CAPTURING HARD-TO-IMAGE TEETH
Certain teeth in dogs and cats are more difficult to image and require specialized imaging techniques.
Maxillary fourth premolar in dogs
To image the mesial roots of the maxillary fourth premolar separately, the tube head must be rotated around the dog's head
in the horizontal plane. This is the most difficult technique in dogs because two precise angles are necessary to create an
accurate image. Proper positioning will separate the mesial roots so they can be visualized independently.3,9
This technique uses the SLOB rule, which stands for same-lingual/opposite-buccal.5,6 The more buccal (or labial) object will move in the opposite direction as the tube shift, whereas the more lingual (or palatal)
object will move in the same direction as the tube shift. This is also known as the tube shift technique.7 Two techniques can be used to create a good image.
The first option is the distal tube shift technique (Figures 12A & 12B). Rotate the tube head around the patient's cranium distally so it is pointing more toward the front of cranium at about
a 30-degree angle. The resultant radiograph will reveal the mesial roots separately. In this case, the mesial-buccal root
is imaged most mesial and the mesio-palatal root is imaged between the mesial-buccal and distal roots. This option is usually
preferred because in most cases the entire tooth can be imaged on one film.
The second option is the mesial tube shift technique (Figures 13A & 13B). With this technique, the tube is shifted about 30 degrees mesially so that the tube head is pointed toward the back of
the patient. The resultant view reveals the mesio-palatal root imaged mesial to the mesio-buccal root. The notable disadvantage
with this technique is that the distal root is now superimposed over the first molar, so an additional exposure will be required
to evaluate the distal root.
Maxillary fourth premolar in cats
In veterinary dental radiography, the most notable difference between dogs and cats is the technique required for imaging
the maxillary cheek teeth.8 In dogs, high-quality radiographs of these teeth can be obtained with the standard bisecting angle technique. In cats, however,
the zygomatic arch does not allow a clear view of the third and fourth premolars since the arch will be superimposed over
these roots.3,8 Because of this limitation, two additional techniques are now being used to get a clear image of these tooth roots in cats—the
extraoral technique and the acute angle technique.
Extraoral technique. The extraoral technique is difficult to master, but it allows the maxillary cheek teeth to be visualized without interference
of the zygomatic arch or elongation of the roots (Figures 14A & 14B).12 Place the film on the table with the embossed dot facing up and the patient in lateral recumbency with the teeth to be imaged
down on the film. Position the film so that the ventral aspect is just visible below the cusp tips of the teeth to be imaged.
This positioning will ensure that most of the film is available to image the roots. Be sure that the film covers the entire
arcade (second premolar to first molar) in the rostrocaudal direction. For this view, a size 4 film is helpful to reduce placement
Next, place a radiolucent gag (e.g. a syringe) between ipsilateral canines to open the mouth relatively wide. Slightly rotate the patient's head so that the
mandible is about 20 degrees above the maxilla. Then position the tube head so it points into the oral cavity, such that the
cusp tips of the opposite arcade will be imaged about 3 mm below the root apices of the arcade to be imaged. The angle produced
is about 25 degrees from perpendicular, which then creates the approximately 45-degree angle necessary to accurately depict
the root length.12 Variation of the angle depends on the anatomy of the particular patient.
With the extraoral technique, the zygomatic arch is visualized apical to the tooth roots, giving a clear picture of the entire
root system. Splitting the tooth roots (SLOB rule) is difficult to achieve in this view. In cases in which this is critical,
use the acute angle technique instead.
An important point to remember when using the extraoral technique is that the film needs to be marked to distinguish right
from left. This is because the embossed dot faces into the mouth, as opposed to out of the mouth as in the intraoral techniques.
So when the films are viewed they will be interpreted as the contralateral arcade. In our practice, the embossed dots on dried
films are filled in with a red permanent marker to denote that the extraoral technique was used. Other practices may mark
an L or R on the film with a permanent marker or use small paper clip markers before exposure.
Acute angle technique. The acute angle technique is similar to the standard bisecting angle; however, the teeth are purposefully elongated to remove
the zygomatic arch interference.3 This technique provides clearer splitting of the mesial roots of the upper fourth premolar than the extraoral technique
does. Perform a standard bisecting angle technique with the beam angle at 30 degrees instead of 45 degrees.3 This technique removes the zygomatic arch interference but results in a slightly elongated image (Figures 15A & 15B). The mesial tube shift technique is preferred in cats because of the proximity of the third premolar mesially and the smaller
first molar distally.