Film Imaging Study Guide

Film Imaging OUTLINE X-Ray Film Composition Intraoral X-Ray Film Screen Film Intensifying Screens Function Composition Formation of the Latent Image Processing Solutions Developing Solution Developer Replenisher Rinsing Fixing Solution Washing Darkroom and Equipment Establishing Correct Exposure Times Management of Radiographic Wastes Image Characteristics Darkroom Safelighting Manual Processing Tanks Thermometer Timer Drying Racks Manual Processing Procedures Rapid-Processing Chemicals Changing Solutions Automatic Film Processing Mechanism Operation A beam of x-ray photons that passes through the dental arches is reduced in intensity (attenuated) by absorption and scattering of photons out of the primary beam. The pattern of the photons that exits the patient, the remnant beam, conveys information about the patient’s anatomy. For this information to be useful diagnostically, the remnant beam must be recorded on an image receptor. The image receptor most often used in dental radiography is x-ray film. This chapter describes x-ray film and film processing and the use of intensifying screens. Digital radiographic systems, which also may be used to make radiographs, are described in Chapter 4. X-RAY FILM COMPOSITION X-ray film has two principal components: (1) emulsion and (2) base. The emulsion, which is sensitive to x rays and visible light, records the radiographic image. The base is a plastic supporting material onto which the emulsion is coated (Fig. 5-1). Emulsion The two principal components of emulsion are silver halide grains, which are sensitive to x radiation and visible light, and a vehicle matrix in which the crystals are suspended. The silver halide grains are composed primarily of crystals of silver bromide. The composition of a dental film emulsion is shown in Table 5-1. The silver halide grains in INSIGHT film and Ultra-speed film (Carestream Dental, a division of Carestream Health, Inc.) are flat, tabular crystals with a mean diameter of about 1.8 µm (Fig. 5-2). The Radiographic Density Radiographic Contrast Radiographic Speed Film Latitude Radiographic Noise Radiographic Sharpness and Resolution Image Quality Common Causes of Faulty Radiographs Mounting Radiographs Duplicating Radiographs tabular grains are oriented parallel with the film surface to offer a large cross-sectional area to the x-ray beam (Fig. 5-3). INSIGHT film has about twice the number of silver grains so that it requires only half the exposure of Ultra-speed film. The silver halide grains are suspended in a surrounding vehicle that is applied to both sides of the supporting base. During film processing (described later in this chapter) the vehicle absorbs processing solutions, allowing the chemicals to reach and react with the silver halide grains. An additional layer of vehicle is added to the film emulsion as an overcoat. This barrier helps protect the film from damage by scratching, contamination, or pressure from rollers when an automatic processor is used. Film emulsions are sensitive to both x-ray photons and visible light. Film intended to be exposed by x rays is called direct exposure film. All intraoral dental film is direct exposure film. Screen film is used with intensifying screens (described later in this chapter) that emit visible light. Screen film and intensifying screens are used for extraoral projections, such as panoramic and cephalometric radiographs. Base The function of the film base is to support the emulsion. The base for dental x-ray film is made of polyester polyethylene terephthalate, which provides the proper degree of flexibility to allow easy handling of the film. The film base must also withstand exposure to processing solutions without becoming distorted. The base is uniformly translucent and casts no pattern on the resultant radiograph. Overcoat Emulsion 0.18 mm Base Emulsion Overcoat FIGURE 5-1 Scanning electron micrograph of INSIGHT dental x-ray film (original magnification 300×). Note the overcoat, emulsion, and base on this double-emulsion film. (Courtesy Carestream Dental, a division of Carestream Health, Inc.) TABLE 5-1 Typical Coating Weights per Film Side (mg/cm2) Silver Bromide Emulsion Vehicle Overcoat Vehicle InSight (E/F speed) 0.8–1.1 0.6–0.75 0.6–0.8 0.1–0.2 Ultra-Speed (D speed) 0.4–0.55 0.6–0.75 0.4–0.7 0.1–0.2 Film Type Data from Carestream Health, Inc., exclusive manufacturer of Kodak dental systems. FIGURE 5-3 Cross-sectional electron microscope image of emulsion of INSIGHT film. The orientation of the tabular crystals in the emulsion is essentially parallel to the film surface to increase the exposure surface of the