Film Imaging PDF

CHAPTER Film Imaging...

CHAPTER Film Imaging 5 m OUTLINE o X-Ray Film Darkroom and Equipment Establishing Correct Exposure Times t.c Composition Darkroom Management of Radiographic Wastes Intraoral X-Ray Film Safelighting Image Characteristics Screen Film Manual Processing Tanks Radiographic Density Intensifying Screens Thermometer Radiographic Contrast po Function Timer Radiographic Speed Composition Drying Racks Film Latitude Formation of the Latent Image Manual Processing Procedures Radiographic Noise Processing Solutions Rapid-Processing Chemicals Radiographic Sharpness and Resolution gs Developing Solution Changing Solutions Image Quality Developer Replenisher Automatic Film Processing Common Causes of Faulty Radiographs Rinsing Mechanism Mounting Radiographs Fixing Solution Washing lo Operation Duplicating Radiographs y.b A beam of x-ray photons that passes through the dental arches tabular grains are oriented parallel with the film surface to offer a is reduced in intensity (attenuated) by absorption and scat- large cross-sectional area to the x-ray beam (Fig. 5-3). INSIGHT r tering of photons out of the primary beam. The pattern of film has about twice the number of silver grains so that it requires the photons that exits the patient, the remnant beam, conveys only half the exposure of Ultra-speed film. ra information about the patient’s anatomy. For this information to The silver halide grains are suspended in a surrounding vehicle be useful diagnostically, the remnant beam must be recorded on that is applied to both sides of the supporting base. During an image receptor. The image receptor most often used in dental film processing (described later in this chapter) the vehicle llib radiography is x-ray film. This chapter describes x-ray film and film absorbs processing solutions, allowing the chemicals to reach processing and the use of intensifying screens. Digital radiographic and react with the silver halide grains. An additional layer of systems, which also may be used to make radiographs, are described vehicle is added to the film emulsion as an overcoat. This in Chapter 4. barrier helps protect the film from damage by scratching, con- tamination, or pressure from rollers when an automatic processor a X-RAY FILM is used. Film emulsions are sensitive to both x-ray photons and visible nt COMPOSITION light. Film intended to be exposed by x rays is called direct expo- X-ray film has two principal components: (1) emulsion and (2) sure film. All intraoral dental film is direct exposure film. Screen base. The emulsion, which is sensitive to x rays and visible light, film is used with intensifying screens (described later in this records the radiographic image. The base is a plastic supporting chapter) that emit visible light. Screen film and intensifying screens de material onto which the emulsion is coated (Fig. 5-1). are used for extraoral projections, such as panoramic and cephalo- metric radiographs. Emulsion The two principal components of emulsion are silver halide grains, Base which are sensitive to x radiation and visible light, and a vehicle The function of the film base is to support the emulsion. The matrix in which the crystals are suspended. The silver halide grains base for dental x-ray film is made of polyester polyethylene tere- are composed primarily of crystals of silver bromide. The composi- phthalate, which provides the proper degree of flexibility to allow tion of a dental film emulsion is shown in Table 5-1. The silver easy handling of the film. The film base must also withstand halide grains in INSIGHT film and Ultra-speed film (Carestream exposure to processing solutions without becoming distorted. The Dental, a division of Carestream Health, Inc.) are flat, tabular base is uniformly translucent and casts no pattern on the resultant crystals with a mean diameter of about 1.8 µm (Fig. 5-2). The radiograph. 63 64 PART II Imaging Overcoat Emulsion 0.18 mm Base o m t.c Emulsion Overcoat FIGURE 5-1 Scanning electron micrograph of INSIGHT dental x-ray film (original magni- fication 300×). Note the overcoat, emulsion, and base on this double-emulsion film. (Courtesy po Carestream Dental, a division of Carestream Health, Inc.) 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.) gs TABLE 5-1 Typical Coating Weights per Film Side (mg/cm2) Emulsion Overcoat Film Type Silver Bromide Vehicle lo Vehicle InSight (E/F speed) 0.8–1.1 0.6–0.75 0.6–0.8 0.1–0.2 y.b Ultra-Speed (D speed) 0.4–0.55 0.6–0.75 0.4–0.7 0.1–0.2 A B Data from Carestream Health, Inc., exclusive manufacturer of Kodak dental systems. r ra FIGURE 5-4 A, The raised film dot (arrow) indicates the tube side of the film and identi- llib fies 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.) a One corner of each dental film has a small, raised dot that is used for film orientation (Fig. 5-4). The manufacturer orients the nt 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 FIGURE 5-2 Scanning electron micrograph of emulsion of INSIGHT film showing flat with the depression is thus oriented toward the patient’s tongue. tabular silver bromide crystals, which capture incident photons. (Courtesy Carestream Dental, a After the film has been exposed and processed, the dot is used to de division of Carestream Health, Inc.) 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 INTRAORAL X-RAY FILM the teeth and anatomic landmarks in the adjacent bone, the films Intraoral dental x-ray film is made as a double-emulsion film—that are arranged in their normal sequential relationship in the mount. is, both sides of the base are coated with an emulsion. With a Intraoral x-ray film packets contain either one or two sheets of double layer of emulsion, less radiation is required to produce an film (Fig. 5-5). When double-film packs are used, the second film image. Direct exposure film is used for intraoral examinations serves as a duplicate record that can be sent to insurance compa- because it provides higher resolution images than screen-film com- nies or to a colleague. The film is encased in a protective black binations. Some diagnostic tasks, such as detection of incipient paper wrapper and then in an outer white paper or plastic wrap- caries or early periapical disease, require this higher resolution. ping, which is resistant to moisture. The outer wrapping clearly  C H A P T E R 5 Film Imaging 65 Inner paper Dental film Inner paper wrap Lead foil backing Outer package o m t.c 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 FIGURE 5-5 Moisture-proof and lightproof packets, paper on the left and vinyl on right, through the lead foil in the film packaging. In such an image, the left and right sides of the po contain an opening tab on the side opposite the tube. Inside is an interleaf paper wrapper that patient are reversed when using the dot as the orientation guide. 