Periapical radiography describes intraoral techniques designed to show individual teeth and the tissues around the apices. Each image 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. 9.2, the image receptor has to be positioned some distance from the tooth.
To prevent the magnification of the image that this separation would cause, an X-ray beam as non-divergent as possible is required (see Fig. 9.3). As explained in Chapter 5, this is achieved by having a long focal spot to skin distance (fsd), ideally of 200 mm.
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. 9.5.
The choice of holder is a matter of personal preference and dependent upon the type of image receptor – film packet or digital sensor (solid-state or phosphor plate) – being used. A selection of different holders is shown in Fig. 9.6.
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. 9.7.
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. 9.8.
Positioning clinically using film packets and digital phosphor plates is shown in Figs 9.9–9.16 for the following different areas of the mouth:
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.
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. 9.17 for different parts of the mouth.
The theoretical basis of the bisected angle technique is shown in Fig. 9.18 and can be summarized as follows: