Section II. BISECTING (SHORT-CONE) PERIAPICAL EXPOSURE TECHNIQUES

4-6. GENERAL

A short cone is used to take x-rays with bisecting angle exposure techniques.

The target-film distance is 8 inches. The resulting image x-ray is somewhat larger using the short cone rather than using a long cone (see figure 4-1). The bisecting plane is halfway between the plane of the dental film and the longitudinal axis of the tooth. The average angle of projection is the angle between the occlusal plane and the angle of the central ray. The angle of the central ray is in relation to the bisecting plane.

Figure 4-1. Comparison of 8-inch and 16-inch target-film distances.

4-7. POSITIONING THE PATIENT

Standard radiographic procedures include precise positioning of the patient's head as one step in placing film. The tissues to be radiographed and the x-ray beam must be in proper relationship to produce an accurate radiographic image. This is particularly important when using the bisecting angle technique. In adjusting the backrest and headrest, it is important to make the patient as comfortable as possible to minimize movement during exposure. As in photography, movement during exposure will result in a blurred image. Blurring may be greatly reduced through the use of ultra-speed film.

1. Head Positioning in Radiography of the Maxilla Using the Bisecting Technique. In radiography of the maxilla, the head should be positioned so that the occlusal surfaces of the maxillary teeth are in a horizontal plane (see figure 4-2). This is done by adjusting the headrest so that the median plane (sagittal plane) is vertical and a line from the ala of the nose to the tragus of the ear is horizontal.

Figure 4-2. Head position for making maxillary periapical radiographs.

2. Head Positioning in Radiography of the Mandible Using the Bisecting Technique. In periapical radiography of the mandible, the head should be positioned so that the occlusal surfaces of the mandibular teeth will be horizontal when the mouth is opened to the position in which the radiographs are to be made (see figure 4-3). This is done by adjusting the headrest so the median plane is vertical and a line from the corner of the mouth to the tragus of the ear is horizontal.

Figure 4-3. Head position for making mandibular periapical radiographs.

3. Angulation. When the cone is adjusted to project the central beam upward, it will be set at a negative (-) degree angulation. When it is adjusted to project the central beam downward, it will be set at a positive (+) angulation.

4-8. CENTRAL RAY ANGULATION

The angle of the x-ray beam, the average angle of projection of the central ray, is essential for successful use of the bisecting exposure techniques. Both vertical and horizontal angulations must be considered.

1. Vertical angulation is the up-and-down movement of the tube head or x-ray beam. The correct vertical angulation exists when the central ray is directed perpendicular to the bisector of the angle formed by the long axis of the tooth and the plane of the film (see figure 4-4). When this angulation is correct, the vertical dimension of the tooth will be as realistic as possible. Incorrect vertical angulation may cause two problems--foreshortening or elongation.

Figure 4-4. Correct projection of central ray.

(1) Foreshortening exists if the vertical angulation is larger than necessary. The image of the teeth appears smaller than normal (see figure 4-5).

(2) Elongation exists if the vertical angulation is less than is necessary. The image of the teeth appears larger than normal (see figure 4-6).

Figure 4-5. Foreshortened image caused by projection of central ray from an angle that is too great.

Figure 4-6. Elongated image caused by projection of central ray from an angle that is too small.

2. Horizontal angulation is the side-to-side movement of the tube head or x-ray beam. Correct horizontal angulation for successful radiographs exists when the central ray is perpendicular to the facial surfaces of the teeth and parallel to the mesial and distal surfaces (see figure 4-7). If the horizontal angulation is incorrect, overlapping will occur on the radiograph. Overlapping results when the proximal surfaces of adjacent teeth are superimposed over one another (see figure 4-8). When this occurs, there will be a light area where the two teeth are overlapped or superimposed. The resultant light area is the inability of the x-ray beam to penetrate the two dense surfaces. Correct horizontal and vertical angulation of the x-ray beam is necessary to obtain radiographs of diagnostic quality.

Figure 4-7. Correct image resulting from proper horizontal projection of the central ray.

Figure 4-8. Overlapping images caused by incorrect horizontal projection of the central ray.

4-9. OVERVIEW OF THE BISECTING ANGLE EXPOSURE TECHNIQUES

a. The bisecting method of periapical radiography is used to varying degrees in Army dental clinics. The dental specialist should be familiar with its techniques. The following paragraphs describe techniques using this method to produce a 14-exposure set of radiographs of an adult dentition. Discussion of the techniques for exposure of each area of the mouth will include illustrations and descriptions of film placement, film holding, direction of the central ray, and average angulation. Slight contouring of a corner may facilitate film placement in restricted areas of the mouth, such as those of the maxillary incisors or the maxillary third molar, and make it more comfortable. To shape the film, press it over the rounded contour of a finger to pre-adapt it to the position in the mouth. Use only slight contouring to prevent distortion of the image. (A flat image produces the best image.) Paragraph 4-7 covers proper head positioning and other instructions for intraoral radiography.

b. Various film holding devices may be used to secure the film in place when using the bisecting technique: the Rinn EEZEE-GRIP Film Holder, a hemostat, a plastic or Styrofoam film holder (with 105º angle) (see figure 4-9). When such film holding devices are not available, the finger or thumb may be used.