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 gs mouth, the mottled image of the foil shows on the resultant image. (Courtesy of Carestream Dental, a division of Carestream Health, Inc.) Size 2 Occlusal lo indicates the location of the raised dot and identifies which side of the film should be directed toward the x-ray tube. y.b A thin lead foil backing with an embossed pattern is between Size 1 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 r quality). It also reduces patient exposure by absorbing some of the Size 0 residual x-ray beam. Most importantly, if the film packet is placed ra 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 posi- tioned between the subject and the film. In this circumstance, most FIGURE 5-7 Dental x-ray film is commonly supplied in various sizes. Left, Occlusal film. llib of the radiation is absorbed by the lead foil, and the resulting Top right, Size 2 for adult posterior film. Middle right, Size 1 for adult anterior film. Bottom radiograph is light and shows the embossed pattern in the lead foil right, Size 0 for child-size film (in vinyl wrapping). (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 designa- Bitewing films often have a paper tab projecting from the a tion indicated by the film dot is reversed. 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 nt Periapical View the diagnostic quality of the image. Film-holding instruments for Periapical views are used to record the crowns, roots, and surround- bitewing projections also are available. ing bone. Film packs come in three sizes: (1) size 0 for small children (22 mm × 35 mm); (2) size 1, which is relatively narrow Occlusal View de and used for views of the anterior teeth (24 mm × 40 mm); and Occlusal film, size 4, is more than three times larger than size 2 (3) size 2, the standard film size used for adults (30.5 mm × film (see Fig. 5-7). It is used to show larger areas of the maxilla or 40.5 mm) (Fig. 5-7). mandible than may be seen on a periapical film. These films also are used to obtain right-angle views to the usual periapical view. Bitewing View The name derives from the fact that the film is held in position Bitewing (interproximal) views are used to record the coronal por- by having the patient bite lightly on it to support it between the tions of the maxillary and mandibular teeth in one image. They occlusal surfaces of the teeth (see Chapter 7). are useful for detecting interproximal caries and evaluating the height of alveolar bone. Size 2 film is normally used in adults; the SCREEN FILM smaller size 1 is preferred in children. In small children, size 0 may The extraoral projections used most frequently in dentistry are be used. A relatively long size 3 is also available. panoramic and cephalometric views. For these projections, screen 66 PART II Imaging o m t.c po FIGURE 5-8 Paper loop placed around a size 2 adult film to support the film when the A B patient bites on the tab for a bitewing projection. This projection reveals the tooth crowns and alveolar crests. FIGURE 5-10 Tablet grains of silver halide in an emulsion of T-MAT film (A) are larger gs 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.) lo silver halide crystals are coated with sensitizing dyes to increase y.b absorption. It is important to use the appropriate screen-film com- bination 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 r silver halide (Fig. 5-10) to capture the image. The tabular grains are oriented with their relatively large, flat surfaces facing the radiation ra source, providing a larger cross section (target) and resulting in increased speed without loss of sharpness. To increase the sharp- ness of images, some manufacturers add an absorbing dye in the llib 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 a Early in the history of radiography, scientists discovered that nt FIGURE 5-9 Cassette for 8 inch × 10 inch film along with a sheet of screen film. When various inorganic salts or phosphors fluoresce (emit visible light) the cassette is closed, the film is supported in close contact between the two white intensifying when exposed to an x-ray beam. The intensity of this fluorescence screens seen on the inside of the cassette. These intensifying screens absorb most of the incident is proportional to the x-ray energy absorbed. These phosphors are incorporated into intensifying screens for use with screen film. The de x-ray beam and then fluoresce and expose the film. sum of the effects of the x rays and the visible light emitted by the screen phosphors exposes the film in an intensifying cassette film is used with intensifying screens (described later in this (see Fig. 5-9). chapter) to reduce patient exposure (Fig. 5-9). Screen film is differ- ent from dental intraoral film. It is designed to be sensitive to FUNCTION visible light because it is placed between two intensifying screens The presence of intensifying screens creates an image receptor when an exposure is made. The intensifying screens absorb x rays system that is 10 to 60 times more sensitive to x rays than the film and emit visible light, which exposes the film. Silver halide crystals alone. Consequently, use of intensifying screens substantially are inherently sensitive to ultraviolet (UV) and blue light (300 to reduces the dose of x radiation to the patient. Intensifying screens 500 nm) and thus are sensitive to screens that emit UV and blue are used with films for virtually all extraoral radiography, including light. When film is used with screens that emit green light, the panoramic, cephalometric, and skull projections. Generally, the  C H A P T E R 5 Film Imaging 67 resolving power of screens is related to their speed: the slower the earth