CHAPTER 1 Introduction to Dental Radiography
Veterinary Dental Radiology
Dental and oral diseases, many of which cause discomfort and inflammation, are common in dogs and cats. These patients present a diagnostic challenge because they often show no outward sign of their discomfort. Furthermore, many of the methods that assist diagnosis in human patients, such as identification and localization of discomfort, thermal and electrical pulp testing, and local anesthesia testing, are not helpful in animal patients. In addition to these obstacles, most of the pathology-related tissues are below the gingival margin, hidden from direct visualization. So while radiography is a narrow method of adding additional information to that which we get during a physical examination, in the oral cavity it is a tremendously important one. In addition to diagnosing problems, dental and oral radiographs also provide critical information for treatment planning, treatment evaluation, and treatment success.
The information on radiographic images must be carefully considered during interpretation. Radiographic opacities and lucencies can be unreliable at best, and outright misleading at worst. Even among experts, there is large interobserver variation in the interpretation of dental radiographs. It has been shown that dentists significantly improved their diagnostic accuracy in finding radiographically visible features when they were given reference images with which to compare their radiographs. This book provides a readily available collection of reference dental and oral radiographic images, both of normal anatomy and of many of the pathological processes commonly found in dogs and cats.
Basics of Radiographic Principles
The basic principles of dental and oral radiography are similar to those for general radiography—the goal is to achieve high-resolution (ability to visualize and differentiate small objects) images with sufficient contrast and gray scale to be able to see and identify the structures.
Radiographs are images of shadows that are cast by tissues and structures of varying radiopacity. Structures that absorb x-rays are radiopaque, and those that transmit them are radiolucent. The final radiograph merges all the structures in a three-dimensional area into a two-dimensional image. It could be considered similar to taking all the slices of a computed tomography study and printing them all on top of each other on a single page. The interpreter needs to mentally reexpand the flat image into its original multilayered size to help make sense of the information on it. Interpretation of radiographs is often facilitated by evaluating two views taken at 90-degree angles. Unfortunately, due to the adjacent anatomy, radiographs of teeth cannot be made along the mesiodistal (side-to-side) axis. Veterinary dental radiographs are mostly taken along the facial-oral axis. When superimposition of structures interferes with radiographic interpretation, oblique projection radiographs should also be made by changing the horizontal angulation (tube shift) while keeping the vertical angulation the same (see “bisecting angle” technique in Chapter 12).
Interpretation also follows the same rules as for general radiographs, including reading the entire film in a consistent manner and, if reading emulsion films, using a good source of illumination and magnification. An example of a good film-reading routine might be to first scan the entire film for an overall impression of the anatomy, looking for things that are present that should not be and for things that are not present but should be. Then evaluate each tooth crown, the pulp chambers, the root canals, the periodontal ligaments, the lamina dura for integrity, the trabecular bone, and finally the cortical bone.
When evaluating a radiograph, always keep in mind the significance of multiple layers of overlapping structures. An important consequence of this is the “summation effect.” The summation effect is the result of superimposed structures and tissues either adding to the radiopacity (addition) or subtracting from it (subtraction) depending on their relative radiopacities (Figure 1-1