrons lost by the bromine when x-ray or light exposure occurs. Therefore these sensitivity specks become negatively charged (4). Because the sensitivity specks are negatively charged, the positive silver ions that are liberated from the AgBr molecules are attracted to them (5). Every silver ion that is attracted to an electron becomes neutralized by that electron, therefore becoming metallic silver (6). The more x-ray or light exposure in a particular area of the film, the more electrons and silver available to be attracted to the sensitivity specks. The bromine liberated by x-ray or light exposure is neutral and is simply absorbed into the gelatin of the emulsion. Several sensitivity specks with many silver ions attracted to them become latent image centers. These latent image centers appear as radiographic density on the manifest image after processing. It is believed that for a latent image center to appear, it must contain at least three sensitivity specks that have at least three silver atoms each. With more exposure to the film, more metallic silver is visualized as radiographic density. Sensitivity Specks and Latent Image Centers Sensitivity specks serve as the focal point for the development of latent image centers. After exposure, these specks trap the free electrons and then attract and neutralize the positive silver ions. After enough silver is neutralized, the specks become a latent image center and are converted to black metallic silver after chemical processing. Film CharacteristicsCurrent manufacturers of medical imaging film offer a wide variety of films. These differ not only in size and general type, but also in film speed, film contrast, exposure latitude, and spectral sensitivity. Film SpeedFilm speed is the degree to which the emulsion is sensitive to x-rays or light. The greater the speed of a film, the more sensitive it is. Because sensitivity increases, less exposure is necessary to produce a specific density. Two primary factors, both relating to the silver halide crystals found in the emulsion layers, affect the speed of radiographic film. The first factor is the number of silver halide crystals present, and the second factor is the size of these silver halide crystals. Radiographic film manufacturers manipulate film speed by manipulating both of these factors in the production of specific speeds of radiographic film. Silver Halide and Film Sensitivity As the number of silver halide crystals increases, film sensitivity or speed increases; as the size of the silver halide crystals increases, film sensitivity or speed increases. The faster the speed of a film, the less radiation exposure needed to produce a specific density. Film Contrast and Film LatitudeFilm contrast refers to the ability of radiographic film to provide a certain level of image contrast. High-contrast film accentuates more black and white areas, whereas low-contrast film primarily shows shades of gray. As discussed in Chapter 9, film latitude is closely related to film contrast. The latitude of film affects the range of radiation exposures that can provide diagnostic optical densities. Films manufactured to display higher contrast have a narrow exposure latitude compared with low-contrast films having a wider exposure latitude. Spectral SensitivitySpectral sensitivity refers to the color of light to which a particular film is most sensitive. In radiography, there are generally two categories of spectral sensitivity films: blue-sensitive and green-sensitive (orthochromatic). When radiographic film is used with intensifying screens, it is important to match the spectral sensitivity of the film with the spectral emission of the screens. Spectral emission refers to the color of light produced by a particular intensifying screen. In radiography, two categories of spectral emission generally exist: blue light–emitting screens and green light–emitting screens. It is critical to use blue-sensitive film with blue light–emitting screens and green-sensitive film with green light–emitting screens. Spectral matching refers to correctly matching the color sensitivity of the film to the color emission of the intensifying screen. An incorrect match of film and screens based on spectral emission and sensitivity results in radiographs that display inappropriate levels of radiographic density. Intensifying Screen CharacteristicsAn intensifying screen is a device found in radiographic cassettes that contains phosphors that convert x-ray energy into light, which then exposes the radiographic film. Its purpose is to intensify the action of the x-rays and thus permit much lower x-ray exposures compared with film alone. As with radiographic film, the construction of screens can be described in layers (Box 12-3). The phosphor layer, or active layer, is the most important screen component because it contains the phosphor material that absorbs the transmitted x-rays and converts them to visible light. The most common phosphor materials consist of chemical compounds of elements from the rare earth group of elements. Rare earth elements are those that range in atomic number from 57 to 71 on the periodic table of the elements; they are referred to as rare earth elements because they are relatively difficult and expensive to extract from the earth. BOX 12-3 Composition of Intensifying Screen Protective layer: Plastic protects the phosphor. Phosphor layer: Absorbs radiation and converts to light. Reflecting layer: Reflects light toward film. Absorbing layer: Absorbs light directed toward it. Base: Provides support and stability for phosphor layer. Intensifying screen systems used in cassettes generally include two screens. The screen that is mounted in the side of the cassette facing the x-ray tube is called the front screen, and the screen that is mounted in the opposite side is called the back screen. With two screens, the film (double-emulsion) is exposed to approximately twice as much light as a single-screen system because the film is exposed to light from both sides. Some screen systems use only a single screen and are used with single-emulsion film. When a single screen is used, it is mounted as a back screen on the side of the cassette that is opposite from the tube side. When loading a single-emulsion film into the appropriate cassette with a single screen, the emulsion side of the film must be placed against the intensifying screen. Film is much more sensitive to visible light than to x-rays. By converting each absorbed high-energy x-ray photon into thousands of visible light photons, intensifying screens amplify film optical density. Without screens, the total amount of energy to which the film is exposed consists of only x-rays. With screens, the total amount of energy to which the film is exposed is divided between x-rays and