Base :The function of the film base is to support the emulsion.

1. X-Ray FilmCOMPOSITIONX-ray film has two principal components: emulsion and base. The emulsion, which is sensitive to x rays andvisible light, records the radiographic image. The base is a plastic supporting material onto which the emulsion is coated

2. Emulsion The two principal components of emulsion are silver halide grains, which are sensitive to x radiation andvisible light, and a vehicle matrix in which the crystals are suspended.

4. Base :The function of the film base is to support the emulsion. • The base must have the proper degree of flexibility • to allow easy handling of the film. The base for • dental x-ray film is O.18mm thick and is made of polyester • polyethylene terephthalate. The film base is uniformly • translucent and casts no pattern on the resultant • radiograph. So

5. Screen film, which is sensitive to visible light, is used with intensifying screens that emit • visible light. • Screen film and intensifying screens are used for extraoral projections such as panoramic and skull radiographs.

6. INTRAORAL X-RAY FILM • A number of manufacturers around the world make intraoral dental x-ray film. • In each case the film is made as a double-emulsion film, that is, coated with an emulsionon each side of the base. • With a double layer of emulsion, less radiation can be used to produce animage.

9. Periapical View • Periapical views are used to record the crowns, roots, and surrounding bone. Film packs corne in three sizes: 0 for small children (22 X 35mm); 1, which is relatively narrow and used for views of the anterior teeth (24 X40mm); and 2, the standard film size used for adults (31 X 41 mm) (Fig. 4-5).

10. Occlusal View Occlusal film is more than three times larger than size 2 film (57 x 76mm) (see Fig. 4-5). 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 usually is held in position by having the patient bite lightly on it to support it between the occlusal surfaces of the teeth

11. 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

12. The silver halide grains are composed primarily of crystals of silver bromide. Film emulsions are sensitive to both x-ray photons and visible light. Film intended to be exposed by x rays is called direct exposurfeil m. All intraoral dental film is direct exposure film.

13. intensifying screen 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 xray energy absorbed. These phosphors have been 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

14. FUNCTION The presence of intensifying screens creates an image receptor system that is 10 to 60 times more sensitive to x rays than the film alone

15. Consequently, use of intensifying • screens means a substantial reduction in the • dose of x radiation to which the patient is exposed. • Intensifying screens are used with films for virtually all • extraoral radiography, including panoramic, cephalometric, • and skull projections. In

16. COMPOSITION • Intensifying screens are made of a base supporting material, a phosphor layer, and a protective polymeric coat . In all dental applications, intensifying screens are used in pairs, one on each side of the film, • and they are positioned inside a cassette

18. Film emulsion consists of photosensitive crystals containing primarily silver bromide suspended in a vehicle and layered on a thin sheet of transparent plastic base. Some cystals also contain small amounts of silver iodide.

19. The crystals are chemically sensitized by the addition of trace amounts of sulfur compounds, which bind to the surface of the crystals. The sulfur compounds playa crucial role in image formation.

20. When the silver halide crystals are irradiated, x-ray photons interact primarily with the bromide ions by Compton and photoelectric interactions. These interactions result in the removal of an electron from the bromide ions. By the loss of an electron, a bromide ion is converted into a neutral bromine atom. The free electrons move through the crystal until they reach a sensitivity site, where they become trapped and impart a negative charge to the site

21. When a silver ion reaches the negatively charged sensitivity site, it is reduced and forms a neutral atom of metallic silver . The sites containing these neutral silver • atoms are now called latent image site

22. The overall distribution of latent image sites in a film after exposure constitutes the latent image. • Film processing converts the latent image into one • that can be visualized . The neutral silver • atoms at each latent image site 4render • the crystals sensitive to development and image formation.

