31
Radiological anatomy of the oral cavity

Chapter contents

31.1 Introduction

The radiographs most frequently taken in general dental practice are of the teeth and their immidiate supporting tissues for detection of dental caries or assessment of bone loss in periodontal disease. Intraoral radiographs are taken by placing the X-ray-sensitive film or receptor in the mouth close to the teeth being investigated. Extraoral radiographs use larger films or receptors positioned externally and produce a view of the entire dentition and its supporting structures on a single film; they are used to ascertain the state of development of the dentitions prior to orthodontic treatment, for example. Dental panoramic tomographs (DPTs) are the most frequent extraoral radiographs.

A radiograph is a negative photographic record. Dense structures such as bone are designated as radio-opaque; they absorb some X-rays and appear white on radiographs. More X-rays pass through less dense radiolucent structures such as air-filled cavities which show up as black areas. The contrast between different tissues of the structures which the X-ray beam passes through is determined by their radiodensity which, in turn, is largely due to their content of metallic elements. Calcium and iron are the prevalent heavy metals in the body. Calcium is combined with phosphate to form hydroxyapatite crystals in bones and mineralized tissues in teeth. Iron is present in haemoglobin in blood, but only large concentrations of blood, such as those found within the heart chambers, show up on X-rays. In sequence from densest to most lucent, the radiodensity of the dental and periodontal tissues are: enamel, dentine, cementum, compact bone, cancellous bone, demineralized carious enamel and dentine, dental soft tissues such as pulp and periodontal ligament, and air; gold and silver–mercury amalgam metallic restorative materials are even denser than enamel.

A radiograph is a two-dimensional representation of a three-dimensional situation. The orientation of anatomical structures relative to the X-ray beam is a major factor determining their appearance on the film. For example, a beam travelling through the long axis of a radiodense structure will produce a whiter image on the film than one passing through its shorter axis because more X-rays are absorbed; the structure will also have a different shape. Orientation also affects the degree to which structures are superimposed on each other on the radiograph; it is necessary to carefully position the part being X-rayed so that it is not obscured by another superimposed radio-opaque structure.

There are many factors that have to be considered when taking radiographs which involve the physics of X-rays and other technical aspects are outside the scope of this book.

Extraoral radiographs reveal extensive anatomy of structures adjacent to the teeth and their supporting tissues whereas smaller intraoral views only show limited areas. It is easier to appreciate the anatomy on extraoral views; the structures visible on different intraoral radiographs can then be related to the bigger picture.

31.2 Dental panoramic tomography

Dental panoramic radiographic machines work on the principle of tomography. This involves simultaneously moving the film and the X-ray source in opposite directions to each other around the patient’s head. The procedure is generally quicker and more comfortable for the patient because there are no intraoral films (see Section 3.4). As you can see in Figure 31.1 the resulting radiograph appears as a flat representation extending from one temporomandibular joint to the other, showing a complete picture of the entire upper and lower dentition and their supporting alveolar processes in the mandible and maxilla; these appear as a sharp image whereas structures in all other planes are blurred and effectively invisible. In DPTs, the teeth and supporting structures do not show with the sharpness and detail seen in intraoral radiographs. Panoramic radiographs are particularly useful when examining developing dentitions, large lesions, or fractures of the jaws when fine detail is often not required. For fractures, panoramic radiography has the great advantage that it can be performed without the patient needing to open their mouth.

Several anatomical structures are visible on D/>

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