Chapter 3 Dental Radiology
In the United States during the 1970s, dental radiology was not incorporated routinely in the course of extractions and other procedures. During the 1980s, practitioners who were interested in performing at a more advanced level found that radiographs helped them. At first, the convenient method was to use the larger veterinary medical units in the office. As radiology was recognized as more useful, these enterprising veterinarians explored the practicality of purchasing human dental radiographic units. It quickly became apparent that a definitive diagnosis often was not made unless intraoral radiographs were taken. Dental radiography has now become an essential part of the veterinary dental diagnostic workup. The diagnostic yield of full mouth radiographs in feline and canine patients is high, and routine full mouth radiography is justified.1,2 These studies found that if disease existed, radiographs were clinically useful in 86.1% of the cases in the study (Tables 3-1 and 3-2). Follow-up radiographs may be required, and professional judgment should be used in determining the type, frequency, and extent of each radiographic examination.1
Table 3-1 VALUE OF RADIOGRAPHS—NO CLINICAL FINDINGS PRESENT
| Dogs | Cats | |
|---|---|---|
| Incidental radiographic findings | 41.7% | 4.8% |
| Clinically important findings | 27.8% | 41.7% |
| Radiographs of no value | 30.5% | 53.6% |
Table 3-2 VALUE OF RADIOGRAPHS—CLINICAL FINDINGS PRESENT
| Dogs | Cats | |
|---|---|---|
| Confirmational only | 24.3% | 13.9% |
| Additional findings | 50.0% | 53.9% |
| Clinically essential findings | 22.6% | 32.2% |
| No value | 3.1% | 0 |
Normal Radiographic Anatomy1–5
Normal and Abnormal
• Often, abnormalities observed on a radiograph can be confirmed or denied by examining a radiograph of the contralateral tooth or teeth.
• Bilateral symmetry suggests that the “pathological” finding may be a normal variant, the sequelae of a systemic condition, or a genetic condition.
Reading the Radiograph
• Before reading a radiograph, it is essential that the view box be clean and the fluorescent tubes be without black discoloration at the ends.
• A systematic approach should be taken when evaluating dental radiographs in order to interpret comprehensively and accurately everything on the film. It is helpful to orient the film so that it matches the orientation of the dental chart, with the cusps of the maxillary crowns pointing down and those of the mandibular crowns pointing up. Additionally, films should be mounted so that the patient’s teeth on the right side are seen to the viewer’s left and the patient’s teeth on the left side are seen to the viewer’s right.
Step 3—Start on one side of the radiograph and view each tooth, evaluating the normalcy of shape and radiopacity, as the examination progresses across the film.
RADIOGRAPHIC FINDINGS
Structures Visible
• The following structures should be identified: crown, neck, pulp chamber, root canal, apex, periodontal space, dentinal wall, cortical bone, interradicular bone, furcation, and mandibular canal (Fig. 3-1).
Normal Young Patient
• As the tooth develops, after apexogenesis is complete, odontoblasts that line the pulp chamber will produce secondary dentine. This will thicken the dentinal wall and reduce the size of the pulp chamber and root canal. The apex may be open, depending on the age of the patient.
• In the young patient, the dense cortical alveolar bone forming the wall of the socket appears radiographically as a distinct, opaque, uninterrupted, white line parallel to the long axis of the tooth root(s). This is known as the lamina dura.
• The radiolucent image between the lamina dura and the tooth root is the periodontal space and is known radiographically as the lamina lucida. It is occupied by the periodontal ligament.
Normal Older Animal
• The dental radiograph of a healthy adult will show a decreased pulp canal size and increased dentinal wall thickness.
To summarize, the normal radiographic signs of aging are as listed below:
Indications
• In young patients, to evaluate the presence and nature of unerupted, embedded, impacted teeth or supernumerary teeth.
• During prophylactic or therapeutic teeth cleaning (periodontal debridement), to evaluate the extent of periodontal disease by measuring bone loss and to assist in treatment planning.
• Intraoperatively, during endodontic therapy, to facilitate evaluation of therapy and to study radicular health and size before, during, and after endodontic therapy, including periodic reexaminations.
• In patients with visually missing teeth, to ascertain the condition, e.g. potentially impacted teeth or areas of spontaneous resorption in roots or unerupted teeth.
• In all types of dental and oral disease, to document and study the progress of therapeutic programs.
• In patients with discolored, worn, or fractured teeth, to evaluate pulp health and root end pathology.
• In cases in which neoplasia or metabolic disease is suspected, to evaluate the involvement of teeth and bone.
• Before or during extractions, to evaluate root and supportive bone structure before extraction and to verify the complete removal of root tips or the location of retained root tips.
Contraindications
• Critically ill patients in whom the risk of anesthesia is so great that the risk of treatment will not be justified.
Radiographic Units
Medical Radiographic Unit
• A medical radiographic unit can be used to obtain extraoral and intraoral films, but its use is limited due to the restricted movement of the radiographic head and because the unit is located distant from the dental operatory, thus requiring the patient to be brought to the machine for each film desired.
• When a medical radiographic unit is used, having the ability to adjust the focal distance and angle of the head is preferred. A focal film distance of approximately 12 inches is preferred, coning down with an adjustable collimator, as appropriate, to reduce scatter.
• The parallel and bisecting angle techniques can be used with a medical radiographic unit. They require, with each film, repositioning the patient with foam wedges and sandbags to achieve the proper alignment with the primary beam.
• To achieve a bisecting angle technique of the caudal maxillary teeth, the patient is placed in sternal recumbency, with the nose supported so that planum nasale is parallel to the table surface. The patient is then rotated obliquely 45 degrees, dental film is placed intraorally, and the radiographic head is adjusted until the primary beam is directed appropriately over the area of interest (see the later section titled Bisecting Angle Technique). The opposite side is taken similarly by rotating the patient in the opposite direction. Anterior maxillary views using the bisecting angle technique are taken with the patient in sternal recumbency, as with the caudal maxillary views, with the film in the mouth. Preferably the radiographic head is tilted approximately 45 degrees, pointing ventrocaudally, and collimated to the appropriate dimensions. The primary beam should be approximately 45 degrees to the plane of the palate.
• Caudal mandibular views are taken on either side by placing the patient in lateral recumbency, with the film placed intraorally lingual to the mandibular teeth to be viewed. This is often a parallel view so the radiographic beam is centered over the area of interest and collimated to limit the x-rays to the space occupied by the film.
• To obtain a rostral oblique mandibular bisecting angle view, the patient is placed in dorsal recumbency with the top of the muzzle parallel to the table and then rotated obliquely 45 degrees. The film is placed intraorally, as close to parallel to the lingual surface of the tooth as possible, and the positioning is adjusted until the beam is directed appropriately in a buccoventrolingual direction. The rostral mandibular bisecting angle view of both canines and incisors is taken similarly, with the patient in straight dorsal recumbency. The primary beam will be directed rostrodorsocaudally at an angle of approximately 20 degrees from vertical.
• Each medical radiograph unit will have different technique ranges. A starting point is 100 mA, 65 kilovolt peak (kVp) at 1/10 second. When the kVp is too low, the result is low density with high contrast. The film will be light and the kVp or the time needs to be increased. When the kVp is too high, the result is high density and poor contrast. The film will be dark and the kVp or the time needs to be decreased. It is necessary to achieve a proper balance between kVp and mA. Practice with a study skull to try several combinations until the most desirable exposure is achieved.
Dental Radiographic Unit
• A dental radiographic unit can be purchased new (or sometimes used from dentists or from dental supply warehouses). It can be wall mounted in the dental area or mounted on a mobile floor stand. It has a compact, rotating, and swiveling tube head, with a multiple jointed cantilevered arm for maneuverability.
• Dental radiograph units are available, varying from 70 to 85 kVp and 7, 10, or 15 mA with several time-exposure capabilities. These units are very versatile and produce a more detailed image due to a smaller focal spot and a fixed focal distance.
• The newest dental radiograph machines are designed to use direct current instead of alternating current power to generate the x-ray beams. This gives greater contrast and less soft radiation to the tissues. These machines are digital and have a programmable keypad. Some of the keypads have diagrams of tooth types and large, medium, and small dog and cat options to choose from for the time exposure that is most appropriate.
• Suggested technique chart using Ultraspeed (type D) film with a 7-mA output and direct current generator.
• If using alternating current, increase time slightly. Upper fourth premolar will use the same technique as the upper molar (Tables 3-3 and 3-4).
• Current state health laws require and, hence, the newer machines have lead-lined or aluminum-lined cylindrical pointing devices to further reduce scatter radiation. This is an important feature to consider if purchasing a used machine. The ease of using a dental radiographic unit and its proximity to the procedure will encourage taking more radiographs, thereby providing a higher percentage of accurate diagnoses and a better service to the patient and client.
• Some machines have timers that are indicated in fractions of seconds. Other machines use “pulses” as an indication for time. One pulse equals 1/60 second.
Table 3-3 60 kV (MAXIMUM IMAGE CONTRAST)
| Small dog or cat | Large dog | |
|---|---|---|
| Incisor | 0.25 second | 0.4 second |
| Canine | 0.25 second | 0.4 second |
| Premolar | 0.32 second | 0.5 second |
| Lower molar | 0.4 second | 0.63 second |
| Upper molar | 0.5 second | 0.8 second |
Table 3-4 70 kV (MAXIMUM GREY LEVEL DEFINITION)
| Small dog or cat | Large dog | |
|---|---|---|
| Incisor | 0.125 second | 0.2 second |
| Canine | 0.16 second | 0.25 second |
| Premolar | 0.2 second | 0.32 second |
| Lower molar | 0.25 second | 0.4 second |
| Upper molar | 0.32 second | 0.5 second |
Materials
Film
• Intraoral radiographic film is inexpensive, small, and flexible, fitting well into the oral cavity and conforming to the area placed (Table 3-5). Intraoral film is nonscreen film, which provides greater detail than the larger high-speed cassettes used in most veterinary general medicine situations. Intraoral film can be processed in 1 to 2 minutes with fresh rapid developer and fixer solutions, and there is minimal loss of detail. It can also be processed in 2 to 4 minutes in dental automatic x-ray processors. Using this film, small areas of interest can be isolated and superimposition of anatomical structures avoided. Each film packet has lead foil backing that reduces fogging and scattered radiation.
• Ultraspeed (D-speed) film is used most commonly because it provides greater detail and is less grainy, although it requires twice the exposure time as Ektaspeed (E-speed) film to produce a comparable image (Eastman Kodak, Rochester, NY). When taking radiographs of giant breed dogs with a dental radiographic unit, it may be helpful to use Ektaspeed film to achieve the x-ray penetration of thicker bone without increasing the exposure time. Kodak InSight6 Intraoral Dental Film (F-speed) reduces radiation dosage to a lower level than Ultraspeed or Ektaspeed. It also delivers excellent image quality because it provides better contrast and is therefore superior to the E-speed and D-speed films.6
Film Identification
• When taking intraoral radiographs, it is important to be able to identify easily which patient they belong to. Several methods of film identification can be used, depending on the clinician’s preferences.
Identification Methods
• A radiograph marking pen that writes in white to mark the client and patient identification on the dry film.
• A small press-apply radiopaque number that correlates in a log to a specific client and patient, placed on the outside of the film packet before film exposure.
• Plastic or cardboard film mounts that are made for the various film sizes are available through dental suppliers. Client and patient identification and other pertinent information can be written on the film mount. Presized punches are available to make cutouts for custom made cardboard holders.
• Small envelopes, with client and patient identification on the outside, to hold one or more films.
