Radiology of the oral cavity in the dog and cat

X-ray examination of the oral cavity is an invaluable diagnostic technique for an in-depth investigation of the real condition of this area. The particular conformation of the mouth of carnivores necessitates the use of specific exposure techniques, each one appropriate for individual anatomical areas.

Radiology of the oral cavity is useful on many occasions, the most frequent being:

The generation of a correct X-ray image is dependent on essentially three factors: the radiation source, the anatomical region of interest and the recording system.

RADIATION SOURCE

It is essential to use specific X-ray equipment for radiographic studies of the oral cavity. Although the radiographic equipment that is normally employed in the clinic can be used to take X-rays of the temporo-mandibular joint, frontal sinuses, nasal cavities and bony portions of the upper and lower jaw, intraoral X-rays cannot be performed correctly with this equipment because of the difficulty in adjusting the inclination of the X-ray tube head and regulating the distance between the unit and the subject.

Specific X-ray units for odontostomatology are available in both mobile and wall-mounted models;  the X-ray tube head is adjustable in every direction. Apart from their use in the field of odontostomatology, these machines can also be used to take X-rays of very small animals (small rodents, birds, reptiles, etc.). X-ray equipment for dental use has particular features that enable the creation of images with a low level of distortion and an excellent level of detail. The technical specifications of these machines are:

• 7 to 15 mA
• 50 to 90 kW
• Focal length no greater than 30 cm
• Small focus
• Tube head that can be oriented in any direction

The exposure techniques which are possible with conventional X-ray units are:

• Closed-mouth dorso-ventral: typically used for the temporo-mandibular joint;
• Open-mouth: useful for the maxilla;
• Skyline: for the visualisation of the frontal sinuses;
• Oblique: used alone or together with other techniques for the temporo-mandibular joint, the nasal cavities, the bony parts of the upper and lower jaws and to complete the radiographic assessment of certain teeth.

The exposure techniques which are possible with specific odontostomatology X-ray units are:

• Intra-oral bisecting angle
• Intra-oral paralleling
• Extra-oral near-paralleling
• Extra-oral oblique lateral exposure
• SLOB (Same – Lingual – Opposite– Buccal) rule

ANATOMICAL REGION

The type of exposure technique used depends on the anatomical region of interest and on the amount of radiation necessary to acquire an optimal radiographic image. The particular conformation of the skull of domestic carnivores (flat palate, the small size of many of these animals and the overlap of the maxillary bones) necessitates particular positions of both the X-ray tube and the recording system (whether analogical or digital).

RECORDING SYSTEM

Analogical systems
Special types of dental radiographic films, without intensifying screens, provide unparalleled detail and are supplied individually in light-tight packets. The chemical treatment of these films requires specific developers and fixers. The radiographs can be processed in small jars placed in a darkroom or with the aid of manual or automatic processors. Darkroom film processing, although cheaper, requires that the operator moves away from the patient, whereas dental film processors, which allow processing in full light, can be placed near the work area. It is important to remember that films have different levels of sensitivity, a factor which must be considered when selecting the exposure time and the focal distance from the film. There are also films supplied with small processing tanks containing both developers and fixers; others are designed for use with a single tank (monobath) used for both development and fixation. In addition to being expensive, these systems come in only one format and do not allow corrections of over- or underexposure.

Traditional X-ray cassettes with fine-grain films (X-fine, mammogram, etc.) can be used for extra-oral radiographs. However, also in the case of extra-oral radiographs, the use of dental films is to be preferred, if possible, because of their unparalleled detail.

Digital systems
Since the mid 1980s digital technology has been progressively introduced in radiology. The main advantages of digital systems are the reduced exposure of patients to radiation and the possibility of processing the images in various ways. Moreover, digital images are not subject to deterioration and can be replicated in multiple copies and transmitted remotely. Advances in digital technology have provided systems for the electronic acquisition of radiological images of such high quality that it can be envisaged that analogical systems will probably be abandoned completely.