24. DEVELOPER SOLUTION • DEVELOPER SOLUTION • The developer reduces all silver ions in the exposed crystals of silver halide (those with a latent image) to metallic silver grains. To produce a diagnostic image, this reduction process must be restricted to crystals containing latent image sites. To accomplish this, the reducing agents used as developers are catalyzed by the neutral silver atoms at the latent image sites

27. The developing solution contains four components, • all dissolved in water: (1) developer, (2) activator, (3) • preservative, and (4) restrainer,

28. The primary function of the developing solution is • to convert the exposed silver halide crystals into metallic • silver grains. This process begins at the latent • image sites, where electrons from the developing agents • are conducted into the silver halide crystal and • reduce the constituent silver ions (approximately 1 • billion to 10 billion) to solid grains of metallic silver

29. Two developing agents are used in dental radiology: a • pyrazolidone-type compound, usually Phenidone (1- • phenyl-3-pyrazolidone), and hydroquinone (paradihydroxy • benzene). Phenidone serves as the first electron • donor that converts silver ions to metallic silver at • the latent image site. 'This electron transfer generates • the oxidized form of Phenidone. Hydroquinone provides • an electron to reduce the oxidized Phenidone • back to its original active state so that it can continue • to reduce silver halide grains to metallic silver. Unexposed • crystals, those without latent images, are unaffected • during the time required for reduction of the • e~posed crystals.

30. Activator The developers are active only at alkaline pH values, usually around 10. This is achieved with the addition of alkali compounds (activators) such as sodium or potassium hydrozide. Buffers are used to maintain this condition-usually sodium bicarbonate. The activators also cause the gelatin to swell so that the developing agents can diffuse more rapidly into the emulsion and reach the suspended silver bromide crystals.

31. Preservative The developing solution contains an antioxidant or preservative, usually sodium sulfite. The preservative protects the developers from oxidation by atmospheric oxygen and thus extends their useful life. The preservative also combines with the brown oxidized developer to produce a colorless soluble compound. If not removed, oxidation products interfere with the developing reaction and stain the film.

32. RINSING Alter development the hImemulsIon swells and becomes saturated with developer. At this point the films are rinsed in water for 30 seconds with continuous, gentle agitation before they aye placed in the fixer. Rinsing dilutes the developer, slowing the development process. It also removes the alkali activator, preventing neutralization of the acid fixer. This rinsing process is typical for manual processing but is not used with automatic processing.

33. FIXING SOLUTION • FIXING SOLUTION • The primary function of fixing solution is to dissolve and remove the undeveloped silver halide crystals from the emulsion . • The presence of unexposed crystals causes film to be opaque. If these crystals are not removed, the image on the resultant radiograph is dark and non diagnostic. a • photomicrograph of film emulsion showing the solid • silver grains after fixer has removed the unexposed

34. A second function of fixing solution is to harden and shrink the film emulsion. As with developer, fix:er should be replenished daily at the rate of 8 ounces per gallon. Fixing solution also contains four components, all dissolved in water: (1) cleaning agent, (2) acidifier, (3) preservative, and (4) hardener.

35. Clearing Agent • Mter development the film emulsion must be cleared • by dissolving and removing the unexposed silver halide. • An aqueous solution of ammonium thiosulfate ("hypo") • dissolves the silver halide grains. It forms stable, watersoluble • complexes with silver ions, which then diffuse • from the emulsion. The clearing agent does not have a • rapid effect on the metallic silver grains in the film • emulsion, but excessive fixation results in a gradual loss • of film density because the grains of silver slowly dissolve • in the acetic acid of the fixing solution

36. Hardener The hardening agent most often used is aluminum sulfate. Aluminum complexes with the gelatin during fixing and prevents damage to the gelatin during subsequent handling. The hardeners also reduce swelling of the emulsion during the final wash. This lessens mechanical damage to the emulsion and limits water FIG. 6-4 Scanning electron micrograph of a processed absorption, thus shortening drying time.

37. WASHING after fixing, the processed film is washed in a sufficient flow of water for an adequate time to ensure removal of all thiosulfate ions and silver thiosulfate complexes. Washing efficiency declines rapidly when the water temperature falls below 600 F. Any silver compound or thiosulfate that remains because of improper washing discolors and causes stains, which are most apparent in the radiopaque (light) areas. This discoloration results from the thiosulfate reacting with silver to form brown silver sulfide, which can obscure diagnostic information.