Identifying Right and Left Sides
• All intraoral films have a small depressed dimple in one corner of the film. It is visible and palpable on both the film and film packet. The side with the raised dimple is called the “dot” and faces the x-ray beam when the film is placed in the mouth.
• One technique is to place the dimple side of the radiograph on the patient’s right in the mouth. When the radiograph is mounted in normal alignment, as when the teeth are radiographed with the raised dot facing you, it will be easy to tell whether the tooth is on the right or left. This is only effective if the entire staff adheres flawlessly to the protocol.
• Identification is still possible for personnel who forget to be consistent with dimple position. To do this, the raised side of the dot is found and oriented so that the convex side of the dimple (the “dot”) faces the viewer while viewing the film. The film is placed in proper alignment with the maxilla or mandible (tooth crown appropriately down or up respectively). Next, the film is oriented with proper alignment of the teeth rostral to caudal. The orientation of the teeth on the film will match the patient’s chart, as seen when the clinician faces the patient that is, in turn, facing the clinician. The right side of the film will be the left side of the patient. In this way, the view that corresponds to the dental chart can be achieved by placing processed films in plastic mounts or plastic sheets, with the dot facing the viewer and with the films of the patient’s right side on the viewer’s left and films of the patient’s left on the viewer’s right, Incisor views are mounted, when the mount is so designed, in the middle. This will reduce charting errors when transposing information from the film to the chart.
Film Storage
• Individual film envelopes or envelopes holding film mounts can be placed in the patient record or stored in a separate radiographic filing system. The patient’s current dental films are stored with its past dental films, so that the course of its dental history can be accessed easily for review. Dental radiograph logs are helpful in retrieving past films, with number identification placed in the chart. To avoid losing these small dental films, it is best to file intraoral films separately and not with the large cassette-size radiographs of the same patient.
TAKING AN INTRAORAL RADIOGRAPH4,5,7–10
General Comments
Step 2—The intraoral film is placed in the proper position. As a supportive aid in positioning the film, a mouth gag, special film wedge, gauze sponge, or other object can be placed behind the film. A tongue depressor can be placed under occlusal size film to prevent bending of the film over the endotracheal tube in maxillary views if the patient is placed in sternal recumbency.
Step 3—The cylinder of the x-ray machine head is centered over the area of interest and placed as close as possible to the structure being evaluated and positioned for the study. If using a medical radiographic unit, a focal distance of 12 inches is maintained from the film to the surface of the collimator.
Whole Mouth Survey: Small Dog or Cat9
• Typical study includes:
Two views (right and left): maxillary premolars and molars; lateral bisecting angle technique for each side.
• Typically, six views may complete the study. Additional views and films may also be required, depending on the disorder involved. It is often helpful to take multiple views when questioning whether a finding is pathology or merely an artifact. Anterior oblique, posterior oblique, and occlusal views can be taken in addition to a whole mouth survey or individual study.
• The parallel technique is the most accurate, because it follows the principle of accurate shadow writing, in which the projected shadow of an object from a light source resembles the height of the object most accurately. If the angle between the film and the structure to be radiographed is greater than 15 degrees, then the x-ray beam should be directed according to the bisecting angle technique.
Whole Mouth Survey: Large Dog9
• Typical study includes:
One view: maxillary incisors and canines; anterior bisecting angle technique. Two views (right and left): maxillary rostral premolars and canine tooth; lateral bisecting angle technique for each side.
Two views (right and left): maxillary caudal premolars and molar teeth; lateral bisecting angle technique for each side. In dolichocephalic dogs, especially in large breeds such as the Collie or Borzoi, oblique bisecting angles are often needed to isolate and visualize well the right and left canine teeth alone. With these breeds, sometimes it is necessary to take two views, at slightly different angles, to accurately visualize the incisors in one film and the canines in the second film.
Two views (right and left): mandibular anterior premolars and canine tooth; lateral bisecting angle technique for each side.
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