Radiography Technique – There are two methods used to expose intraoral dental radiographs.
In this technique, the central X-ray strikes at a 900 angle (perpendicular) to the film. This allows the oral structures to be projected at close to normal size, which is important for diagnosis and treatment.
The Rinn XCP is a special instrument designed to take radiographs using the paralleling radiography technique. It is made up of a series of rings, rods, and film holders.
BISECTED-ANGLE Radiography Technique
The bisected-angle technique is another method used when an XCP (extension cone paralleling) instrument is not available. The bisected-angle technique uses a geometric rule called Cieszynski’s rule. The rule states “distortion of the dental structures in a radiograph can be lessened by directing the CR in a 90° angle to a line which bisects the angle formed by the plane of the film and the long axis of the tooth.”
Bisecting-Angle Instrument (BAT), developed for the bisected-angle radiography technique, is composed of rods, rings, and film holders, which are assembled the same way as for the paralleling XCP.
In the bisected-angle radiography technique, the film is placed in a different relationship to the teeth than for the paralleling technique. Even though radiographs taken using either technique will show the same teeth, the bisecting-angle technique requires the film to be placed as close as possible to the lingual surface of the teeth. In the paralleling radiography technique, the film is placed far into the mouth.
ROUTINE TO BE FOLLOWED FOR RADIOGRAPHIC EXPOSURE
When taking all types of radiographs, be sure the patient is seated comfortably and properly. The level of the chair should be raised so it is easy for the operator to see into the mouth while placing the films. Various anatomic landmarks are used to make sure that the patient is positioned correctly. One is the midline, or midsagittal plane, dividing the body in half.
For taking all exposures, this line must be perpendicular to the floor. Another landmark is a line from the ala of the nose (corner flare of the nostril) to the tragus of the ear (tab of cartilage at the opening to the ear). This line must be parallel to the floor for maxillary exposures.
For a mandibular exposure, a line from the corner of the mouth to the tragus should be parallel to the floor. The occlusal plane is considered parallel to the floor when the head is in this position.
Exposure techniques that should be incorporated in the operator’s routine, along with infection control procedures, include the following:
1. Make sure the patient is comfortable and seated properly.
2. Have the patient remove earrings, glasses, lipstick, and partial dentures—anything that will be in the way of the moving tube head, collimator, or area of exposure.
3. Place the lead apron with thyroid collar on the patient.
4. Check the controls to be sure they are set properly for the type of exposure to be made.
5. Place the film as according to the exposure radiography technique followed.
6. Move to the safe area (preferably out of the room) and press the timer button. A beeping noise will indicate that the exposu1 has been made. Release the button when the beep stops.
7. Remove the holder and film. Place the film in a paper towel or cup in a designated area outside the room until other exposures are taken. This protects the film from exposure to secondary radiation. Place the patient’s name on the cup or paper towel to prevent misidentification.
8. Repeat the process until all necessary exposures are taken.
9. Remove lead apron, turn off machine, and take films to be processed.
Film Size and Placement
To get the accurate image the proper size film must be selected and placed in the proper position behind the area to be radiographed. The plain side of the film packet should face the primary beam (or central ray, CR). After the radiograph is properly exposed, the film is taken to the darkroom to be chemically processed.
It is developed, rinsed, fixed, washed, and dried (if a manual method is used) or processed in machine (if an automatic method is used). The film is then mounted on a cardboard or plastic radiographic mount for the dentist to interpret.
Proper film size and placement is important for a number of reasons. If the film is too small or it is placed improperly, some parts of the teeth may not show on the radiograph, resulting in a partial image. If the film is placed with the wrong side toward the primary beam, the film will be covered with a light gray haze (fog), and there will be a herringbone pattern at one end of the film because the radiation has passed through the lead shield before affecting the film.
If the collimator is angled incorrectly, the size and shape of the teeth will be distorted. In a good radiograph, the teeth and tissues are close to normal size. If the angle of the CR is too steep, the teeth will appear shorter than normal (foreshortened). If there is not enough of an angle, the teeth will appear longer than normal (elongated).
Exposure time is also important. If it is too long, the film will be too dark (dense); if the time is too short, the film will be light (thin image).
Film Processing and Handling
Likewise, development time is crucial. Over development will cause the film to be dark; underdevelopment will cause the film to be too light. If it is not fixed for the proper length of time, the film will fade and become too light for comparison with other films.
Splashes of chemicals on the film can cause spots which could be misdiagnosed as caries or other problems. If the film is not washed and dried properly, the film will have a brown haze over it because fixer has dried on the film. These are all problems that can occur if the manual processing method is used.
If the film is not handled properly during processing or mounting, fingernail scratches or fingerprints may appear on the film. If the films are mounted incorrectly, the wrong teeth could be diagnosed with problems and treated by mistake. Films that are misidentified or misplaced usually get lost. It is almost impossible to match the films with the correct patient again.
As with all procedures conducted in a dental office, infection control measures should be followed when exposing radiographs. During most radiography technique, the operator is most likely to come into contact with saliva, which can contain contagious diseases such as viral infections and tuberculosis. To have proper protection against it, one needs to take the following precautions:
• Disinfect all parts of the X-ray unit and chair that will come into contact with your hands during the exposure procedure.
• Always use disposable or sterilizable film holders and bite blocks.
• Disinfect surfaces, which will come into contact with the film packet, before and after exposure to the patient’s mouth.
• Do not keep radiographs in your pocket, before or after exposure. Place them in a paper cup or towel outside of the exposure area.
• When taking X-rays, wear gloves, a mask, and glasses. When developing X-rays, wear glasses and gloves.