Digital systems can be direct or indirect: direct digital dental radiology exploits modern X-ray units which, unlike traditional film equipment, use CCD sensors for image acquisition. The high sensitivity and excellent detail of these sensors make it possible to obtain high quality images, while limiting the patient’s exposure to radiation. The images acquired by digital studies may be printed on a single sheet in an orderly and standard format, or they may be stored on a normal compact disc, a solution which is more practical and economical. Another advantage offered by direct digital systems is that the image acquisition is immediate, which makes it possible to view the images in real-time, on a computer, and hence to monitor the appropriateness of the data and the correct position of the patient. Once on the computer, the veterinarian can optimise the image in order to facilitate the diagnostic interpretation; on the other hand, direct digital systems come in only two formats (2x3 cm and 3x4 cm), which is totally inadequate to meet the diagnostic needs of veterinary medicine.

Indirect digital systems are based on phosphor plates which take the place of conventional film. The phosphor plate is a thin layer of a material, such as alkaline halide mixed with rare earths, on which there are “traps”, or lacunae that are capable of attracting the electrons released from radiation beams. These “traps” essentially retain energy in a semi-stable condition, which can be released by the “traps” following exposure to light. The energy is released at a specific wavelength, depending on the energy level of the “trap”. The latent image is thus acquired by scanning the plate with a laser; the light emitted by a photomultiplier is “recorded” and converted into a digital image. The key feature of this process is that the signal being transmitted essentially has a linear correlation with the intensity of the X-rays (as opposed to the film). The latent image may be analysed using a low intensity reading cycle;  the material is re-usable after each high intensity cancellation cycle that completely removes the latent image (15” of exposure to the light source). The resolution of the system depends on the size of the laser, on the thickness and type of the photosensitive material and on the support material of the plate. The sensitivity of the process (signal/noise ratio on the final image) depends on the thickness and type of material but also on the reading time. To date, the spatial resolution of phosphor plates does not surpass 25 line pairs per millimetre, while the scanners designed for reading them have a spatial resolution of up to 50 line pairs per millimetre. Indirect digital systems, while not offering the immediacy of direct systems, come in all formats (Table 1) and have a higher definition, which is equal to or greater than that of analogical systems.

After processing is completed, the patient’s image may be printed, sent by e-mail to colleagues, or saved on a hard disc or CD-ROM for subsequent duplication, if required. It should also be noted that digital technology is environmentally friendly. In fact, dry printing on films or on special paper has eliminated the traditional X-ray film, avoiding the need for the highly polluting liquid chemicals that were used for development and fixation.

STORAGE OF IMAGES

After examination, X-rays on film should be washed with neutral soap and water (or with a specific cleaner), rinsed with demineralised water, dried and placed in appropriate transparent envelopes (Fig. 1) which allow subsequent consultation of the films without having to remove them. Digital images, in addition to being included in the patients’ clinical records, can be stored on external devices (hard discs, pen drives, etc.). An additional good practice is to give the client a copy on a magnetic device (CD or DVD) in order to allow consultation during future visits also by colleagues at other facilities.

RADIOGRAPHIC ANATOMY

During a clinical evaluation of teeth, only a small proportion of the anatomical structures involved is visualised. Radiology helps by providing an overall picture. In odontostomatology, radiographs are taken to evaluate several anatomical elements (Figs. 2 and 3):

• pulp chamber
• alveolar bone
• lamina dura
• periodontal ligament
• cement (imperceptible, as it has the same density as dentine)
• dentine
• enamel
• maxillary and mandibular bones
• mandibular canal
• foramina
• soft tissues

EXPOSURE TECHNIQUES

Closed-mouth dorso-ventral exposure
The patient is placed in sternal recumbency with the neck and limbs extended. Special attention should be paid to symmetry. With the help of a sandbag placed between the skull and shoulders, the X-ray is taken while keeping the main radiation beam parallel (in italiano c’è “parallelo” ma non coincide con il disegno – sembra “perpendicular” – è da controllare) to the sensor (or X-ray film) (Fig. 4).

