A patient arrives in your chair, and tells you that his lower left molar has been keeping him up at night, hurting. However, he does not want a “x-ray” because his finances are scarce. It is important to let your patient know that the dentist, under the guidance from the Oregon Board of Dentistry, cannot diagnose the problem, or perform any treatment without a radiograph. Definitions ANSI film rating: desigrates film speed (sensitivity) by using letters A-F (A-slowest, F-fastest) Characteristic curve: Film contrast is usually expressed in the slope of the line in the diagnostically useful part of the haracteristic curve.
Cassette: A container for x-ray film that is light-tight yet permits penetration of x-rays. Double emulsion: The emulsion with its protective coating is attached to the acetate base by an adhesive. The emulsion is sensitive to x-rays, visible light, and static electricity. The film base is coated on both sides and thus is referred to as a double emulsion. Double-film packet: A packet with 2 films inside and may require more exposure time. The packet contains waterproof outer package, black paper, two films, black paper and lead foil backing.
Extraoral projection: A radiograph produced on film place xtra-orally. Film base: The cellulose acetate sheet on which the emulsion is coated. Film contrast: The characteristic of the x-ray film that enables it to portray differences in subject contrast. Film duplication: Duplicating film for insurance companies or to a patient’s new dentist is an important task. A dental office always needs to keep a record of a patient’s films. The original copy should never leave the office.
Film emulsion: X-ray film is composed of a clear cellulose acetate film base that is coated with an emulsion of silver halide (usually silver bromide) grains suspended in a layer of gelatin. The emulsion with its protective coating is attached to the acetate base by an adhesive. Film reversal: Film reversal is when the radiologist puts the film backwards in the patient’s mouth. Film reversal is sometimes referred to as the “herringbone effect” because the herringbone pattern that is embossed on the lead foil blacking is transferred to the processed reversed radiograph.
The x-rays are attenuated ina pattern by the lead foil before striking the film. Film sensitivity: Film sensitivity is film speed. The size of silver halide crystals, the thickness of emulsion, and the presence of pecial radiosensitive dyes determine the film speed, or film sensitivity. Film-screen system: The imaging system used in extraoral radiography is a film-screen system. The film is used in combination with intensifying screens. Today with the improved film quality and heightened concern for radiation safety, all extroral films should be taken using intensifying screens.
Fluorescence: The property of emitting visible light when struck by radiation. Fog: X-ray film fog occurs when all or part of the radiograph is darkened by sources other than the primary beam of radiation to which the film was exposed. Fog may occur chemical fog results from an imbalance or exhaustion of processing solutions, light fog results from unintentional exposure from light leaks and improper safelighting, and scatter radiation fog results from radiation striking the film from sources other than the intentional exposure of the primary beam.
Intensifying screen: A coating of fluorescent material on a suitable base that intensifies the radiation thus permitting a decrease in exposure time. Intensifying screen speed: As the name implies, these screens intensify or increase the radiation and thus decrease the exposure time needed. They vary in their speed or exposure time requirements, just as does film. Orientation dot: Button or dot that is a small convex, concave area that indicates which side of film is closest to the tube for mounting. Phosphor: A substance that has the property of fluorescence.
It emits light when struck by x-rays. Rare earth screen: Four times more efficient in converting x-ray energy into light. Rare earth elements produce green light and are new phosphors (not calcium tungstate). It must be used with compatible film that is sensitive to light in the green portion of the light spectrum. Lecture It is important to know and understand the imaging systems available in dental radiography. After this chapter, you will be able to apply this knowledge to clinical situations that call for the use of introral panoramic or extraoral projections.
You will also learn how to process and duplicate the film. Film consists of waterproof outer package, black paper, film, another black paper and lead-foil backing. The lead-foil backing is placed on the side of the film away from the x-ray tube to absorb any unused radiation and back scatter secondary to irradiation and prevent them from affecting the patient or ogging the film. Film speed is determined by the size of the silver halide crystals, the thickness of the emulsion, and the presence of special radio-sensitive dyes that determine the film speed, or film sensitivity.
The size of the silver bromide crystals is the main factor in film speed: The larger the crystals, the faster the film. Fast film equals less exposure to produce a desired density than a slow film. Film speed is designated by group from the American National Standards Institute (ANSI) using the letters A-F. Fis the fastest film. D,E and Fare the only film speeds that should be used. More sensitive films require fewer mAs are said to have greater film speed, these are fast films. Therefore, fewer mAs are needed for E-speed film than D- speed film.
A double-film packet contains the same articles as a regular film packet, but has two films in it. After radiation and processing, two films are available for viewing. “X-ray film is composed of a clear cellulose acetate film base that is coated with an emulsion of silver halide (usually silver bromide) grains suspended in a layer of gelatin. The emulsion with its protective coating is attached to the acetate base by an adhesive” (Frommer, p. 6). An intensifying screen enhances the development of film by intensifying or increasing the radiation and thus decreasing the exposure time needed.
The film sandwiched between the intensifying screens in the cassette is affected by both x-rays and light. A screen possesses a coating of fluorescent material on a suitable base that intensifies the radiation, thus decreasing the exposure time. Operators should take special care when loading or unloading cassettes in the darkroom, as not to scratch the intensifying screens with sharp objects. Damaged screens and cassettes should be thrown away. If an intensifying screen is badly scratched, the phosphor is removed and a dark streak will appear on the film taken with this screen.
Also, the film used has to be sensitive to the type of light emitted by the particular screen. The speed of the screen depends on the type of phosphor and size of the crystal. The larger the crystal, the faster the screen but the poorer the definition. The rare earth elements used in intensifying screens are four times more efficient in converting x-ray energy into light than calcium tungstate crystals, thus the screens are faster and require less exposure time. The size of the silver halide crystals, thickness of the emulsion, and the presence of special radiosensitive dyes determine film speed (or sensitivity).
The larger the silver bromide crystals, the faster the film. Therefore, film speed F has larger silver bromide crystals than film speed E. Duplicating film for insurance companies, for a court document or a patient’s new dentist is an important task. A dental office should keep a record of a patient’s films. The original copy should never leave the office. Duplicating films do not have an orientation dot as standard intraoral films do, so ilms must be labeled “right” and “left.
The process of radiographic duplication involves additions of normal darkroom equipment such as duplication film, appropriate size film hangers, a light source (ultraviolet is preferable) and a photographic printing frame. To begin, the emulsion side of the duplicating film must be identified. Under safelight conditions, the emulsion side is dull, whereas the non-emulsion side is shiny. The radiographs to be duplicated are placed on the emulsion side of the duplicating film, so the light can strike the original film first. The films are exposed to light for 6-8 seconds.
The duplicating film is then processed in the same manner as a normal film. It is important that the film used is sensitive to the type of light emitted by the particular screen. Film fog is when the radiograph has all or part of it darkened by sources other than the primary beam. Three main sources of film fog are chemical imbalance or exhaustion of processing solution, light leaks (light fog) for improper safe lighting before or after processing, or scatter radiation where radiation striking the film from sources other than the primary beam, or poor storage of film.