Dental radiography

Chapter 7


Dental radiography



Introduction


Radiography is a vital diagnostic tool in veterinary dentistry. The bulk of the tooth, i.e. root and most of the periodontium, can only be visualized by means of radiographs. Consequently, a lot of pathology will remain undiscovered if clinical examination does not involve radiography. While lesions such as caries can be recognized without radiography, it is not possible to assess the full extent of the lesions or if there is pulpal and periapical involvement. In other words, a clinical examination is incomplete without radiography.


Periodontal disease, endodontic disease, caries, resorptive lesions, fractures, bone pathology and neoplastic conditions all require radiography for a more complete diagnosis, thus allowing optimal planning of treatment. It is also necessary to know the normal radiographic anatomy to be able to identify the abnormal. Many dental procedures can only be carried out under radiographic control. Checking adequacy of procedures and success of treatment also relies heavily on radiography. In short, radiographs are required to reach a diagnosis and thus plan treatment optimally, to be able to perform certain procedures, and to be able to assess the outcome of treatment performed. Practicing dentistry without radiography as a tool would be considered negligent in human dentistry. The same applies to veterinary dentistry.


Pathologic radiographic changes are usually discrete and therefore clarity and detail are essential. For a dental radiograph to be diagnostic, it should be an accurate representation of the size and shape of the tooth without superimposition of adjacent structures (Figs 7.1, 7.2). Intraoral radiographic techniques are therefore required – a parallel technique for the mandibular premolars and molars, and a bisecting angle technique for all other teeth. Contralateral (same teeth, opposite side) views should be taken as routine.




image


Fig. 7.2 A diagnostic view
For a dental radiograph to be diagnostic, it should be an accurate representation of the size and shape of the tooth without superimposition of adjacent structures. Intraoral placement of dental film and parallel technique gives an accurate representation of the mandibular 3rd and 4th premolars and the 1st molar, as well as detail of the mandibular bone, in a cat. The mesial surface of the 3rd premolar is not on the film and a second view with the film placed further rostrally in the mouth is required to assess this tooth fully. In this view, the component structures of the tooth and its supporting tissues are well defined. The enamel is seen as an incompletely visualized radiodense band that covers the crown and tapers to a fine edge at the cervical margin of the tooth. The dentine is less radiodense than enamel and accounts for the bulk of the hard tissues of the tooth. The cementum is not visible. The pulp cavity is the continuous radiolucent space in the center of the tooth which extends from the coronal portion to the apex of the roots. The wall of the alveolar tooth socket (the lamina dura) is the radiodense line which runs parallel to the root of the tooth. The periodontal ligament space is the fine radiolucent line between the lamina dura and the root of the tooth. The cortical bone on the crest of the alveolar ridge is continuous with the lamina dura. The mandibular canal is clearly visible. The horizontal bone loss affecting the distal root of the molar is obvious.


Intraoral radiographic techniques do require some time and patience to master, but once this has been achieved they provide invaluable information, with minimal loss of time, which allows optimal planning and performing of dental treatments. Attending a practical course is particularly valuable in learning these techniques.



Equipment and materials (Box 7.1)


Dental radiographs can be taken using conventional (film) or digital (digital sensors) methods. Digital radiography refers to computer-generated images of radiographs. In indirect digital systems the exposure is made on a photostimulatable phosphor plate (Fig. 7.3), rather than on a dental film. The latent image from the plate is then transferred to a computer. Direct-to-digital equipment uses a sensor (Fig. 7.4) as a data input device that sends the image directly to the computer. Most modern X-ray machines are compatible with most digital software and hardware packages. Older X-ray machines may not be capable of short enough exposure times (as low as 0.02 s for small patients).





Digital radiography has many advantages as compared to conventional. The main advantages of digital radiography are the following:



I would strongly recommend that all veterinarians in small animal practice invest in a digital dental radiography system. The whole procedure of taking radiographs becomes so much simpler and quicker. In my experience, the resistance to using dental radiography as a diagnostic tool has been largely due to the ‘hassle’ factor (time consuming, messy, storage issues). With digital radiography, all these problems are removed and radiographs are thus more likely to be taken and disease processes identified and treated.