Open-mouth exposure
This technique is indicated for the examination of the temporo-mandibular joints. After positioning the patient in dorsal recumbency, the mouth is opened to its full extent and the X-ray is taken, avoiding as much as possible involvement of the mandible. If possible, the sensor (or X-ray film) should be inclined at 15-25° and the radiation tube angled accordingly, until the angle of incidence of the main radiation beam becomes perpendicular to the recording system (Fig. 6). Unfortunately, when using X-ray machines not specifically designed for odontostomatology, it is not always possible to angle the radiation tube adequately and the images obtained are consequently interpreted with the resulting distorsions (Fig. 5). These projections can also be obtained with general purpose X-ray equipment.

Skyline exposure
This technique is of little interest in odontostomatology; it is mainly used to study the frontal sinuses. After positioning the patient in dorsal recumbency, the X-ray is taken making sure that the head is placed perpendicularly to the radiographic cassette and being careful to avoid superimposition of the nasal structures on the frontal sinuses. These projections can also be obtained with general purpose X-ray equipment.

Latero-lateral projections
Latero-lateral projections are rarely helpful, although occasionally they are used to complete the odontostomatological study. These projections can also be obtained with general purpose X-ray equipment (Figs. 22 and 32).

Oblique projections
Oblique views are useful for the study of tumours, fractures and other diseases that mainly affect the bony parts of the maxilla. More specific projections are usually preferred forthe analysis of the teeth(Figs. 21 and 31).

Intraoral bisecting angle exposure
The shape of the palate in dogs and cats prevents the sensor (or X-ray film) from being placed at an appropriate angle with respect to the maxillary teeth, thereby preventing proper formation of the X-ray image. If the sensor (or X-ray film) is placed in an intraoral position and the main radiation beam is aimed perpendicular to it, the radiographic image of the tooth is shortened. If, on the other hand, the main radiation beam is aimed perpendicular to the long axis of the tooth, the image of the tooth is elongated.

It is, therefore, preferable to aim the main radiation beam perpendicular to the bisector of the angle formed by the long axis of the tooth and the recording system; this compromise restores the correct dimensions of the parts of the tooth to be investigated.

This technique (Figs. 7, 8, 9 and 10) is applicable to the incisors and canines of the maxillary and mandibular arches and to the premolars and molars of the maxillary arch. When using this technique, care should be taken not to bend the sensor (or film X-ray), in order to avoid deformities, and not to superimpose the image of the investigated teeth on other dental elements or other structures (the mental foramen, etc.). This technique is not easily performed in brachycephalic breeds, or in animals with canines that are extremely vertical with respect to the mandibular axis.

Intraoral paralleling exposure
After positioning the patient in lateral recumbency, with the side to be X-rayed facing upward, a sensor (or X-ray film) of adequate size is placed in close contact with the premolars and/or mandibular molars, pressing it as much as possible against the ventral part of the mandible. This ensures that the recording system is parallel with the long axis of the teeth and that the apices of the teeth are included. The main radiation beam must be perpendicular to the film. The sensor (or X-ray film) can be held in place using soft, elastic materials such as sponges, gauzes, crumpled paper towels or the like.

This technique (Figs. 11 and 12) is clearly possible only when the size of the animal allows it. If the subject is too small for an intraoral sensor (or X-ray film), a different technique must be used.

The intraoral paralleling technique does not always allow the correct visualisation of the first mandibular premolars. The presence of the mandibular symphysis and soft tissues often makes it impossible to place the X-ray films in a ventral enough position to include the apices of these teeth in the radiograph.

Extraoral near-paralleling exposure
By exploiting the width of the maxilla and the divergence of the long axis of the molars and premolars of the two arches, the premolars and molars of one arch can be X-rayed without risking  superimposition of the contralateral teeth.

