Periapical radiography describes intraoral techniques designed to show individual teeth and the tissues around the apices. Each film usually shows two to four teeth and provides detailed information about the teeth and the surrounding alveolar bone.
• The image receptor should be positioned with its long axis vertically for incisors and canines, and horizontally for premolars and molars with sufficient receptor beyond the apices to record the apical tissues.
The anatomy of the oral cavity does not always allow all these ideal positioning requirements to be satisfied. In an attempt to overcome the problems, two techniques for periapical radiography have been developed:
This positioning has the potential to satisfy four of the five ideal requirements mentioned earlier. However, the anatomy of the palate and the shape of the arches mean that the tooth and the image receptor cannot be both parallel and in contact. As shown in Fig. 7.2, the image receptor has to be positioned some distance from the tooth.
To prevent the magnification of the image that this separation would cause, a parallel, non-diverging, X-ray beam is required (see Fig. 7.3). As explained in Chapter 3, this is achieved usually by having a long focal spot to skin distance (fsd), ideally of 200 mm.
A variety of holders has been developed for this technique. The choice of holder is a matter of personal preference and dependent on the type of image receptor – film packet or digital sensor (solid-state or phosphor plate) – being used. The different holders vary in cost and design, as shown in Fig. 7.4 but essentially consist of three basic components:
The different components of the various holders usually need to be assembled together before the holder can be used clinically. The holder design used depends upon whether the tooth under investigation is:
These variables mean that assembling the holder can be confusing, but it must be done correctly. To facilitate this assembly some manufacturers now colour-code the various components. Once assembled correctly the entire image receptor should be visible when viewed through the beam-aiming device, as shown in Fig. 7.5. A selection of different holders is shown in Fig. 7.6.
Fig. 7.5 A The appearance of the film packet when viewed through the locator ring of a correctly assembled Rinn XCP holder. B The appearance when the image receptor holder has been assembled incorrectly.
Fig. 7.6 A A selection of film packet and digital phosphor plate holders designed for the paralleling technique. Note how some manufacturers use colour coding to identify holders for different parts of the mouth. B Holders incorporating additional rectangular collimation – the Masel Precision all-in-one metal holder and the Rinn XCP holder with the metal collimator attached to the locator ring. C Blue anterior and yellow posterior Rinn XCP-DS solid-state digital sensor holders. D Green/yellow anterior and red/yellow posterior Hawe – Neos holders suitable for film packets and digital phosphor plates (shown here). (See colour plates section)
Typically, the same anterior holder can be used for right and left maxillary and mandibular incisors and canines utilizing a small image receptor (22 × 35 mm) with its long axis vertical. Four images in the maxilla and three images in the mandible are usually required to cover the right and left incisors and canines, as shown in Fig. 7.7.
Fig. 7.7 A The anterior Rinn XCP holder suitable for imaging the maxillary incisors and canines. B Diagram showing the four small image receptors required to image the right and left maxillary incisors and canines. C The same anterior Rinn XCP holder suitable for imaging the mandibular incisors and canines. D Diagram showing the three small image receptors required to image the right and left mandibular incisors and canines. (See colour plates section)
Typically different holders are required for the right and left premolar and molar maxillary and mandibular posterior teeth. The different designs allow the holders to hook around the cheek and corner of the mouth. A large image receptor (31 × 41 mm) is ideally utilized with its long axis horizontal. Two images are usually required to cover the premolar and molar teeth in each quadrant, as shown is Fig. 7.8.
Fig. 7.8 A The posterior Rinn XCP holder assembled for imaging the RIGHT maxillary premolars and molars. B The posterior Rinn XCP holder assembled for imaging the LEFT maxillary premolars and molars. C Diagram showing the two large image receptors required to image the right and left premolars and molars in each quadrant. D The posterior Rinn XCP holder assembled for imaging the RIGHT mandibular premolars and molars. E The posterior Rinn XCP holder assembled for imaging the LEFT mandibular premolars and molars. (See colour plates section)
2. When using film packets and digital phosphor plates the end of the receptor with the orientation dot should be placed opposite the crowns of the teeth to avoid subsequent superimposition of the dot over an apex.
Positioning using solid-state digital sensors: Clinical positioning of holders for the paralleling technique when using solid-state digital sensors can be more difficult because of the bulk and absolute rigidity of the sensor. Those systems employing cables also require extra care with regard to the position of the cable to avoid damaging it. Once the holder is inserted into the mouth, the positioning of the tubehead is the same as described previously when using other types of image receptors and is shown in Fig. 7.17 for different parts of the mouth.
The theoretical basis of the bisected angle technique is shown in Fig. 7.18 and can be summarized as follows: