Positioning: Dog and Cat

The anatomic complexity of the skull makes it imperative that radiographic positioning be standardized and repeatable; this removes a source of variation that can complicate radiographic interpretation. At a minimum, this standardization necessitates sedation, but general anesthesia is preferable. Making skull radiographs without sedating or anesthetizing the patient leads to the production of poor-quality images that cannot be interpreted intelligently and is a waste of time, effort, and money.

Lateral and dorsoventral (DV) views are the minimum radiographic projections of the canine and feline skull to acquire. The DV view is preferred over the ventrodorsal (VD) view, because for the DV view the mandibles can be compressed mildly against the x-ray table, which facilitates obtaining a symmetrically positioned radiograph (Fig. 7-1). This assumes that there is not mandibular asymmetry, as from a fracture or a mass for example, which would make it impossible to position the head symmetrically using the mandibles as a guide. As the dorsal surface of the head is rarely flat, extreme care must be taken to position the head symmetrically if a VD view is attempted.

For the lateral view, the dog or cat cannot be allowed to rest naturally on the x-ray table. Radiolucent sponges will be needed to elevate the nose in most patients. Sponges may also be needed to elevate the mandibles. The goal is to have left-sided and right-sided structures superimposed perfectly in the resultant radiograph (Fig. 7-2). Care must be taken to keep the positioning devices free of dirt and debris that can introduce artifacts into the image (see Fig. 7-2). The mouth can be either left closed or held open with a speculum. If the disease affects the mandible or maxilla, then there is value in having the mouth open to reduce superimposition.

Because of the complexity of the skull, many ancillary projections have been devised to increase the conspicuity of certain regions. These are listed in Table 7-1, and a few selected examples are included here (Figs. 7-3 through 7-9). Space does not permit a thorough discussion of each of the ancillary views that are available for use in the skull, but other sources are available.^1,2 These ancillary views are not used routinely but are selected based on the purpose of the radiographic examination. Many ancillary views of the skull involve having the skull at an angle with respect to the primary x-ray beam; this requires changing the position of the patient. Some other ancillary views actually require that the x-ray beam be angled from its normal perpendicular perspective with respect to the x-ray table; this requires having an x-ray machine with an x-ray tube head that can be (1) moved longitudinally along the top of the x-ray table and (2) rotated clockwise or counterclockwise to angle the primary x-ray beam.

An important aspect regarding oblique radiographs of the skull is making sure that a reliable external marking system has been used so that left can be distinguished from right in the image. It is helpful to use both “L” and “R” markers on the same image to designate which structures are being projected (Fig. 7-10). Taking the time to define a suitable marking system for your practice and explaining it thoroughly to personnel will save confusion in the long run. This is a situation where identification of a radiology supervisor, discussed in Chapter 1, is valuable because he or she can become familiar with the marking system and ensure that it is used correctly and routinely.

Positioning: Horse

Sedation is not required for equine skull radiography, but sedation will facilitate radiographic acquisition, especially for the DV view, where the dropped position of the head will make it easier to position the x-ray tube dorsally. Skull radiographs of most horses are acquired with the horse standing. It may be possible to position a sedated or anesthetized foal on the x-ray table for skull radiography, as for a dog or cat, but this is not done commonly.

For the lateral view, the cassette is positioned against the side of the head and the x-ray beam directed from the opposite side (Fig. 7-11, A). The position of the cassette and x-ray beam can be adjusted for acquiring radiographs of different regions of the skull, such as the nasal cavity versus the brain case versus the guttural pouches. Another thing to keep in mind for the lateral view is the effect of magnification. Because of the thickness of the equine skull, a lesion on the x-ray tube side of the skull will be magnified considerably and may not be conspicuous because of blurring. Thus if the location of the lesion is known, the cassette should be held against that side of the head and the x-ray beam directed from the opposite side. If the location of the lesion is not known, it is good practice to obtain both the left-right and right-left radiographs of the skull to make sure that a lesion is not overlooked because of magnification.

For the DV view, the cassette is held ventral to the mandibles and the x-ray beam directed onto the dorsal aspect of the head (Fig. 7-11, B). It is important to keep the x-ray beam perpendicular to the cassette. As noted before, sedation is very helpful for obtaining the DV view because as the horse lowers its head in response to the sedative, the positioning of the x-ray tube dorsally is facilitated. Without sedation one might be able to acquire a DV view of the nasal passage, but a DV view of the calvaria or brain case will not be possible. VD views of the skull are not made in the standing horse.

Radiation safety becomes paramount when radiographing the equine skull. Placing the cassette in a cassette holder with an extension handle is impractical because the force needed to keep the large cassette in the proper position increases as the hands move further distant on the handle; this leads to cassette motion and blurring of the image. In many instances, the only way to acquire an equine DV skull radiograph is to hold the cassette by hand. If this is done, the hands should be shielded with lead gloves, the body with a lead apron, and a thyroid shield and protective glasses should be worn. The x-ray beam must also be collimated such that the gloved hands are never in the primary x-ray beam. Protective lead gloves are adequate for protection only against scattered photons. For the lateral view, these same precautions apply, but care must also be taken to make sure that no part of the body is directly behind the cassette, where it will be in the direct line of fire of photons passing through the patient and then through the cassette.

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