Once the patient has been placed in lateral recumbency with the arch to be investigated resting on the film, the mouth is opened and the head tilted until achieving the maximum possible parallelism with the sensor (or X-ray film) allowed by the tooth, or group of teeth, to be X-rayed (Figs. 13 and 13bis).

In dogs with a sufficiently wide mandible, this technique can also be used for radiographs of the molars and premolars of the mandibular arch. The images thus obtained are substantially free from distortion. Unfortunately, unlike the intraoral bisecting angle technique, the arch being studied is not on the side on which dental work is possible, consequently patients must be moved from one recumbency position to the other with all the difficulties that this entails. In addition to having to rotate the tracheal tube to avoid tracheal lesions, care must be taken to avoid ripping the intravenous drip tube and to replacing monitoring probes. In large dogs, it is advisable to invert the recumbency by turning the dog onto its belly, rather than on its back, to avoid the risk of torsion of the stomach.

Extra-oral oblique lateral exposure
This exposure technique is easy to perform (Fig. 14), but provides radiographs that must be evaluated with care, because of the shortening of the length of the dental images and, in small subjects, the possible superimposition of the dental arches. That said, when the mandible to be investigated is so small as to not allow the intra-oral paralleling exposure technique, the extra-oral oblique lateral technique remains the only alternative for radiography of the first premolars. It can be used for both the lower and upper dental arches but, as with the extra-oral paralleling technique, the arch to be investigated is not the one on which dental work is possible, with the problems that this entails.

The patient is placed in lateral recumbency, with the head resting on a wedge of foam rubber or on another adequate and comfortable support, with the part to be X-rayed in contact with the film. The mouth is kept open, preferably with radiolucent materials (foam rubber, etc.) and, as far as possible, the dental arches should be positioned so as to avoid superimposition. The X-ray is taken with the radiation beam perpendicular to the film lying on the table.

The SLOB (Same – Lingual – Opposite – Buccal) rule
The SLOB rule, also known as Clarke's rule, allows radiological identification of the dental roots when these are superimposed. The underlying concept is that if several X-rays are taken, moving the X-ray tube horizontally, the roots closest to the radiation source move in the same direction as the tube, while the more distant ones move in the opposite direction.

In the projection with the tube in an oblique and rostral direction, the lingual (palatal) root moves, on the radiograph, towards the tube itself. In the projection with the tube in an oblique and distal direction, the lingual (palatal) root moves, on the radiograph, distally with respect to the unit head.

This technique is particularly useful in the radiographic study of the fourth premolar in the dog and cat.

When taking an X-ray of the fourth upper premolar, if the radiation source is positioned rostro-distally with respect to the tooth, the X-ray image obtained shows the roots (read in a distal-rostral direction) arranged as follows: mesial > buccal > palatal (Fig. 15). When taking an X-ray of the fourth upper premolar, if the radiation source is positioned distal-rostrally with respect to the tooth, the X-ray image obtained shows the roots (read in a distal-rostral direction) arranged as follows: mesial > palatal > buccal (Fig. 16).

RADIOGRAPHIC IMAGES

Suggested readings

1. DeForge D.H., Colmery B.H. (2000) An atlas of veterinary dental radiology –Iowa State University Press, First edition, 2000
2. Atlas of Canine and Feline Dental Radiology, TW Mulligan, DVM, Dipl. AVDC, MS Aller, DVM, Dipl AVDC, CA Williams, DVM, Dipl AVDC, VLS, Trenton NJ 1998 Oral-dental
3. Radiographic Examination Technique. In: Clinics of North America, Small Animal Practice - Dentistry (ISSN 0195-5616 begin_of_the_skype_highlighting            0195-5616      end_of_the_skype_highlighting). ER Eisner, Holmstrom SE Guest ed. WB Saunders, Philadelphia, September,1998; 28:5:1063-1087."

External links

Introduction to veterinary radiology, Dentalaire Products

Dental radiology, My pets dentist

Radiology, Veterinary dental center

Dental radiology, VIN