Introduction

The short answer to the question posed in the title is that one can see anything that is radio-opaque. That creates a problem; the oral cavity has too many radio-opaque structures. Added to this is the positioning challenge imposed by the size and shape of the oral cavity. Intraoral technique using dental film or small sensors eliminates some superimposition. However, an ideal radiographic study includes two views (using parallel technique) that are perpendicular to each. The second view repositions the third dimension that was collapsed (the axis parallel to the X-ray beam) to the expanded dimension that is perpendicular to the X-ray beam. This allows three-dimensional reconstruction. In the mouth we rarely use parallel technique, and perpendicular views are usually not possible.

Radiographs are needed to identify and characterise many of the dental and oral problems of animals. They also provide important information related to treatment planning, procedural decision making, follow-up evaluation and medical record keeping.

Making a good-quality radiograph requires appropriate patient and tube positioning, proper film or sensor positioning, and correct exposure settings. It also requires proper film handling and developing if film is used. But even the perfect radiograph is of little value without the ability to interpret the information on the image; it can appear as a mysterious blur of blended and overlapping shadows. Druin Burch stated: 'Nothing is quite so strange as that which is half familiar.' Familiarity with normal and abnormal radiographic images is a tremendous help when reading dental films.

Indications for making dental or oral radiographs:

 Fractured teeth--to look for endodontic disease (evidence of periapical lucency, internal resorption, immature pulp)

 Discoloured teeth--to look for endodontic disease that resulted from the trauma that caused the previous pulp haemorrhage

 Worn teeth--to look for evidence of endodontic disease caused by ingress of bacteria

 Dental resorptive lesions--to determine the type, extent and best treatment option

 Regions affected by periodontitis--to evaluate the extent of bone loss, and to look for endodontic extension

 Super-erupted teeth--to evaluate alveolar health and the best treatment option

 Prior to most extractions--to look for root fractures, complex root anatomy or fragile supportive bone

 Missing teeth--to find submerged or impacted teeth or persistent root fragments

 Anomalous or malformed teeth--to evaluate the nature and severity of the anomaly

 Sneezing or nasal discharge--to evaluate turbinate integrity and to look for intranasal masses or foreign bodies

 Oral trauma--to evaluate mandibular fractures, temporomandibular joint injuries or displaced teeth

 Routine full mouth radiographs (possibly?)--for identifying unapparent lesions and for baseline data

Interpretation

Radiographs are the shadows made by tissues and structures that absorb X-rays travelling from the tube to the film or sensor. All the superimposed structures along the way add together to make a sometimes confusing muddle of two-dimensional (2-D) shadows as the third dimension is collapsed to a plane (Figure 1). Consider that a CT study of teeth and the surrounding skull may form a separate 2-D image for every millimeter of depth. If all these images were transferred to transparencies that were laid on top of each other, that would approximate the image that is made with a radiograph.

Interpretation of the image requires the practitioner to re-inflate the hodge-podge of black, white and shades of grey to make a mental reconstruction of its original 3-D structure.

Most veterinary dental radiographs are taken along the facial-oral axis (vestibular-lingual, buccolingual) (Figures 2 and 3). Additional radiographs can be made by changing the horizontal angulation (tube shift) while keeping the vertical angulation unchanged to modify the relationship of superimposed structures to each other.

Interpretation of dental radiographs is similar to reading other types of radiographs:

 Make good quality images (positioning, technique)

 Develop properly for a high-quality image

 Read the entire film in a consistent manner

 If there is any uncertainty or doubt, make an additional view

 When interpreting emulsion films, use magnification and a good source of illumination

 When interpreting digital images, take advantage of image manipulation (magnification, contrast/brightness, etc.) when needed

 Scan the radiograph for an overall impression of the anatomy

 Look for shadows that are present but should not be or that appear abnormal

 Look for shadows that are not present but should be there

 Then begin a detailed evaluation of each tooth crown, pulp chamber, the roots, the root canal spaces, the periodontal ligaments, the laminae durae, the trabecular bone and the cortical bone

The 'summation effect' is the result of superimposed structures and tissues either adding to, or subtracting from, other structures depending on their radio-opacity. Multiple overlapping tissue-dense structures can add together to make a bone-dense shadow; or a soft-tissue structure over bone can give the appearance of increased bone density in the area. Alternately, a radiolucent structure superimposed over bone can make it appear regionally less dense.