crystals to the incident x-ray beam. (Courtesy Carestream Dental, a division of Carestream Health, Inc.) A B FIGURE 5-4 A, The raised film dot (arrow) indicates the tube side of the film and identifies the patient’s right and left sides. B, The location of this dot is clearly marked with a black circle on the outside of every film packet. (Courtesy of Carestream Dental, a division of Carestream Health, Inc.) FIGURE 5-2 Scanning electron micrograph of emulsion of INSIGHT film showing flat tabular silver bromide crystals, which capture incident photons. (Courtesy Carestream Dental, a division of Carestream Health, Inc.) INTRAORAL X-RAY FILM Intraoral dental x-ray film is made as a double-emulsion film—that is, both sides of the base are coated with an emulsion. With a double layer of emulsion, less radiation is required to produce an image. Direct exposure film is used for intraoral examinations because it provides higher resolution images than screen-film combinations. Some diagnostic tasks, such as detection of incipient caries or early periapical disease, require this higher resolution. One corner of each dental film has a small, raised dot that is used for film orientation (Fig. 5-4). The manufacturer orients the film in the packet so that the convex side of the dot is toward the front of the packet and faces the x-ray tube. The side of the film with the depression is thus oriented toward the patient’s tongue. After the film has been exposed and processed, the dot is used to orient the patient’s right and left side images properly. When mounting radiographs, each film is oriented with the convex side of the dot toward the viewer, and on the basis of the features of the teeth and anatomic landmarks in the adjacent bone, the films are arranged in their normal sequential relationship in the mount. Intraoral x-ray film packets contain either one or two sheets of film (Fig. 5-5). When double-film packs are used, the second film serves as a duplicate record that can be sent to insurance companies or to a colleague. The film is encased in a protective black paper wrapper and then in an outer white paper or plastic wrapping, which is resistant to moisture. The outer wrapping clearly Inner paper Dental film Inner paper wrap Lead foil backing Outer package FIGURE 5-5 Moisture-proof and lightproof packets, paper on the left and vinyl on right, contain an opening tab on the side opposite the tube. Inside is an interleaf paper wrapper that is folded around the film as well as a sheet of lead foil. Film is packaged with one or two sheets of film. The foil is positioned between the back side of the packet and the paper wrapper. In this position, it absorbs radiation that has passed through the film and prevents scatter radiation from blurring the image. If the film packet is inadvertently placed backward in the patient’s mouth, the mottled image of the foil shows on the resultant image. (Courtesy of Carestream Dental, a division of Carestream Health, Inc.) FIGURE 5-6 Placing a film backward in the patient’s mouth when the exposure was made results in a radiograph that is too light and shows the characteristic markings caused by exposure through the lead foil in the film packaging. In such an image, the left and right sides of the patient are reversed when using the dot as the orientation guide. Size 2 Occlusal indicates the location of the raised dot and identifies which side of the film should be directed toward the x-ray tube. A thin lead foil backing with an embossed pattern is between the wrappers in the film packet. The foil is positioned in the film packet behind the film, away from the tube. This lead foil serves several purposes. It shields the film from backscatter (secondary) radiation, which fogs the film and reduces subject contrast (image quality). It also reduces patient exposure by absorbing some of the residual x-ray beam. Most importantly, if the film packet is placed backward in the patient’s mouth so that the tube side of the film is facing away from the x-ray machine, the lead foil will be positioned between the subject and the film. In this circumstance, most of the radiation is absorbed by the lead foil, and the resulting radiograph is light and shows the embossed pattern in the lead foil (Fig. 5-6). This combination of a light film with the characteristic pattern indicates that the film packet was exposed backward in the patient’s mouth and that the patient’s right side–left side designation indicated by the film dot is reversed. Periapical View Periapical views are used to record the crowns, roots, and surrounding bone. Film packs come in three sizes: (1) size 0 for