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The practical aspects of dental radiography

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01 August 2017, at 12:00am

MATTHEW OXFORD explains the differences between the two main types of dental x-ray systems and evaluates the two broad options for positioning for best results

DENTAL RADIOGRAPHY IS A GROWING FIELD in general veterinary practice, and rightly so. Dental radiography opens a huge volume of information that allows accurate decision making regarding disease processes and treatment options. For a long time, dental radiography was only used by those professionals who carried out advanced dental procedures. But now, in a straw poll of delegates attending various CPD events and lectures, my estimation is that perhaps a third of veterinary  practices now have dental radiography. This article is going to look at some of the practical aspects of dental radiography, including different types of system that are available, and how best to get diagnostic images. 

Dental radiography systems 

As with standard x-ray systems, there are two broad categories of dental radiographic equipment for image capture. There are indirect (or CR) systems and direct (or DR) systems. 

CR systems 

CR systems can be further broken down into dental-specific systems (Figure 1) and systems used to adapt existing standard image capture cassettes (Figure 2;  verleaf). These systems utilise digital phosphor films, which are exposed to x-rays while positioned within the mouth. The film is then removed from the mouth and placed within a processor which reads the film and produces an image on a computer screen. For each exposure, a barrier envelope is required to protect the film, which is then removed as the film is placed in the processor. For dentalspecific systems, films are processed individually. For the adaptor cassettes, several films are mounted within a standard CR cassette which can then be processed together. CR systems usually have more than one size of film, often with size 2 and size 4 films available. CR films have a lifespan of approximately one to two years depending on their use, as they will become scratched and will therefore need to be replaced.

DR systems

DR systems utilise a digital sensor which is connected to the computer by a wire, usually to the USB port (Figure  3). The sensor is exposed to x-rays while positioned within the mouth. The sensor then processes the image, which will appear on the computer screen within a few seconds, without any need for further processing. A new barrier envelope is required for each patient. DR systems in general only have a single size 2 sensor available. DR sensors are very hard-wearing and will last many years. Their weak point is often the point where the cable attaches, which can often be replaced if damaged.

Image quality (Table 1)

For the most part, dental-specific systems, either CR or DR, have a very similar image quality. Image quality and resolution in dental radiography is measured in line pairs per mm (lp/ mm). The higher the number, the better the resolution. Most dental-specific systems have a resolution of over 25lp/mm.1,2 Some CR systems quote higher theoretical lp/mm resolutions, but this is achieved using single-use films. One must be very careful of some dental-specific
systems, as not all offer such a high resolution. Systems used to adapt existing standard CR x-ray do, however, often fall behind the dental-specific systems. Typical CR conversion systems have a
resolution of 5-8lp/mm, which is only 20-30% of the dental-specific systems. Reduced image resolution can make assessment of pathology difficult. In particular, assessment of tooth resorption in cats can be much more challenging, as the clearest image possible of the periodontal ligament space is required to be able to assess
external root resorption (Figure 4).

Flexibility

With their variety of film sizes, CR systems are more flexible than DR systems. Indeed, CR systems can be used outside of just dental
imaging and can be used in the radiography of distal limbs and for exotic animal work. 

Ease of use 

DR systems, in the author’s opinion, are far easier to use. In particular, they make it far easier to learn dental radiography techniques. As the DR sensor stays within the mouth for processing,
all the equipment (sensor and generator) stays in the same place between exposure and the image appearing. This is important, as especially when learning dental radiography positioning techniques, the position of either the sensor or the generator can be wrong. As the equipment doesn’t move, it is very simple to adjust the positioning to improve the image. With CR systems, as the film is removed from the patient’s mouth for processing each time, an additional variable is created. Therefore, the clinician must remember where the film was positioned if any adjustments need to be made to positioning. Secondly, the speed of image production is far faster with a DR system than with a CR system. There is always an element of time taken for film processing with a CR system that is not required for DR.

Positioning for dental radiography

There are again two broad options for learning how to position for dental radiography: either the clinician can position the patient according to a set of predetermined angles, or they can use a bisecting angle technique. Learning radiography from a set of predetermined angles may at first seem the most straightforward way of producing images. However, like many shortcut techniques, it has its limitations. As vets, we treat a vast number of dog breeds with a huge variation in their size and relative anatomy. Therefore, if we just try to use a set of predetermined angles, then we will soon find that many of our patients don’t fit those angles. If having taken the image the clinician isn’t happy with the result, unless they understand the principals of how the original angle was generated, there is little chance of being able to improve that image by repositioning. Sometimes this can be overcome by taking
an image of a full dental arcade. The drawback of this is that it goes against our need for dental radiography, whereby we are aiming to focus down on one or two teeth to avoid the problems
of superimposition and artefact from beam divergence. This is where the necessity to learn the bisecting angle technique becomes apparent. By learning this technique, the clinician can calculate the correct angle that the x-ray generator should be placed at by assessing the position of the film/sensor relative to the long access of the tooth. It can be used therefore for any tooth with any film/ sensor position. It is not a difficult technique to learn and once it is mastered, dental radiographic positioning becomes very straightforward. Numerous texts, articles and courses are available that cover the bisecting angle technique (Gorrel, 2004).

Full mouth radiography or not?

Full mouth radiography is mandatory for dental assessment in all cats undergoing dental treatment for the assessment of tooth resorption. This is incredibly common and the only way that pathology can be assessed and treatment options decided upon is by using dental radiography (Lommer and Verstraete, 2000). A large proportion of lesions cannot simply be assessed by oral examination
as the lesions cannot be seen in their full extent, and this will affect treatment decision-making. The situation in dogs is different. While much of the disease processes still affect the roots or their surrounding bone, there is often an indicator that pathology is present that can be assessed by dental examination under anaesthetic. The pathology still needs to be assessed radiographically to assess the most appropriate treatment option. It is therefore more commonplace to simply radiograph teeth identified as being affected by pathology to formulate a treatment plan.

Conclusion

This article has covered some of the more practical aspects of dental radiography posed to practices and clinicians. These are becoming more apparent as more and more practices look to invest in dental radiography systems. 

References

  1. Lommer, M. J. and Verstraete, F. J. M. (2000) Prevalence of odontoclastic resorption lesions and periapical radiographic lucencies in cats: 265 cases (1995-1998). Journal of the American Veterinary Medical Association 217 (12): 1,866-1,869.
  2. Gorrel, C. (2004) Dental Radiography. In: Veterinary Dentistry for the General Practitioner; Elsevier; pp57-68.