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Dental Radiology for Dental Assisting Exam Study Guide (page 2)

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Updated on Jun 23, 2011

Processing Intraoral Radiographs

Conventional intraoral radiographs require a process that brings out the latent image, making it visible.

Latent Image

A latent image is an unseen image that is on the film from the exposure time to the time that the image appears on the film. This invisible image is made when the X-radiation in the X-ray tube head strikes the silver halide crystals on the film.

Film Composition

Dental X-ray film is covered with an emulsion on both sides of the film. The emulsion consists of a mixture of silver halide crystals in a gelatin base. These halide crystals change when they are exposed to radiation. When the film is processed, the developer and fixer chemicals react with these exposed and unexposed silver halide crystals to produce an image.

Chemicals Involved in Processing

The three forms of processing chemicals are ready-to-use, concentrate, and powder. The concentrate form is most commonly used in dental offices.

Developer

The developer reacts with the exposed silver halide crystals and is responsible for creating the black or dark tones on the image. The reducing agents in the developer are metol and hydroquinone.

Fixer

The fixer reacts with unexposed silver halide crystals and removes them from the film, which causes a white or clear area. The fixing agent is ammonium thiosulfate.

Water

Water is used in the processing of dental X-rays to rinse and remove any developer or fixer chemicals on the film.

Automatic Film Processing

Automatic processing systems can develop intraoral films in four to five minutes or less. There is much less room for operator error as the temperature and developing and fixing time are controlled by the processor and not the operator. Some automatic processors have a daylight loader attached to them so that a darkroom is not necessary. The solutions in the automatic processors are much warmer than in manual processing. The solutions are approximately 85–105° F (30–40° C). The films move through the processor on rollers. Maintenance of the automatic processor is very important. Solutions must be replenished daily and changed every two to six weeks, depending on the rate of use. The processor rollers must be cleaned each time the solutions are changed. Chemicals must be disposed of according to local regulation, and documentation should be filed. There is less chance of error using automatic processors.

Manual Film Processing

Films may be processed manually in a darkroom with an X-ray utility “red” safety light over the traditional tank. Intraoral films are clipped to a film rack and processed on the rack. Accurate temperature of the developer and fixer solutions and timing is mandatory to obtain a diagnostic image. Optimum developing time and temperature for manual processing is four-and-a-half to five minutes at 68° F (20° C). Increasing the time that the films are in the developer and increasing the temperature of the developer cause a denser, darker image. Fixing time is usually twice the developing time. The films are then washed for at least 20 minutes in a fresh running water tank, then hung to dry. The process includes developing, rinsing, fixing, washing, and drying.

Mounting Films

There are 18–20 individual films mounted into a full mouth survey mount. A dot (pimple, not a dimple) is utilized to determine the front side of the film. This is known as labial mounting. Anatomical landmarks are used to aid in mounting. Note that on an edentulous patient, the maxillary tuberosity and the outer corner of the eye would serve as the landmarks of the maxillary molar radiograph.

Helpful hints include:

  • Maxillary molars have three roots.
  • Mandibular molars have two roots.
  • Maxillary central incisor films show the median palatine suture, a radiolucent line between the maxillary central incisors. 
  • Anterior films are orientated vertically.
  • Posterior films are orientated horizontally.
  • Maxillary films have large radiolucent areas: nasal fossa and maxillary sinuses.
  • Maxillary molar films show the maxillary tuberosity.
  • Mandibular molar films show the retromolar pad and the external oblique ridge.
  • The overall appearance of the full mouth survey is in an upward curve (smile).
  • Radiolucent structures, such as dental pulp, permit the passage of X-rays and thus appear dark on the film. On the other hand, radiopaque materials, such as gold and amalgam, block the penetration of X-rays and therefore appear lighter on the film.
  • If the operator uses too much vertical angulation, the result is an X-ray image that is shorter than the actual tooth; this is known as foreshortening.
  • The correct vertical angulation in a bitewing radiograph is +10 degrees.

