Dental Radiology

Sandra Manfra Marretta, DVM, Diplomate ACVS, AVDC
University of Illinois

 

Extra- and intra-oral dental radiographic techniques can be utilized to obtain diagnostic dental films utilizing standard or dental radiographic units. Proper positioning of the dental patient provides optimal imaging of normal anatomic structures and pathologic conditions. A variety of techniques can be utilized for development of dental films and proper magnification of films can help permit the diagnosis of previously undetected canine and feline dental diseases.

Standard Radiographic Unit Dental Radiography

Standard radiographic units can be utilized to take dental films however dental radiographic equipment is much easier to use when taking radiographs of teeth. When using standard radiographic units with standard cassette film and a high speed screen the mA should be set at 100 and the kVp will vary depending on the size of the animal (85kVp/150 pound dog, 75kVp/50 pound dog, 70 kVp/25 lb/dog, and 65kVp/10 pound animal). Exposure time should be set at 1/20 second with a focal film distance of 30 inches. Six radiographic views can be utilized to obtain a complete dental series with a standard radiographic unit including: lateral obliques of both mandibles and maxillae and intraoral views of the rostral incisive and maxillary and rostral mandibular teeth. Sponge wedges can be used to help obtain proper positioning. Alternatively, gauze can be used to hold the jaws open to help obtain proper positioning.

Dental Radiographic Unit

A dental x-ray unit is ideal for taking dental radiographs in the dog and cat because it permits accurate positioning of the film with minimal adjustment in patient positioning. Utilization of intraoral dental film and a dental radiographic unit has several advantages over techniques utilizing stationary machines found in most veterinary practices. These advantages include the following: (1) smaller and more maneuverable, (2) lead-lined cylinder cones, resulting in a more concentrated and nearly parallel radiation with less scatter, (3) greater detail, and (4) greater convenience. A variety of veterinary dental x-ray positioning devices are available to hold all different sizes of intra-oral film in place anywhere in the animal's mouth.

The film best suited for taking dental radiographs in the dog and cat are the small periapical films, Nos. 0-3 and the larger, No.4 occlusal film. These films come in packets which are marked to indicate which side of the film should face the x-ray tube and the depressed dot should be placed toward the tongue. Packets of dental x-ray film contain an outer moisture-resistant film packet with the film located between two layers of black paper and a layer of stippled lead foil at the bottom of the film packet. Ultraspeed film requires twice the exposure time of Ektaspeed (Eastman Kodak Company, Rochester, NY 14650). However, Ultraspeed film is usually prefered because it is less grainy and therefore provides greater detail.

There are two specific intraoral radiographic dental techniques: the parallel technique and the bisecting angle technique. The ideal dental radiograph is produced by utilizing the parallel technique. When using the parallel technique the plane of the radiographic film is parallel to the long axis of the tooth and perpendicular to the plane of the radiographic beam. The parallel technique in dogs and cats can only be achieved with the mandibular premolars and molars. The flat shallow palate and the shallow caudally extending mandibular symphysis in these species prevents utilization of the parallel technique when radiographing the maxillary teeth and rostral mandibular teeth. In these teeth the bisecting angle technique can be utilized. The film is placed as parallel as possible to the teeth being radiographed. An imaginary line that bisects the angle between the long axis of the tooth and the film is the bisecting angle line. The x-ray beam should be directed perpendicular to the bisecting angle line. Improper utilization of the bisecting angle technique will result in an elongated, foreshortened, or overlapping radiographic dental image.

A basic radiographic dental survey consists of six views: the rostral maxillary and mandibular projection, the right and left maxillary projections and the right and left mandibular projections. Additional radiographs are necessary in some cases. The upper fourth premolar requires additional radiographs to permit adequate visualization of all three roots. A 30-degree rostral oblique position needs to be added to the bisecting angle technique to permit adequate visualization of the mesiobuccal and palatal roots. This position on occasion may result in a superimposition of the apex of the distal root over the first molar. The distal root may be isolated with a 30-degree caudal oblique projection if necessary.

