Variables of Intraoral Imaging
There are four variables involved in acquiring radiographic images: horizontal angulation, vertical angulation, sensor placement, and head position. When aiming instruments are used, the horizontal and vertical angulations as shown by the position of the aiming ring, but as we will learn, the aiming ring is not always in the ideal position. When using simple sensor holders, such as sticky tabs, without an aiming device, the operator must understand how to manage the four variables.
Horizontal angulation is responsible for ‘opening’ interproximal contacts and embrasures, both of which are critical for caries diagnosis and periodontal bone assessment. The horizontal angulation of the PID (position indicating device, see figure 1) is ultimately determined by the position of the teeth. Changing the horizontal angulation can capture the distal of a forward canine or a posterior molar and can separate multiple endodontic files and master cones.
Vertical angulation is responsible for capturing the entire tooth and root and surrounding bone. Creating an image that is equal or nearly equal to the actual length of the tooth is critical for periapical diagnosis and endodontic procedures. Elongation and foreshortening are distortions that result from insufficient and excessive vertical angulation, respectively. When acquiring bitewings, vertical angulation is responsible for capturing maxillary and mandibular alveolar crest levels. When the patient has periodontal bone loss, a vertical bitewing holder increases the chances of capturing alveolar crest bone levels on the image, as the length of the sensor is positioned ‘up and down’, as opposed to the normal horizontal position.
|Table 1. Suggested Vertical Angulations for Periapical Images. |
Occlusal Plane Parallel To The Floor
|Maxillary||+40 degrees||+45 degrees||+30 degrees||+20 degrees|
|Mandibular||-15 degrees||-20 degrees||-10 degrees||-5 degrees|
Sensor placement: The operator may have little ability to change the sensor’s position in the mouth because intraoral anatomy and the amount of room available in the oral cavity dictate where the sensor will rest. When the arch is larger and there is extra room in the mouth, the operator has some flexibility when placing the sensor. When the mouth is small, the sensor can only position where there is room; the sensor and aiming ring’s placement for the teeth of interest may be compromised. Blindly following an out-of-position aiming ring will create poor images.
Head position should be managed by the operator, even when using aiming instruments. The operator’s perspective is better when the patient’s skeletal midline is straight, perpendicular to the floor, and the occlusal plane is parallel with the floor. Placing the PID requires determination of the horizontal and vertical angulations needed for the particular area and the purpose of the intended image. It is much easier to assess the position of the teeth in the jaws and to place the sensor parallel with the teeth when the occlusal plane is at 0 degrees (parallel with the floor), as in Figure 4.