Duplicating Films

Intraoral film is available in double-pack films, which creates two originals simultaneously. However, duplicating is completed using a duplicating film. Duplication is completed in a darkroom using a duplicator that shines a bright ultraviolet light onto the films that then produces an image onto the duplicating film.

The Generation of X-rays in the X-ray Tube Head

X-rays are produced in the tube head and are the result of high-speed electrons stopping or slowing. The electron kinetic energy is changed into electromagnetic energy by Bremsstrahlung radiation. The X-ray photons produced in the tube head have many different wavelengths. Photons striking a living organism break molecules into smaller pieces, disrupt molecular bonds, and form new ones within molecules and between new molecules.

Properties of X-rays

X-rays are energy that travels in a wave-like motion. They penetrate matter, produce fluorescence in some materials, cause ionization of matter, and produce a latent image on the film. Ionization is the loss of electrons from a substance.

Parts of the Dental X-ray Tube

There are five main parts in a dental X-ray tube: anode-tungsten target, cathode-tungsten filament, aluminum filter, lead collimator, and position indicating device (PID). Each is discussed in turn. The X-ray tube creates the X-ray production conditions of: a source of electrons, high voltage for electron speed, and a target that can stop the electrons.

Anode-Tungsten Target

This is the positively charged end of the X-ray tube head. The kilovoltage (kV) setting controls the current in the anode or the quality.

Cathode-Tungsten Filament

This is the negatively charged side of the X-ray tube head where the electrons are boiled off of the tungsten filament. The milliampere (mA) setting controls the number of electrons or quantity.

Aluminum Filter

This filter is located in the position indicating device (PID) between the PID and the X-ray tube head. This filter removes the long wave—low energy wavelengths that are not needed to produce X-rays.

Lead Collimator

Also referred to as the lead disc, the lead collimator restricts the spread of the X-ray beam to no more than 2.75 inches (70 mm) at the patient’s face. Lead is used because it is resistant to the penetration of ionizing radiation.

Position Indicating Device (PID)

The position indicating device (PID) is also referred to as the cone. The PID is used for aiming the central beam toward the patient’s face and anatomical landmarks.

Production of X-rays

Thermionic emission occurs at the tungsten filament in the cathode. A cloud of electrons is boiled off the filament. The negatively charged electrons are attracted to the tungsten target in the anode, the positive side of the tube. When the electrons collide with the target, energy in the form of X-rays and heat is produced. Ninety-nine percent of the energy is heat and only 1% of the energy produced is X-rays. The X-rays then escape the tube head through the aluminum filter and collimator and travel down the PID to strike the matter and the film.

Characteristics of the Image

There are four characteristics of an X-ray image. These are: contrast, density, detail, and geometric distortion. Each is discussed in turn. The dentist requires properly processed radiographs with minimal distortion to be diagnostically acceptable. For example, a film that has been exposed to radiation twice, known as double exposure, is of no use.

Contrast

This refers to the varying shades of gray present in the image. Contrast is dependent upon density and can be influenced by processing. Contrast is difference in densities. It is controlled by the voltage (kV) setting.

Density

Radiographic density is the degree of darkness in the image. Density depends on the total amount of radiation that the film receives, the thickness of the bone, the developing/processing conditions, and the distance between the X-ray tube head and the patient. Density is controlled by the amperage (mA) setting.

Detail

Detail is the sharpness and clarity of the image. Detail is affected by patient movement or X-ray tube head movement. Any movement during the exposure of an X-ray will cause the image to appear blurry and out of focus.

Geometric Distortion

By increasing the object-to-film distance, a penumbra will be present. A penumbra is the lack of sharpness that surrounds the shadow. This results in an inaccurate duplication of the tooth since it is geometrically distorted. Also, if the patient makes any slight movement, the result will be a blurry image.

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