screen and the spectral sensitivity of an appropriate film. speed of a screen, the greater its resolving power, and vice versa. Other intensifying screens have a major peak at 350 nm (UV) and Intensifying screens are not used intraorally with periapical or at 450 nm (blue). It is important to match green-emitting screens occlusal films because their use would reduce the resolution of the with green-sensitive films and blue-emitting screens with blue- resulting image below that necessary for diagnosis of much dental sensitive films. disease. Fast screens have large phosphor crystals and efficiently convert x-ray photons to visible light but produce images with lower resolu- COMPOSITION tion. As the size of the crystals or the thickness of the screen Intensifying screens are made of a base supporting material, a decreases, the speed of the screen also declines, but image sharp- m phosphor layer, and a protective polymeric coat (Fig. 5-11). In all ness increases. Fast screens also have a thicker phosphor layer and dental applications, intensifying screens are used in pairs, one on a reflective layer, but these properties also decrease sharpness. In each side of the film, and they are positioned inside a cassette (see deciding on the combination to use, the practitioner must consider Fig. 5-9). The purpose of a cassette is to hold each intensifying the resolution requirements of the task for which the image will o screen in contact with the x-ray film to maximize the sharpness of be used. Screen-film combinations are rated for speed, a measure the image. of the amount of radiation required for a proper exposure. For t.c dental extraoral diagnostic tasks, it is recommended to use screen- Base film combinations that have a speed of 400 or faster. The base material of most intensifying screens is some form of polyester plastic that is about 0.25 mm thick. The base provides Protective Coat po mechanical support for the other layers. In some intensifying A protective polymer coat (≤15 µm thick) is placed over the phos- screens, the base also is reflective; thus it reflects light emitted from phor layer to protect the phosphor and to provide a surface that the phosphor layer back toward the x-ray film. This reflective base can be cleaned. The intensifying screens should be routinely increases the light emission of the intensifying screen but also cleaned because any debris, spots, or scratches may cause light gs results in some image “unsharpness” because of the divergence of spots on the resultant radiograph. light rays reflected back to the film. Phosphor Layer The phosphor layer is composed of phosphorescent crystals sus- lo pended in a polymeric binder. When the crystals absorb x-ray TABLE 5-2 Rare Earth Elements Used photons, they fluoresce (see Fig. 5-11). The phosphor crystals often in Intensifying Screens y.b contain rare earth elements, most commonly lanthanum and gado- linium. Their fluorescence can be increased by the addition of Emission Phosphor small amounts of elements, such as thulium, niobium, or terbium. Common phosphor combinations used in intensifying screens are Green Gadolinium oxysulfide, terbium activated shown in Table 5-2. Rare earth screens convert each absorbed x-ray Blue and UV Yttrium tantalite, niobium activated r photon into about 4000 lower energy, visible light (green or blue) photons. These visible photons expose the film. ra Different phosphors fluoresce in different portions of the spec- trum. For example, light emission from Lanex rare earth intensify- ing screens ranges from 375 to 600 nm and peaks sharply at llib 545 nm (green). Figure 5-12 shows the spectral emission of a rare a Base nt Phosphor Coat Film de Coat Phosphor Base FIGURE 5-11 Image on the left shows a schematic of two intensifying screens enclosing a film (yellow). An intensifying screen is composed of a supporting base (purple), a layer FIGURE 5-12 Relative sensitivity of T-MAT film (orange line) and emission lines (shown containing the phosphors (light blue), and a protective coat (orange). The detailed view on the in their visual colors) of Carestream Dental LANEX and EV screens (gadolinium oxysulfide, right shows x-ray photons entering at the top, traveling through the base, and striking phosphors terbium activated). Intensifying screens emit light as a series of relatively narrow line emissions. in the base. The phosphors emit visible light, exposing the film. Some visible light photons may The maximal emission of the screen at 545 nm (green) corresponds well to a high-sensitivity reflect off the reflecting layer of the base. region of the film. (Data from Carestream Dental, a division of Carestream Health, Inc.) 68 PART II Imaging FORMATION OF THE LATENT IMAGE in a film after exposure constitutes the latent image. Processing the exposed film in developer and fixer converts the latent image into When a beam of photons exits an object and exposes an x-ray film the visible radiographic image. (either direct-exposure film or screen film exposed by light photons), it chemically changes the photosensitive silver halide crystals in the film emulsion. These chemically altered silver PROCESSING SOLUTIONS bromide crystals constitute the latent (invisible) image on the film. Film processing involves the following procedures: Before exposure, film emulsion consists of photosensitive crystals 1. Immerse exposed film in developer. containing primarily silver bromide (Fig. 5-13, A). These silver 2. Rinse developer off film in water bath. m halide crystals also contain a few free silver ions (interstitial silver 3. Immerse film in fixer. ions) and trace amounts of sulfur compounds bound to the surface 4. Wash film in water bath to remove fixer. of the crystals. Along with physical irregularities in the crystal 5. Dry film and mount for viewing. produced by iodide ions, sulfur compounds create sensitivity Following exposure, each grain of silver halide in film emulsion o sites, sites in the crystals that are sensitive to radiation. Each crystal (Fig. 5-14, A) contains neutral silver atoms at their latent image has many sensitivity sites. When the silver halide crystals are irradi- sites (Fig. 5-14, B). These latent image sites render the crystals sensi- t.c ated, x-ray photons release electrons from the bromide ions (Fig. tive to development and image formation. Developer converts 5-13, B). The free electrons move through the crystal until they silver bromide crystals with neutral silver atoms deposited at the reach a sensitivity site, where they become trapped and impart a latent image sites into black, solid silver metallic grains (Fig. 5-14, negative charge to the site. The negatively charged sensitivity site C). These solid silver grains block light from a viewbox. Fixer po attracts positively charged free interstitial silver ions (Fig. 5-13, C). removes unexposed, undeveloped silver bromide crystals (crystals When a silver ion reaches the negatively charged sensitivity site, it without latent image sites), leaving the film clear in unexposed is reduced and forms a neutral atom of metallic silver (Fig. 5-13, areas (Fig. 5-14, D). Thus the radiographic image is composed of D). The sites containing these neutral silver atoms are now called light (radiopaque) areas, where few photons reached the film, and gs latent image sites. This process occurs numerous times within a dark (radiolucent) areas of the film that were struck by many crystal. The overall distribution of crystals with latent image sites photons. lo Photon Sensitivity site Sensitivity site y.b Br Ag Br Br Ag Br e Ag Br Ag Br Ag Br Ag Br Ag+ Ag+ Br Ag Br Ag Br Br Ag Br Ag Br r Br Br Br Br ra Ag Br Ag Br Ag Ag Br Ag Br Ag Ag Ag Ag Ag Ag Ag A B llib Sensitivity site Latent image site Br Ag Br Br Ag Br Ag a Ag Br Ag Br Ag Br Ag Br Ag+ nt Br Ag Br Ag Br Br Ag Br Ag Br Br Br Br Br Ag Br Ag Br Ag Ag Br Ag Br Ag Ag Ag Ag Ag de Ag Ag C D FIGURE 5-13 A, A silver bromide crystal in the emulsion of an x-ray film contains mostly silver and bromide ions in a crystal lattice. There are also free interstitial silver ions and areas of trace chemicals that form sensitivity sites. B, Exposure of the crystal to photons in an x-ray beam results in the release of electrons, usually by interaction of the photon with a bromide ion. The recoil electrons have sufficient kinetic energy to move about in the crystal. When electrons reach a sensitivity site, they impart a negative charge to this region. C, Free interstitial silver ions (with a positive charge) are attracted to the negatively charged sensitivity site. D, When the silver ions reach the sensitivity site, they acquire an electron and become neutral silver atoms. These silver atoms now constitute a latent image site. The col- lection of latent image sites over the entire film constitutes the latent image. Developer causes the neutral silver atoms at the latent image sites to initiate the conversion of all the silver ions in the crystal into one large grain of metallic silver. The bromine dissolves in the developer.  C H A P T E R 5 Film Imaging 69 The development of unexposed crystals results in chemical fog on DEVELOPING SOLUTION the film. The interval between maximal density and fogging The developer reduces all silver ions in the exposed crystals of explains why a properly exposed film does not become overdevel- silver halide (crystals with a latent image) to metallic silver grains oped, although it may be in contact with the developer longer than (see Fig. 5-14). To produce a diagnostic image, this reduction the recommended interval. process must be restricted to crystals containing latent image sites; The developing solution contains four components, all dis- to accomplish this, the reducing agents used as developers are cata- solved in water: (1) developer, (2) activator, (3) preservative, and lyzed by the neutral silver atoms at the latent image sites (see Fig. (4) restrainer. 5-14, B). Individual crystals are developed completely or not at all Developer m during the recommended developing times (see Fig. 5-14, C). Varia- tions in density on the processed radiographs are the result of Developer converts exposed silver halide crystals into metallic different ratios of developed (exposed) and undeveloped (unex- silver grains. Two developing agents, usually phenidone and hydro- posed) crystals. Areas with many exposed crystals are darker quinone, are used in dental radiology. Phenidone serves as the first o because of their higher concentration of black metallic silver grains electron donor that converts silver ions to metallic silver at the after development. latent image site. This electron transfer generates the oxidized form t.c When an exposed film is developed, the developer initially has of phenidone. Hydroquinone provides an electron to reduce the no visible effect (Fig. 5-15). After this initial phase, the density oxidized phenidone back to its original active state so that it can increases, rapidly at first and then more slowly. Eventually, all the continue to reduce silver halide grains to metallic silver. Unex- exposed crystals develop (are converted to black metallic silver), posed crystals—crystals without latent images—are unaffected po and the developing agent starts to reduce the unexposed crystals. during the time required for reduction of the exposed crystals. Activator The developers are active only at pH values around 10. This pH gs is achieved with the addition of alkali compounds (activators) such A B as sodium or potassium hydroxide. Buffers are used to maintain this condition. The activators also cause the gelatin to swell so that the developing agents can diffuse more rapidly into the emulsion lo to reach silver bromide crystals. Preservative y.b The developing solution contains an antioxidant or preservative, C D usually sodium sulfite, which extends the useful life of the solu- tion. The preservative also combines with oxidized developer to produce a compound that subsequently stains images brown if not FIGURE 5-14 Emulsion changes during film processing. A, Before exposure, many silver washed out. r bromide crystals (gray) are present in the emulsion. B, After exposure, the exposed crystals Bromide-containing compounds are added to the developing solution to restrain development of unexposed silver halide crys- ra containing neutral silver atoms at latent image sites (orange dots within some crystals) constitute the latent image. C, The developer converts the exposed crystals containing neutral silver atoms tals. Restrainers act as antifog agents and increase contrast. at the latent