light. When intensifying screens are used, approximately 90% to 99% of the total energy to which the film is exposed is light. X-rays account for the remaining 1% to 10% of the energy. Intensifying screens operate by a process known as luminescence.Luminescence is the emission of light from the screen when stimulated by radiation. The desired type of luminescence in imaging is fluorescence.Fluorescence refers to the ability of phosphors to emit visible light only while exposed to x-rays. Screen SpeedThe purpose of intensifying screens is to decrease the radiation dose to the patient. Because screen phosphors can intensify the action of the x-rays by converting them to visible light, the use of screens allows the radiographer to use considerably lower mAs. The disadvantage of using screens is the reduction in recorded detail. Screen Speed and Recorded Detail As screen speed increases, recorded detail is decreased; as screen speed decreases, recorded detail increases. Screen manufacturers produce a variety of intensifying screens, which differ in how well they intensify the action of the x-rays and therefore differ in their capacity to produce accurate recorded detail. The capability of a screen to produce visible light is called screen speed. A faster screen produces more light than a slower screen given the same exposure. Although very fast screens reduce patient exposure, they also degrade image resolution and increase quantum noise, so a balance must be chosen. Screen Speed, Light Emission, and Patient Dose The faster an intensifying screen, the more light is emitted for the same intensity of x-ray exposure. As screen speed increases, less radiation is necessary and radiation dose to the patient decreases; as screen speed decreases, more radiation is necessary and radiation dose to the patient increases. Several factors affect how fast or slow an intensifying screen is, including absorption efficiency, conversion efficiency, thickness of the phosphor layer, and size of the phosphor crystal (Table 12-1). The presence of a reflecting layer, an absorbing layer, or dye in the phosphor layer also affects screen speed. TABLE 12-1 Factors Affecting Screen Speed, Recorded Detail, and Patient Dose
Phosphor layer thickness • Thicker • Thinner Increased Decreased Decreased Increased Decreased Increased Phosphor crystal size • Larger • Smaller Increased Decreased Decreased Increased Decreased Increased Reflecting layer Increased Decreased Decreased Absorbing layer Decreased Increased Increased Dye in phosphor layer Decreased Increased Increased Absorption efficiency refers to the screen’s ability to absorb the incident x-ray photons. A rare earth phosphor screen absorbs approximately 60% of the incident photons. Conversion efficiency describes how well the screen phosphor takes these x-ray photons and converts them to visible light. Increased absorption and conversion efficiency mean that rare earth phosphors have increased speed when compared with a previously used screen phosphor, calcium tungstate. This increased speed allows the radiographer to substantially reduce the x-ray exposure needed to produce images with the appropriate amount of density. The thickness of the phosphor layer and the size of the crystal also have an effect on screen speed. A thicker phosphor layer contains more phosphor material than a thinner phosphor layer. The phosphor is the material that converts x-rays into light, so if more phosphor material is present in a screen, more light will be produced, increasing the screen speed. The size of the phosphor material crystals also affects screen speed. Larger phosphor crystals produce more light than smaller phosphor crystals. Again, more light being produced means that the screen is faster. The final factors that affect screen speed are the presence or absence of a reflecting layer, a light-absorbing layer, or light-absorbing dyes in the phosphor layer. A reflecting layer is used to increase screen speed by reflecting light back toward the film (Figure 12-1). A light-absorbing layer or light-absorbing dyes present in the phosphor layer are used to decrease screen speed by absorbing light that would otherwise reach and expose the film. FIGURE 12-1 Reflecting Layer. The reflecting layer redirects the visible light emitted by the screen phosphor toward the film emulsion to increase screen speed. The ability of the screen to produce visible light can also be described in terms of its relative speed. Relative speed results from comparing screen-film systems based on the amount of light produced for a given exposure. Most radiology departments that use film-screen technology have at least two different speeds of intensifying screen systems. A fast system usually is available with a relative speed of about 400. A 400-speed system is a good compromise between the beneficial effect of decreasing the patient dose and the detrimental effect of decreasing the recorded detail. A slower system is usually available, and it is sometimes labeled on the outside of the cassette as detail or extremity. The relative speed of this system typically is 100. Detail or extremity screen systems are relatively slow, and therefore require greater exposure and result in higher patient doses. However, the anatomic parts imaged with detail or extremity screen systems generally are small; therefore they do not require large exposures. Detail or extremity screen systems produce excellent recorded detail. The radiographer must be careful in selecting the appropriate screen system for the examination ordered. Cassettes with extremity and detail screens should be used only for tabletop examinations. They should never be used in the Bucky tray because of the excessive amount of exposure needed. What is an intensifying screen in radiography?The intensifying screen converts the x-ray energy into light, which reaches the film and forms the latent image. In dentistry, this type of film is used in panoramic imaging and cephalometric radiography.
What is intensifying screen and how does it work?Intensifying screens intensify the effect of the X-ray beam energy on the film by energy conversion. Some X-ray energy is absorbed by the screen and re-emitted as u. v. and visible light energy (Figure 5.1), to which the film has a greater sensitivity, thus producing a greater film response.
What is the purpose of the intensifying screen quizlet?What is the purpose of an intensifying screen? To reduce patient dose by using light as a intermedary.
Why is it important to maintain screen film contact?Without good contact between the film and screens of the cassette, detail will be lost on the radiograph. Therefore, it is necessary to routinely perform a screen contact test on all of your cassettes.
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