39. MANUAL PROCESSING TANKS All dental offices should have the capability to develop film by tank processing, if only as a backup for an automatic processor or digital imaging system. The tank must have hot and cold running water and a means of maintaining the temperature between 600 and 750 F. A practical size for a dental office is a master tank about 20 x 25cm (8 x 10 inches) that can serve as a water

40. jacket for two removable inserts that fit inside The insert tanks usually hold 3.8L (1 gallon) of developer or fixer and are placed within the outer, larger master tank. The outer tank holds the running water for maintaining the temperature of the developer and fixer in the insert tanks and for washing films. The developer customarily is placed in the insert tank on the left side of the master tank and the fixer in the insert tank on the right. All three tanks should be made of stainless steel, which does not react with the processing solutions and is easy to clean. The master tank should have a cover to reduce oxidation of the processing solutions, protect the developing film from accidental exposure to light, and minimize evaporation of theprocessing solutions.

44. Image Characteristics Processing an exposed x-ray film causes it to become dark in the exposed area The degree and pattern of film darkening depend on numerous factors, including the energy and intensity of the x-ray beam, composition of the subject imaged, film emulsion used, and characteristics of film processing. This section describes the major imaging characteristics of x-ray film.

45. RADIOGRAPHIC DENSITY When a film is exposed by an x-ray beam (or by light, in the case of screen-film combinations) and then processed, the silver halide crystals in the emulsion that were struck by the photons are converted to grains of metallic silver. These silver grains block the transmission of light from a viewbox and give the film its dark appearance. The overall degree of darkening of an exposed film is referred to as radiographic density

46. Radiographic density is influenced by exposure and the thickness and density of the subject. Exposure The overall film density depends on the number of photons absorbed by the film emulsion. Increasing the milliamperage (rnA), peak kilovoltage (kVp), or exposure time increases the number of photons reaching the film and thus increases the density of the radiograph. Reducing the distance between the focal spot and film also increases film density.

47. Subject Thickness The thicker the subject, the more the beam is attenuated and the lighter the resultant image. If exposure factors intended for adults are used on children or edentulous patients, the resultant films are dark because a smaller amount of absorbing tissue is in the path of the x-ray beam. The dentist should vary exposure (either kVp or time) according to the patient's size to produce radiographs of optimal density

48. Variations in the density of the subject exert a profound influence on the image. The greater the density of a structure within the subject, the greater the attenuation of the x-ray beam directed thro~gh that subject or area. In the oral cavity the relative densities of various natural structures, in order of decreasing density, are enamel, dentin and cementum, bone, muscle, fat, and air. Metallic objects (e.g., restorations) are far denser than enamel and hence better absorbers. Because an x-ray beam is differentially attenuated by these absorbers, the resultant beam carries information that is recorded • Subject Density

49. RADIOGRAPHIC CONTRAST Radiographic contrast is a general term that describes the range of densities on a radiograph. It is defined as the difference in densities between light and dark regions on a radiograph. Thus an image that shows both light areas and dark areas has high contrast. This also is referred to as a short gray scale of contrast because few shades of gray are present between the black and white images on the film. A radiographic image composed only of light gray and dark grey zones has low contrast, also referred to as having a long gray scale of contrast . The radiographic contrast of an image is the result of the interplay of subject contrast, film contrast, and scattered radiation

50. Subject Contrast Subject contrast is the range of characteristics of the subject that influences radiographic contrast. It is influenced largely by the subject's thickness, density, and atomic number. The subject contrast of a patient's head and neck exposed in a lateral cephalometric view is high. The dense regions of the bone and teeth absorb most of the incident radiation, whereas the less dense soft tissue facial profile transmits most of the radiation. Subject contrast also is influenced by beam energy and intensity.