image sites into solid grains of metallic silver (black). D, The fixer dissolves the DEVELOPER REPLENISHER unexposed, undeveloped silver bromide crystals, leaving only the solid silver grains that form the llib radiographic image. The developing solution of both manual and automatic developers should be replenished with fresh solution each morning to prolong the life of the used developer. The recommended amount to be added daily is 8 ounces of fresh developer (replenisher) per gallon of developing solution. This assumes the development of an a average of 30 periapical or 5 panoramic films per day. Some of the Chemical used solution may need to be removed to make room for the nt fog replenisher. Film density Full development RINSING After development, the film emulsion swells and becomes satu- de rated with developer. At this point, the films are rinsed in water for 30 seconds with continuous, gentle agitation before they are placed in the fixer. Rinsing dilutes the developer, slowing the development process. It also removes the alkali activator, prevent- ing neutralization of the acid fixer. This rinsing process is typical 5 10 for manual processing but is not used with most automatic Development time (minutes) processors. FIGURE 5-15 Relationship between film density and development time. The density of FIXING SOLUTION film increases quickly initially in the developer. After full development, the density continues to Fixing solution removes undeveloped silver halide crystals from increase slowly because of chemical fogging. the emulsion (see Fig. 5-14, D). If these crystals are not removed, 70 PART II Imaging the image on the resultant radiograph is dark and nondiagnostic Fixing solution also contains four components, all dissolved in (Fig. 5-16). Figure 5-17 is a photomicrograph of film emulsion water: (1) clearing agent, (2) acidifier, (3) preservative, and (4) showing the solid silver grains after fixer has removed the unex- hardener. posed silver bromide crystals. (Compare it with Fig. 5-2, which shows the unprocessed emulsion.) Fixer also hardens and shrinks Clearing Agent the film emulsion. As with developer, fixer should be replenished An aqueous solution of ammonium thiosulfate (“hypo”) dissolves daily at the rate of 8 ounces per gallon. the unexposed silver halide grains. It forms stable, water-soluble complexes with silver ions, which diffuse from the emulsion. Excessive fixation (hours) results in a gradual loss of film density m because the grains of silver slowly dissolve in the acetic acid of the fixing solution. Acidifier o The fixing solution contains an acetic acid buffer system (pH 4 to 4.5) to keep the fixer pH constant. The acidic pH is required to t.c promote good diffusion of thiosulfate into the emulsion and of silver thiosulfate complex out of the emulsion. The acid-fixing solution also inactivates any residual developing agents in the film emulsion, blocking continued development of any unexposed crys- po tals while the film is in the fixing tank. Preservative Ammonium sulfite is the preservative in the fixing solution, as it gs is in the developer. It prevents oxidation of the thiosulfate clearing agent, which is unstable in the acid environment of the fixing lo solution. Hardener A hardening agent, usually aluminum sulfate, complexes with the gelatin during fixing and prevents damage to the gelatin y.b during subsequent handling. The hardeners also reduce swelling of the emulsion during the final wash. This reduction of swelling lessens mechanical damage to the emulsion and shortens drying time. WASHING r After fixing, the processed film is washed in water to remove all ra FIGURE 5-16 Incomplete fixation results in images that are dark and discolored, making thiosulfate ions and silver thiosulfate complexes. Washing effi- them diagnostic. This film was also poorly positioned in the patient’s mouth, cutting off most ciency declines rapidly when the water temperature decreases to apices of the teeth. Staining may also be caused by using depleted developer or fixer or using less than 60° F. Any silver compound or thiosulfate that remains llib contaminated solutions. because of improper washing discolors and causes stains, which are most apparent in the radiopaque (light) areas. DARKROOM AND EQUIPMENT a A conventional darkroom with manual wet processing tanks should be convenient to the x-ray machines and dental opera- nt tories and should be at least 4 feet × 5 feet (1.2 m × 1.5 m) (Fig. 5-18). DARKROOM de One of the most important requirements is that the darkroom be lightproof. If it is not, stray light can cause film fogging and loss of contrast. To make the darkroom lightproof, a light- tight door or doorless maze (if space permits) is used. The door should have a lock to prevent accidental opening, which might allow an unexpected flood of light that can ruin opened films. The darkroom must also be well ventilated for the comfort of people working in the area and to exhaust moisture from FIGURE 5-17 Scanning electron micrograph of a processed emulsion of Ultra-speed dental drying films. Also, a comfortable room temperature helps main- x-ray film (500×). Note the white-appearing solid silver grains above the base. (Courtesy of tain optimal conditions for developing, fixing, and washing Carestream Dental, a division of Carestream Health, Inc.) solutions.  