Figure 4-9. Film holding devices for the bisecting technique.

4-10. MAXILLARY MOLARS

Adjust the head as described for radiographs of maxillary teeth (refer to paragraph 4-7a). Place the film packet in the mouth so that its long axis is horizontal, the anterior border of the film is lingual to the mesial border of the second bicuspid, and the lower border of the film is parallel to and slightly below (approximately 1/4 inch) the occlusal surfaces of the molars. The upper corner of the packet may be contoured slightly but the film packet should not be bent. Adjust the tube to an average angulation of +20º. Direct the central ray straight through the interproximal spaces in the area of the second molar and perpendicular to the bisecting plane (see figure 4-10). Follow the manufacturer's instructions for all exposure times.

Figure 4-10. Maxillary molar area.

4-11. MAXILLARY BICUSPIDS

Adjust the head as described for radiographs of maxillary teeth. Place the film packet in the mouth so that its long axis is horizontal and its anterior border is lingual to the mesial surface of the cuspid. Have its lower border paralleled to, and slightly below, the occlusal surfaces of the teeth (approximately 1/4-inch). Adjust the cone to an average angulation of +30º. Direct the central ray straight through the interproximal spaces of the first and second bicuspids at the center of the film and perpendicular to the bisecting plane (see figure 4-11). Follow the manufacturer's instructions for all exposure times.

Figure 4-11. Maxillary bicuspid area.

4-12. MAXILLARY CUSPIDS

Adjust the head as described for radiographs of maxillary teeth. Place the film packet in the mouth so that its long axis is vertical and its lower border is parallel to and slightly below (approximately 1/8 inch) the incisal edges of the lateral incisor and cuspid teeth. The anterior border of the film should lie lingual to the central incisor of the same side. The upper anterior corner of the film may be contoured slightly to fit the curvature of the maxillary arch. Adjust the cone to an average angulation of +45º. Direct the central ray straight at the cuspid at the level of the root and perpendicular to the bisecting plane (see figure 4-12). Follow the manufacturer's instructions for exposure times.

Figure 4-12. Maxillary cuspid area.

4-13. MAXILLARY INCISORS

Adjust the head as described for radiography of maxillary teeth. Gently contour both long borders of the film slightly in the direction of the curvature of the palate. Place the film packet in the mouth so that its long borders are vertical and its center is in line with the median plane of the upper arch. Have the lower border of the packet slightly below (approximately 1/8 inch) and parallel to the incisal edges. Adjust the tube to an average angulation of +40º. Direct the central ray to pass through the tip of the nose in line with the median plane and perpendicular to the bisecting plane (see figure 4-13). Follow the manufacturer's instruction for exposure times.

Figure 4-13. Maxillary incisor area.

4-14. MANDIBULAR MOLARS

Adjust the head as described for radiographs of mandibular teeth (paragraph 4-7b). Place the packet in the mouth with the long axis horizontal and the upper border of the film parallel to, and slightly above (approximately 1/4 inch) the occlusal surfaces of the molar teeth. Relieve the lower anterior border by contouring. Place the packet alongside the tongue and far enough distally to include the entire third molar area. Impacted or malposed mandibular teeth may require special positioning of the film packet. Adjust the tube to an average angulation of -5º. Direct the central ray straight through the interproximal spaces at the center of the film and perpendicular to the bisecting plane (see figure 4-14). Follow the manufacturer's instructions for exposure times.

Figure 4-14. Mandibular molar area.

4-15. MANDIBULAR BICUSPIDS

Adjust the head as described for radiographs of mandibular teeth. Place the film packet in the mouth with its long axis horizontal and its upper border parallel to and slightly above (approximately 1/4 inch) the occlusal surfaces of the teeth. Locate the anterior border of the film lingual to the mesial surface of the cuspid. The lower anterior border of the film should be contoured slightly to fit the curvature of the mandibular arch. Adjust the tube to an average angulation of -10º. Direct the central ray straight through the interproximal spaces at the center of the film and perpendicular to the bisecting plane (see figure 4-15). Follow the manufacturer's instructions for exposure times.

Figure 4-15. Mandibular bicuspid area.

4-16. MANDIBULAR CUSPIDS

Adjust the head as described for radiographs of mandibular teeth. Place the film packet in the mouth with its long axis vertical and its upper border parallel to and slightly above (approximately 1/8 inch) the incisal edges of the lateral incisor and cuspid teeth. The film's anterior border should be located lingual to the distal surface of the opposite central incisor. Adjust the tube to an average angulation of -20º. Direct the central ray straight through the bisecting plane (see figure 4-16). Follow the manufacturer's instructions for exposure times.

Figure 4-16. Mandibular cuspid area.

4-17. MANDIBULAR INCISORS

Adjust the head as described for radiographs of mandibular teeth. Place the film packet in the mouth with the long axis vertical. Both the long borders of the packet should be placed under the tongue with the center of the film opposite the midline of the arch and the upper border parallel to and slightly above (approximately 1/8 inch) the incisal edges of the incisor teeth. Adjust the tube to an average angulation of -15º. Direct the central ray straight through the interproximal spaces at the center of the film and perpendicular to the bisecting plane (see figure 4-17). Follow the manufacturer’s instructions for exposure times.

Figure 4-17. Mandibular incisor area.