Proper exposure of dental film is easily obtainable with a dental x-ray unit. Most dental x-ray machines have a fixed kVp (70-90) and a fixed milliamperage (10-15). Overexposed and underexposed films are usually the result of inappropriate time selection. Adjustments in exposure time should be made depending on patient size and thickness of tissues. Dental charts can be created for each specific tooth following test exposure and appropriate adjustments. A Minxray P200D dental radiographic is used in the University of Illinois teaching laboratory and the technique chart developed for use in this laboratory utilizes exposure times of 0.3-0.4 seconds in cats and 0.4-0.6 seconds in large dogs with the mandibular incisor teeth requiring the slightly lower exposure times and the caudal maxillary teeth requiring the slightly higher exposure times.

The most time efficient method of processing dental film is a "chairside darkroom" kit. The "chairside darkroom" consists of a box with a special lid that permits visualization of the developing process but prevents exposure of the film. This box contains too hand-entry portals so that the operator may develop the film in the box without exposure of the film to light. The film is removed from its protective packet and is then attached to a film clip. The film is placed in water for 5 seconds and then placed in the developer and agitated to remove bubbles. The time in the developer is dependent upon the type of solution used and the temperature. The film is then placed in water agitated for 30 seconds. The film is then transfered to the fixer and agitated during fixing. It can be read in 1 minute but for archival-quality radiographs, the film should be replaced in the fixer for 10 minutes and washed in clean wash water for 30 minutes prior to drying to ensure the removal of all fixer from the film. After adequate rinsing the film is dried and stored in a film mount and/or envelope. Alternatively dental films may be developed in an automatic dental film processor or the Veterinary Dental Film System (Hanskin Technical Laboratory, Ltd.) may be used in which chemicals are injected directly into a specially packaged dental film packet.

Several errors in the processing of dental films can be easily corrected. A light film is caused by underexposure or underdevelopment of the film and can be corrected by increasing exposure time or changing the developing solution. A dark film is caused by overexposure or overdevelopment and can be corrected by decreasing exposure time and development time. A brownish discoloration of the film can be prevented by adequately fixing and rinsing the film. Fogged films with poor detail may be caused by improper safelight or light leaks, old film or contaminated chemicals. This problem can be resolved by checking the film expiration date, thoroughly rinsing the film between developer and fixative and checking for light leaks. Improper intraoral film placement with the dipple rather than the dot facing the x-ray tube will produce a film with a lighter stippled appearance because the x-ray beam must pass through the lead foil prior to exposing the dental film.

Accurate interpretation of dental radiographs requires careful observation and excellent radiographic technique. A dental x-ray view box and a magnifying glass are helpful in the interpretation of dental films. Also it must be remembered that bone loss is always greater than the radiographs depict and variations in the radiographic technique can obscure the presence and extent of the pathologic process. Therefore, complete assessment of the dental problem requires both a thorough clinical examination including periodontal probing and careful radiographic assessment of the dentition.

Frequent abnormal radiographic findings include changes associated with periodontal and endodontic disease. Radiographic pathologic alterations that can be associated with periodontal disease may involve changes in the entire dentition or it may affect individual teeth or root surfaces. The type of bone loss can be defined as resorption of the alveolar margin, horizontal bone loss, vertical bone loss or cup-like bone loss. The extent of destruction can be assessed by evaluating the distance from the alveolar bone margin to the cemento-enamel junction, the presence of furcation involvement, and the remaining amount of attachment in relationship to the length of the root.

Radiographic changes associated with endodontic disease include:

1.  periapical lysis/apical lysis

2.  large endodontic systems (failure in normal development or resorption)

3.  radiographic loss of tooth structure to the pulp canal

4.  secondary destruction of periodontal structures.

Other abnormal radiographic findings include:

1.  external root resorptive lesions

2.  retained root tips

3.  missing teeth

4.  abnormally located teeth

5.  malformed teeth

6.  osteomyelitis

7.  boney lysis secondary to neoplasia

8.  metabolic bone disease/traumatic injuries

References

1.  Mulligan TW, Aller MS, Williams CA. Atlas of Canine and Feline Dental Radiography. Veterinary Learning Systems, Trenton, New Jersey, 1998