C H A P T E R 5 Film Imaging 71 light only at the red end of the spectrum (Fig. 5-20). Film handling SAFELIGHTING under a safelight should be limited to about 5 minutes because The processing room should have both white illumination and film emulsion shows some sensitivity to light from a safelight with safelighting. Safelighting is low-intensity illumination of relatively prolonged exposure. The older ML-2 filters (yellow light) are not long wavelength (red) that does not rapidly affect open film but appropriate for fast intraoral dental film or extraoral panoramic or permits one to see well enough to work in the area (Fig. 5-19). To cephalometric film. minimize the fogging effect of prolonged exposure, the safelight should have a frosted 15-watt bulb or a clear 7.5-watt bulb and MANUAL PROCESSING TANKS should be mounted at least 4 feet above the surface where opened It is wise for dental offices to have the capability to develop m films are handled. film by tank processing, if only as a backup for an automatic X-ray films are very sensitive to the blue-green region of the processor or digital imaging system. The tank must have hot spectrum and are less sensitive to red wavelengths. The red GBX-2 and cold running water and a means of maintaining the tem- filter is recommended as a safelight in darkrooms where either perature between 60° F and 75° F. A practical size for a dental o intraoral or extraoral films are handled because this filter transmits office is a master tank about 20 cm × 25 cm (8 inches × 10 t.c po gs FIGURE 5-18 Darkroom work area. Left, Film mounting area, timer, film racks, and safelight above. Middle, Developing lo and fixing tanks below the viewbox and stirring paddles. Right, Sink and drying racks with fan. (Courtesy C. L. Crabtree, DDS, Bureau of Radiological Health, Rockville, MD.) r y.b ra llib 15 watts a nt 4 feet FIGURE 5-19 A, A safelight may be mounted on the wall or ceiling in the darkroom de and should be at least 4 feet from the work surface. B, The safelight uses a GBX-2 filter and 15-watt bulb. A B 72 PART II Imaging inches) that can serve as a water jacket for two removable the tank. Thermometers may contain alcohol or metal, but they inserts that fit inside (Fig. 5-21). The insert tanks usually hold should not contain mercury because they could break and con- 3.8 L (1 gallon) of developer or fixer and are placed within taminate the processor or solutions. the outer, larger master tank. The outer tank holds the water for maintaining the temperature of the developer and fixer in TIMER the insert tanks and for washing films. The developer customar- The x-ray film must be exposed to the processing chemicals for ily is placed in the insert tank on the left side of the master specific intervals. An interval timer is indispensable for controlling tank, and the fixer is placed in the insert tank on the right. development and fixation times. All three tanks should be made of stainless steel, which does DRYING RACKS m not react with the processing solutions and is easy to clean. The master tank should have a cover to reduce oxidation of Two or three drying racks can be mounted on a convenient wall the processing solutions, protect the developing film from acci- for film hangers. Drip trays are placed underneath the racks to dental exposure to light, and minimize evaporation of the catch water that may run off the wet films. An electric fan can be o processing solutions. used to circulate the air and speed the drying of films, but it should not be pointed directly at the films. t.c THERMOMETER The temperature of the developing, fixing, and washing solutions should be closely controlled. A thermometer can be left in the MANUAL PROCESSING PROCEDURES water circulating through the master tank to monitor the tempera- Manual processing of film requires the following eight steps: po ture and ensure that the water temperature regulator is working 1. Replenish solutions. The first step in manual tank processing is to properly. The most desirable thermometers clip onto the side of replenish the developer and fixer. Check the solution levels to gs lo y.b FIGURE 5-20 Spectral sensitivities of EVG film (green line) and INSIGHT film (blue line) shown with the transmission characteristics of a GBX-2 filter (red line). The films are more sensitive in the blue-green portion of the spectrum (shorter than 600 nm), whereas the GBX-2 filter transmits primarily at the red end of the spectrum (longer than 600 nm). r ra a llib Unit cover nt de FIGURE 5-21 Processing tank. The developing and fixing tanks Insert tank are inserted into a bath of running water with an overflow drain. The for fixing water bath may be maintained at a stable and optimal temperature Insert tank for chemicals for film processing. developing chemicals Water bath/ rinsing tank Overflow pipe  C H A P T E R 5 Film Imaging 73 ensure that the developer and fixer cover the films on the top 7. Fix. Place the hanger and film in the fixer solution for 2 to 4 clips of the film hangers. minutes and agitate for 5 of every 30 seconds. Agitation elimi- 2. Stir solutions. Stir the developer and fixing solution to mix the nates bubbles and brings fresh fixer into contact with the emul- chemicals and equalize the temperature throughout the tanks. sion. When the films are removed, drain the excess fixer into To prevent cross-contamination, use a separate paddle for each the wash bath. solution. It is best to label one paddle for the developer and 8. Wash. After fixation of the films is complete, place the hanger the other for the fixer. in running water for at least 10 minutes to remove residual 3. Mount films on hangers. Using only safelight illumination in the processing solutions. After the films have been washed, remove darkroom, remove the exposed film from its lightproof packet surface moisture by gently shaking excess water from the films m or cassette. Hold the films by their edges only to avoid damage and hanger. to the film surface. Clip the bare film onto a film hanger, one 9. Dry. Dry the films in circulating, moderately warm air. After film to a clip (Fig. 5-22). Label the film racks with the patient’s drying, the films are ready to mount. name and the exposure date. o 4. Set timer. Check the temperature of the developer, and set the interval timer to the time indicated by the manufacturer for the RAPID-PROCESSING CHEMICALS t.c solution temperature, typically: Rapid-processing solutions typically develop films in 15 seconds and fix them in 15 seconds at room temperature. They have the Temperature (° F) Development Time (minutes) same general formulation as conventional processing solutions 68 5 but often contain a higher concentration of hydroquinone. They po 70 also have a more alkaline pH than conventional solutions, which 4 12 causes the emulsion to swell more, thus providing greater access 72 4 to developer. These solutions are especially advantageous in end- 76 3 odontics and in emergency situations, when short processing 80 gs 2 12 time is essential. Although the resultant images may be satisfac- tory, they often do not achieve the same degree of contrast as Processing films at either higher or lower temperatures and films processed conventionally, and they may discolor over time for longer or shorter times than recommended by the manu- if not fully washed. After viewing, rapidly processed films are facturer reduces the contrast of the processed film. lo placed in conventional fixing solution for 4 minutes and washed 5. Develop. Start the timer mechanism, and immerse the hanger for 10 minutes; this improves the contrast and helps keep them and films immediately in the developer. Agitate the hanger stable in storage. Conventional solutions are preferred for most y.b mildly for 5 seconds to remove air bubbles from the film. Do routine use. not agitate the film during development. 