A similar type of summation (addition or subtraction) occurs due to the 'tangential effect'. When a structure is perpendicular to the X-ray beam its effect appears diminished. However, when a structure is parallel to the X-ray beam its effect is emphasised. This is true of both radio-opaque as well as radiolucent structures. An example of this effect is the white line, or lamina dura, that represents the wall of the alveolus on a radiograph (Figure 3). This is visible where the bony plate parallels the X-ray beam, but it disappears where it is perpendicular to the beam. Another example is the obvious black line visible at the site of a root fracture when the fracture plane is aligned parallel to the beam. The radiographic lesion can completely disappear, however, when the fracture plane is perpendicular or oblique to the beam.

The technique of 'tube shift' is employed to make a second radiograph with a different view from the first. While we cannot make a 90 degree angle view while maintaining correct horizontal angulation, we can make an oblique-view radiograph. When this is done, objects that are close to, or superimposed over, each other in the 2-D radiograph will remain close if they are in the same tissue plane that is perpendicular to the X-ray beam (same tissue depth along the beam) but will move relative to each other if they are at a different tissue depth. For example when the middle mental foramen is superimposed over the apex of the mesial root of the mandibular 2nd premolar tooth it can appear similar to a periapical lucency of endodontic origin. Changing the tube angle by shifting the tube and repeating the exposure will separate the lucency of the foramen from the root tip confirming that the 2-D radiographic 'periapical' lucency was actually a subtraction effect of a superimposed lucency at a different depth from the root tip.

One of the most common pathology-related radiographic abnormalities is lucency around a root tip caused by endodontic disease (Figure 4). Mediators of inflammation that exit through the apical delta or foramen, or through lateral canals, cause rarefying osteitis. Every untreated tooth that is fractured with pulp chamber exposure to the oral cavity will develop endodontic disease.

Another common radiographic finding is alveolar marginal bone loss from periodontitis as shown in Figure 5. This is seen as apical migration of interdental or inter-radicular alveolar bone.

Click on the image to see a larger view

	Figure 1. Illustration of left maxillary incisor, canine, and premolar area. (1. Incisors. 2. Canine. 3. Premolars. 4. Vomer. 5. Palatine fissure. 6. Conchal crest. 7. Incisivomaxillary suture. 8. Nasoincisive suture. 9. Root canal. 10. Labial alveolar margin. 11. Palatal alveolar margin. 12. Chevron-shaped lucency)
	Figure 2. Illustration of left maxillary 3rd and 4th premolar and 1st molar region. (1. Premolars. 2. Molar. 3. Palatal root. 4. Mesiobuccal root. 5. Distal root. 6. Pulp horns. 7. Pulp chamber. 8. Root canal. 9. Ventral surface of pterygopalatine fossa. 10. Nasal surface of alveolar process. 11. Conchal crest)
	Figure 3. Illustration of left mandibular 4th premolar and molar region. (1. Premolar. 2. Molars. 3. Mandibular canal. 4. Interalveolar margin. 5. Radicular groove. 6. Interdental alveolar septum. 7. Inter-radicular septum. 8. Lamina dura)
	Figure 4. Illustration of lesions of endodontic origin affecting a mandibular 1st molar tooth. There are radiolucent (dark) areas around the root tips of the tooth indicating bone loss caused by chronic inflammation.
	Figure 5. Oblique and vertical bone loss indicating infrabony pockets.

Summary

Familiarity of the interpreter with normal and abnormal findings on dental radiographic images is a powerful resource to improve diagnostic accuracy. In this session we will view dental and oral radiographs of dogs to identify normal structures and abnormal findings caused by developmental, traumatic and infectious processes.