small children (22 mm × 35 mm); (2) size 1, which is relatively narrow and used for views of the anterior teeth (24 mm × 40 mm); and (3) size 2, the standard film size used for adults (30.5 mm × 40.5 mm) (Fig. 5-7). Bitewing View Bitewing (interproximal) views are used to record the coronal portions of the maxillary and mandibular teeth in one image. They are useful for detecting interproximal caries and evaluating the height of alveolar bone. Size 2 film is normally used in adults; the smaller size 1 is preferred in children. In small children, size 0 may be used. A relatively long size 3 is also available. Size 1 Size 0 FIGURE 5-7 Dental x-ray film is commonly supplied in various sizes. Left, Occlusal film. Top right, Size 2 for adult posterior film. Middle right, Size 1 for adult anterior film. Bottom right, Size 0 for child-size film (in vinyl wrapping). Bitewing films often have a paper tab projecting from the middle of the film on which the patient bites to support the film (Fig. 5-8). This tab is rarely visualized and does not interfere with the diagnostic quality of the image. Film-holding instruments for bitewing projections also are available. Occlusal View Occlusal film, size 4, is more than three times larger than size 2 film (see Fig. 5-7). It is used to show larger areas of the maxilla or mandible than may be seen on a periapical film. These films also are used to obtain right-angle views to the usual periapical view. The name derives from the fact that the film is held in position by having the patient bite lightly on it to support it between the occlusal surfaces of the teeth (see Chapter 7). SCREEN FILM The extraoral projections used most frequently in dentistry are panoramic and cephalometric views. For these projections, screen FIGURE 5-8 Paper loop placed around a size 2 adult film to support the film when the patient bites on the tab for a bitewing projection. This projection reveals the tooth crowns and alveolar crests. A B FIGURE 5-10 Tablet grains of silver halide in an emulsion of T-MAT film (A) are larger and flatter than the smaller, thicker crystals in an emulsion of older conventional film (B). The flat surfaces of the tablet grains are oriented parallel with the film surface, facing the radiation source. (Courtesy of Carestream Dental, a division of Carestream Health, Inc.) silver halide crystals are coated with sensitizing dyes to increase absorption. It is important to use the appropriate screen-film combination recommended by the screen and film manufacturer so that the emission characteristics of the screen match the absorption characteristics of the film. Contemporary screen films use tabular-shaped (flat) grains of silver halide (Fig. 5-10) to capture the image. The tabular grains are oriented with their relatively large, flat surfaces facing the radiation source, providing a larger cross section (target) and resulting in increased speed without loss of sharpness. To increase the sharpness of images, some manufacturers add an absorbing dye in the film emulsion. This dye reduces light from one screen crossing through the film to reach the emulsion on the opposite side. EVG film from Carestream Dental is an example of this type of film. INTENSIFYING SCREENS FIGURE 5-9 Cassette for 8 inch × 10 inch film along with a sheet of screen film. When the cassette is closed, the film is supported in close contact between the two white intensifying screens seen on the inside of the cassette. These intensifying screens absorb most of the incident x-ray beam and then fluoresce and expose the film. film is used with intensifying screens (described later in this chapter) to reduce patient exposure (Fig. 5-9). Screen film is different from dental intraoral film. It is designed to be sensitive to visible light because it is placed between two intensifying screens when an exposure is made. The intensifying screens absorb x rays and emit visible light, which exposes the film. Silver halide crystals are inherently sensitive to ultraviolet (UV) and blue light (300 to 500 nm) and thus are sensitive to screens that emit UV and blue light. When film is used with screens that emit green light, the Early in the history of radiography, scientists discovered that various inorganic salts or phosphors fluoresce (emit visible light) when exposed to an x-ray beam. The intensity of this fluorescence is proportional to the x-ray energy absorbed. These phosphors are incorporated into intensifying screens for use with screen film. The sum of the effects of the x rays and the visible light emitted by the screen phosphors exposes the film in an intensifying cassette (see Fig. 5-9).