Prior to presenting technique errors and the means to troubleshoot them, a clear understanding of technique must be established. Although the bisecting angle technique is still utilized and may be necessary in certain circumstances, the paralleling technique is the method of choice for intraoral radiography. However, the use of bisecting angle technique remains relevant due to the increasing use of rigid digital receptors for intraoral radiography. Rigid digital receptors are more difficult to place comfortably in the mouth particularly in children and the posterior regions of the mouth in adults.9-12 In addition, rigid receptors are difficult to place truly parallel to the teeth due to their thickness and rigid construction which contributes to a higher rate of retakes. The use of rigid digital receptors results in more retakes due to technical errors than phosphor plate receptors or film.13 A good understanding of bisecting angle technique is necessary to correct dimensional errors associated with placement difficulties as well as instances when occlusal techniques are utilized for intraoral radiographic imaging particularly with rigid digital receptors.
As previously discussed, the paralleling technique is the most accurate intraoral radiographic technique, meeting four of the five principles of accurate image projection. The paralleling technique does not allow for as short as possible object-to-receptor distance (Principle Three) as desired. In order to achieve parallelism, it is necessary to place the receptor more lingual to the structures where there is adequate depth to place the receptor. The use of receptor-holding devices helps to standardize and execute this technique but attention to the relationship of the receptor to the object(s) of interest is critical to successful outcomes.
The paralleling technique is accomplished by placing the receptor parallel to the long axis of the tooth. After this parallel relationship has been established, the central ray must be directed perpendicular to both the tooth and receptor. Because the receptor cannot always be placed as close as possible to the tooth due to the receptor-holding device or anatomical obstacles, image magnification may occur. However, this can be compensated for by using an increased source-object distance, which decreases magnification and unsharpness. The paralleling technique, when performed correctly, is superior to the bisecting angle technique by producing an image with both linear and dimensional accuracy.
Bisecting Angle Technique
The bisecting angle technique is accomplished by placing the receptor as close to the tooth as possible. The central ray of the x-ray beam should be directed perpendicular to an imaginary line that bisects or divides the angle formed by the long axis of the tooth and the plane of the receptor. Principle Three of accurate image projection is met by using this technique.
Images produced by the bisecting angle method are in true linear relationship. However, this technique does not follow several of the principles of accurate image projection and also requires accurate visualization of the bisected angle in order to accomplish it properly. Bisecting angle images are not anatomically accurate and are prone to shape distortion. However, the bisecting angle provides an alternative technique when parallel placement cannot be achieved or occlusal techniques are employed.
Occlusal techniques may need to be utilized when the arch is too narrow to permit placement of the receptor behind the structures of interest or when anatomical obstacles like tori fill the palate or floor of the mouth.
The bitewing radiographic image is used to examine the interproximal surfaces of the teeth and is particularly useful for the detection of dental caries and alveolar bone levels. The receptor is placed into the mouth parallel to the crowns of the maxillary and mandibular posterior teeth. The patient stabilizes the receptor by biting on a tab or bitewing holder. The central ray of the x-ray beam is then directed through the contacts of the posterior teeth and at a +5º to +10º vertical angle.
Receptor-holding devices or bitewing tabs may be used to stabilize the receptor in the mouth. Receptors may be positioned in the horizontal or vertical dimension with this technique, depending upon the area to be examined. Bitewings may be taken in the anterior segments of the dentition as well. Some dentists, particularly periodontists, may prescribe a 7 image bitewing survey consisting of 4 vertical bitewings in the posterior with size 2 receptor and 3 vertical bitewings in the anterior using size 1 receptor. This permits evaluation for both alveolar bone loss and caries detection.