Step 3: Positioning the beam head
Accurate beam head placement in relation to the patient and film is the most challenging aspect of veterinary dental radiography.
However, once correct positioning is mastered, dental radiography becomes much easier and more efficient.
There are two common techniques for positioning the beam head in dental radiography—the parallel and the bisecting angle techniques.
Parallel technique. By strict definition, this technique is used only on the mandibular premolar and molar teeth. All other teeth have anatomical
structures (palate or mandibular symphysis) that preclude parallel film placement. For this technique, place the film parallel
to the tooth and perpendicular to the X-ray beam (Figure 3).7 This technique is similar to standard radiographic techniques for other parts of the body (e.g. spine, thorax). The parallel technique provides the most accurate image.
Bisecting angle technique. This positioning technique is the most common one used in veterinary dentistry since the parallel technique cannot be used
for most of the teeth. For this technique, place the film as parallel as possible to the tooth root. Then measure or approximate
the angle between the long axis of the tooth root and film. Finally, divide this angle in half (bisect the angle) and direct
the incident X-ray beam perpendicular to this imaginary angle (Figure 4) (see "Understanding bisecting angles").7
Understanding bisecting angles
Step 4: Setting the exposure
This step involves determining the amount of radiation that will be used to expose the image. Exposure settings in dental
radiography differ from those in standard radiography in that the kVp and mA are held constant. Only the time is adjusted.
Table 1 Dental Radiography Technique Charts
Dental radiography machines used in people* require manually setting the exposure; you will need to create a technique chart
for your system. This chart is similar to the one used for a standard radiography machine, except only one variable, exposure
time, needs to be adjusted. Start with the sample charts provided, and adjust to your individual machine (Table 1). Note that the settings for direct digital systems (sensors) are much lower than those for standard film (see "Digital dental radiography advantages and disadvantages"). With indirect digital systems, such as phosphor plate, the settings are similar to or higher than those for standard film,
and phosphor plates have a wide exposure range.
Digital dental radiography advantages and disadvantages
If you are using a computer-controlled system,** set the controls for the species, size of the patient, and tooth to be imaged.
If the setting is proper and the exposure is incorrect, the easiest way to make corrections is to change the f setting. By pressing the f button, both numbers in the windows will go up. The one on the left is the f number, and the one on the right is the exposure time. If you continue to press the button, it will continue to increase
the exposure time until the f number reaches 9, when it will markedly decrease and the f number will go back to 1. If the radiograph is overexposed (too dark), decrease the f number by 1. If it is underexposed (too light), increase the number by 1. Continue this process until you have correct film
exposure. Generally, the f number will be the same for all radiographs on a given machine.