The X-ray unit


A dental X-ray machine is preferable to a veterinary X-ray machine. The dental unit has a freely maneuverable head that allows accurate positioning of the film with minimal adjustment in patient position. The cone of the dental unit will collimate the beam and provide the optimal film–focus distance.


Most veterinary X-ray machines can be used for dental radiography, but the film–focus distance will need to be adjusted to between 30 and 50 cm. The more maneuverable the head (in angulation and positioning) the better it is for intraoral techniques. With the less maneuverable units, it is necessary to position the animal differently for each area requiring investigation.


Ideally a dental X-ray machine should be installed in the designated dental theatre (Fig. 7.5). They are available as wall mounted or free-standing units. These machines are cost-effective and their outlay is rapidly recouped. They usually have a fixed potential of 50–70 kV and a fixed tube current of 8–10 mA. Electronic timers are used to set the desired exposure time. Modern dental X-ray machines are capable of exposures as low as 0.02 s, which is required for digital radiography. Do remember to ensure that the X-ray machine is compatible with the digital system you purchase.




X-ray film


Single emulsion, non-screen or screen film can be used to take dental radiographs. To allow intraoral film placement and achieve high definition, dental film should be used. Dental film is single emulsion, non-screen, and is available in three sizes (Fig. 7.6): occlusal, periapical and pediatric. The dental film is packed in either a paper or a plastic envelope and the film is flanked by black paper and backed by a thin lead sheet (foil) that reduces scattered radiation.



Dental film is available in a range of ‘sensitivities’ (film speeds) from A (least sensitive) to F (most sensitive). The sensitivity of the film determines the required exposure time – faster speed (higher sensitivity) requires less radiation to expose (blacken) the film. The film sensitivity is increased by using larger silver halide crystals in the emulsion. Fast speed (higher sensitivity) produces images with a slightly lower image quality (cf larger and fewer pixels reduce the image quality of digital camera images). The decreased the image quality is only significant when the radiographs are viewed under magnification. In practice, depending on the type of processing used, D (ultra) and E (ekta) speeds are often used. D speed film requires slightly longer exposure and can be processed manually. E speed film requires a lower exposure but can only be developed in an automatic processor.



Orientation


Ensure the correct side of the film envelope is facing the incident beam; the envelope is marked or labeled. If exposed through the back of the envelope, the lead sheet will absorb much of the X-ray beam, resulting in an underexposed radiograph with the pattern of the lead sheet imposed on it.


Each film has a raised dot in one corner. The dot helps with orientation when viewing and mounting dental radiographs if the following procedure is adhered to. First, the dot should face the incident beam. Second, the film should be placed in the mouth so that the dot is always facing a specific direction. I position the dot so that it is always facing forward in the mouth. Another way is to ensure that the film is placed so that the dot is always in the same position, i.e. facing forward in the mouth on one side and backward in the mouth on the contralateral side.



Digital systems


In indirect digital systems the conventional film is replaced by a photostimulatable phosphor plate (Fig. 7.3). The plate is exposed and the latent image is transferred to a computer. The plates come in three sizes equivalent to dental film sizes. The plates are reusable in that the latent image can be cleared by placing them in light, e.g. against a viewing box.


In direct digital systems, the conventional film is replaced by a sensor that sends the X-ray image directly to the computer (Figs 7.4, 7.7). In my opinion, this is the easiest and most cost-efficient method of obtaining radiographs. The sensors are available in two sizes, equivalent to periapical and pediatric film. The occlusal size is not available. In my experience, the size 2 sensor is the only one needed for dogs and cats. The smaller, size 1 sensor may be useful in cats and small exotics.


Oct 9, 2016 | Posted by in GENERAL | Comments Off on Dental radiography
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