6. Rinse. After development, remove the film hanger from the developer, draining excess into the water bath, and place in the CHANGING SOLUTIONS running water bath for 30 seconds. Agitate the films continu- All processing solutions deteriorate as a result of continued use r ously in the rinse water to remove excess developer, thus and exposure to air. Although regular replenishment of the devel- slowing development and minimizing contamination of the oper and fixer prolongs their useful life, the buildup of reaction ra fixer. products eventually causes these solutions to cease functioning properly. Exhaustion of the developer results from oxidation of the developing agents, depletion of the hydroquinone, and buildup llib of bromide. With regular replenishment, solutions may last 3 or 4 weeks before they must be changed. A simple procedure can help determine when solutions should be changed. A double film packet is exposed on one projection for the first patient radiographed after new solutions have been pre- a pared. One film is placed in the patient’s chart, and the other is mounted on a corner of a viewbox in the darkroom. As successive nt films are processed, they are compared with this reference film. Loss of image contrast and density become evident as the solutions deteriorate, indicating when it is time to change them. The fixer is changed when the developer is changed. de AUTOMATIC FILM PROCESSING Equipment that automates all processing steps is available (Fig. 5-23). Although automatic processing has numerous advantages, the most important is the time saved. Depending on the equip- ment and the temperature of operation, an automatic processor requires only 4 to 6 minutes to develop, fix, wash, and dry a film. FIGURE 5-22 Films are mounted securely on film clips. Film is always held by its edges Many dental automatic processors have a light-shielded (daylight to avoid fingerprints on the image. (Courtesy C. L. Crabtree, DDS, Bureau of Radiological Health, loading) compartment in which the operator can unwrap films and Rockville, MD.) feed them into the machine without working in a darkroom. 74 PART II Imaging However, special care must be taken to maintain infection control Whether automatic processing equipment is appropriate for a when using these daylight-loading compartments (see Chapter 15). specific practice depends on the dentist and the nature and volume When extraoral films are processed, the light-shielded compart- of the practice. The equipment is expensive and must be cleaned ment is removed to provide room for feeding the larger film into frequently, as described by the processor manufacturer. Also, auto- the processor. Another attractive feature of the automatic system mated equipment may break down, and conventional darkroom is that the density and contrast of the radiographs tend to be equipment may still be needed as a backup system. consistent. However, because of the higher temperature of the developer and the artifacts caused by rollers, the quality of films MECHANISM processed automatically often is not as high as the quality of films Automatic processors have an in-line arrangement consisting of m carefully developed manually. With automatically processed films, a transport mechanism that picks up exposed, unwrapped film if more grain is evident in the final image, the correct choice of and passes it through the developing, fixing, washing, and drying processing solutions may be able to help minimize the issue. sections (Fig. 5-24). The transport system most often used is a series of rollers driven by a motor that operates through gears, o belts, or chains. The rollers often consist of independent assem- blies of multiple rollers in a rack, with one rack for each step t.c in the operation. Although these assemblies are designed and positioned so that the film crosses over from one roller to the next, the operator may remove them independently for cleaning and repairing. po The primary function of the rollers is to move the film through the developing solutions, but they also serve at least three other purposes. First, their motion helps keep the solutions agitated, which contributes to the uniformity of processing. Second, in the gs developer, fixer, and water tanks, the rollers press on the film emul- sion, forcing some solution out of the emulsion. The emulsions rapidly fill again with solution, thus promoting solution exchange. Finally, the top rollers at the crossover point between the developer lo and fixer tanks remove developing solution, minimizing carryover of developer into the fixer tank. This feature helps maintain the uniformity of processing chemicals. y.b FIGURE 5-23 Dent-X 810 AR film automatic film processor. The operator opens the film The chemical compositions of the developer and fixer are modi- packet in a darkroom and inserts the film into the opening on the left end of the machine. The fied to operate at higher temperatures than the temperatures used exposed film is carried on a roller apparatus through processing solutions, and the processed for manual processing and to meet the more rapid development, and dried film is returned through the upper right opening in 4.5 minutes. (Courtesy Image- fixing, washing, and drying requirements of automatic processing. Works, Elmsford, NY.) The quality of the fixer is very important. High quality fixers r ra Film Drying llib exit elements Film entry a A nt Developing Fixing Water solution solution wash Film exit de Film Dryer entrance Developer Fixer Wash C B FIGURE 5-24 A, Automatic film processors typically consist of a roller assembly that transports the film through developing, fixing, washing, and drying stations. B, Assembly of film transport mechanism. C, One roller assembly. (B and C, Courtesy ImageWorks, Elmsford, NY.)  C H A P T E R 5 Film Imaging 75 contain an additional hardener that helps the emulsion withstand crystals in the emulsion that were struck by the photons are con- the rigors of the transport system and improves transport. Poor verted to grains of metallic silver. These silver grains block the quality fixers containing no hardener produce more film artifacts, transmission of light from a viewbox and give the film its dark and film may slip and jam during transport. appearance. The degree of darkening or opacity of an exposed film is referred to as optical density. The optical density of an area of OPERATION an x-ray film can be measured as follows: Successful operation of an automatic processor requires standard- ized procedures and regular maintenance. The processor and sur- Io Optical density = Log10 rounding area should always be kept clean so that no chemicals It m contaminate hands or films. The solution level and temperature should be checked each morning before films are processed. Hands where Io is the intensity of incident light (e.g., from a viewbox), should be dry when handling film, and films should be touched and It is the intensity of the light transmitted through the film. on their edges only. The better processors have automatic replen- With an optical density of 0, 100% of the light is transmitted; with o ishment systems. A daily, weekly, and quarterly maintenance a density of 1, 10% of the light is transmitted; with a density of 2, routine (see Chapter 15) should be followed, including cleaning 1% of the light is transmitted; and so on. t.c the rollers and other working parts. It is vital to run a large roller A plot of the relationship between film optical density and transport clean-up film daily through the processor to clean the exposure is called a characteristic curve (Fig. 5-25). It usually top and bottom rollers. is shown as the relationship between the optical density of the film and the logarithm of the corresponding exposure. As expo- po ESTABLISHING CORRECT EXPOSURE TIMES sure of the film increases, its optical density increases. A film is of greatest diagnostic value when the structures of interest When radiographs are first made with a new x-ray machine, it is are imaged on the relatively straight portion of the graph, between important to examine the exposure guidelines that come with the 0.6 and 3.0 optical density units. The characteristic curves of gs machine. Typically, such guidelines provide a table listing the films reveal much information about film contrast, speed, and various anatomic regions (incisors, premolars, or molars), patient latitude. size (adult or child), and the length of the aiming cylinder. For An unexposed film, when processed, shows some density. This each of these combinations, there is a suggested exposure time. It appearance is caused by the inherent density of the base and added lo is also important to start out using fresh processing chemicals and tint and the development of a few unexposed silver halide crystals. optimal processing conditions as previously described. After the This minimal density is called base plus fog and typically is 0.2 first images are made on patients, it may be necessary to adjust to 0.3. Radiographic density is influenced by exposure and the y.b exposure time. If optimal film processing techniques are being thickness and density of the subject. followed and the images are consistently dark, exposure times should be decreased until optimal images are obtained. If images Exposure are consistently light, exposure times should be increased. When The overall film density depends on the number of photons the optimal times have been determined, these values should be absorbed by the film emulsion. Increasing the milliamperage (mA), r posted by the control panel. peak kilovoltage (kVp), or exposure time increases the number of photons reaching the film and thus increases the density of the ra MANAGEMENT OF RADIOGRAPHIC WASTES radiograph. Reducing the distance between the focal spot and film also increases film density. To prevent environmental damage, many communities and states llib have passed laws governing the disposal of wastes. Such laws often derive from the federal Resource Conservation and Recovery Act of 1976. Although dental radiographic waste constitutes 3.5 only a small potential hazard, it should be discarded properly. The primary ingredient of concern in processing solutions is 3.0 a the dissolved silver found in used fixer. Another material of concern is the lead foil found in film packets. Dental offices 2.5 nt also should consider using companies licensed to pick up waste Optical density materials. 2.0 IMAGE CHARACTERISTICS de 1.5 Processing an exposed x-ray film causes it to become dark in the 1.0 exposed area. The degree and pattern of film darkening depend on numerous factors, including the energy and intensity of the x-ray 0.5 beam, composition of the subject imaged, film emulsion used, and characteristics of film processing. This section describes the major 0.0 imaging characteristics of x-ray film. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Log relative exposure RADIOGRAPHIC DENSITY When a film is exposed by an x-ray beam (or by light, in the case FIGURE 5-25 Characteristic curve of direct exposure film. The contrast (slope of the of screen-film combinations) and then processed, the silver halide curve) is greater in the high-density region than in the low-density region. 76 PART II Imaging densities are weak absorbers. They allow most photons to pass Subject Thickness through, and they cast a dark area on the film that corresponds to The thicker the subject, the more the beam is attenuated, and the the radiolucent object. lighter the resultant image (Fig. 5-26). If exposure factors intended for adults are used on children or edentulous patients, the resultant RADIOGRAPHIC CONTRAST films are dark because a smaller amount of absorbing tissue is in Radiographic contrast is a general term that describes the range the path of the x-ray beam. The dentist should vary exposure time of densities on a radiograph. It is defined as the difference in