Principles of Radiographic Interpretation of the Axial Skeleton



Principles of Radiographic Interpretation of the Axial Skeleton


Donald E. Thrall



Skull


Skull radiography in dogs and cats has been replaced in specialty practices by computed tomography (CT) and magnetic resonance imaging (MRI), and very few skull radiographs are made currently in that setting. There is more of a need for skull radiography in general practice, but skull radiography will still be one of the more infrequent radiographic examinations performed. Thus, because of the low demand, radiology personnel may become out of practice with regard to acquiring diagnostic canine and feline skull radiographs. Given the complexity of the skull and the number of possible special views that can be acquired, it is easy to understand how this might occur. The framework provided here will assist radiographers in acquiring a skull radiographic study of adequate quality.


Although CT and MRI of the skull are feasible in the horse, skull radiography is still performed commonly in that species because of the need for general anesthesia for CT and MRI and for economic reasons.



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.



Table • 7-1


Ancillary Radiographic Projections of the Canine or Feline Skull

































































VIEW NO. PART OF INTEREST NAME OF PROJECTION BRIEF DESCRIPTION
1 Frontal sinuses Rostrocaudal projection of frontal sinuses Subject in dorsal recumbency. Head flexed 90 degrees to spine such that nose points directly at x-ray tube. Center x-ray beam on frontal sinuses.
2 Right maxillary dental arcade or right tympanic bulla. Open mouth left 20-degree ventral–right dorsal* Subject in right recumbency. Secure mouth open with speculum. Elevate mandibles 20 degrees with sponge wedge. Center x-ray beam on right maxillary arcade or tympanic bulla.
3 Left maxillary dental arcade or left tympanic bulla. Open mouth right 20-degree ventral–left dorsal Subject in left recumbency. Secure mouth open with speculum. Elevate mandibles 20 degrees with sponge wedge. Center x-ray beam on left maxillary arcade or tympanic bulla.
4 Right mandibular dental arcade Open mouth left 20-degree dorsal–right ventral Subject in right recumbency. Secure mouth open with speculum. Elevate maxilla 20 degrees with sponge wedge. Center x-ray beam on right mandibular arcade.
5 Left mandibular dental arcade Open mouth right 20-degree dorsal–left ventral Subject in left recumbency. Secure mouth open with speculum. Elevate maxilla 20 degrees with sponge wedge. Center x-ray beam on left mandibular arcade.
6 Nasal cavity Open mouth ventral 20-degree rostral–dorsocaudal Subject in dorsal recumbency. Secure maxilla against x-ray table such that hard palate is parallel to tabletop. Open mouth widely with speculum or gauze traction on mandible. Angle x-ray beam 20 degrees rostrally and center x-ray beam on hard palate.
7 Nasal cavity Intraoral dorsoventral Subject in ventral recumbency. Slide cassette into mouth. Center x-ray beam on dorsal aspect of maxilla.
8 Tympanic bullae Open mouth rostrocaudal Subject in dorsal recumbency. Head flexed 90 degrees to spine such that nose points directly at x-ray tube. Open mouth by retracting maxilla and mandible equally with gauze or tape. Center x-ray beam in back of mouth.
9 Tympanic bullae (cats and brachycephalic dogs) Rostral 10 degrees ventral–caudodorsal Subject in dorsal recumbency. Head flexed 90 degrees to spine such that nose points directly at x-ray tube. Angle head/neck 10 degrees caudally, keeping mouth closed. Center x-ray beam at base of skull.
10 Right tympanic bulla/right temporomandibular joint. Left 20 degrees rostral–right caudal Subject in right recumbency. Elevate nose 20–30 degrees with sponge. Center x-ray beam on region of temporomandibular joint.
11 Left tympanic bulla/left temporomandibular joint. Right 20 degrees rostral–left caudal Subject in left recumbency. Elevate nose 20–30 degrees with sponge. Center x-ray beam on region of temporomandibular joint.


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*Any angle specified in this table for this or any other ancillary skull radiograph is an estimate. This will vary among dogs and should be selected based on visual determination of the angle that results in optimal isolation of the area of interest.


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Fig. 7-3 Rostrocaudal frontal sinus view (see Table 7-1, View No. 1). The patient is in dorsal recumbency with the nose pointing directly at the x-ray tube. This view, which provides an unobstructed view of each frontal sinus, is useful only in subjects with developed frontal sinuses. Brachycephalic breeds of dogs and some cats have incompletely developed, and/or nonpneumatized, frontal sinuses. In those patients, this projection has no value to assess the sinus cavity but may be useful for assessing the frontal bone itself. If the sinus cavity contents are of interest, the frontal sinuses should be examined in the lateral view to make sure they are present and pneumatized before going to the trouble of acquiring this rostrocaudal view. L, left side; R, right side.

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Fig. 7-4 Open-mouth rostrocaudal view of tympanic bullae (see Table 7-1, View No. 8). The patient is in dorsal recumbency with the nose pointing at the x-ray tube. The mouth is then opened by retracting the mandible and maxilla with gauze. Centering the x-ray beam in the back of the open mouth provides a relatively unobstructed view of the tympanic bullae (black arrows).

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Fig. 7-5 Intraoral DV radiograph of maxilla (see Table 7-1, View No. 7). The patient is in ventral recumbency with the cassette inserted into the mouth and the x-ray beam directed onto the dorsal surface of the maxilla. Using a thin paper cassette allows the film to be inserted further caudally than when using a conventional film cassette or a computed radiography cassette. Some paper cassettes have no intensifying screen, meaning that x-rays are being used to expose the film rather than visible light from the intensifying screen. This will require considerably higher mAs values than when an intensifying screen is used, but the detail in the image will be exquisite, as seen here. This projection provides an unobstructed view of the nasal cavity.

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Fig. 7-6 Open mouth, left 20-degree ventral–right dorsal projection of the right maxillary dental arcade (see Table 7-1, View No. 2). The patient is in right recumbency and the mouth secured open with a speculum. A portion of the speculum is visible (white arrow). The mandibles are elevated 20 degrees from the tabletop with a radiolucent sponge. This positions the left maxillary arcade dorsal to the right. The x-ray beam, which is vertical to the tabletop, is centered on the right maxillary arcade.

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Fig. 7-7 Open-mouth, left 20-degree ventral–right dorsal projection of the right tympanic bulla and temporomandibular joint (see Table 7-1, View No. 2). The patient is positioned exactly as described in the legend for Figure 7-6, except the x-ray beam has been centered on the middle ear. This allows assessment of the right (dependent) tympanic bulla (white arrows) and right temporomandibular joint (black arrows). The left (nondependent) tympanic bulla and left temporomandibular joint are located dorsal to the right tympanic bulla and right temporomandibular joint and are superimposed on the skull. Only the right tympanic bulla and temporomandibular joint can be assessed using this positioning. If the left-sided structures are of interest, then the view described in Table 7-1, View No. 3, should be used.

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Fig. 7-8 Left 20-degree rostral–right caudal projection of the right temporomandibular joint (single white arrow) and right tympanic bulla (double white arrows) (see Table 7-1, View No. 10). The patient is in right recumbency, and the tip of the nose is elevated 20 degrees from the tabletop. The x-ray beam is perpendicular to the tabletop and centered on the tympanic bulla region. Elevating the nose positions the upper, left, nondependent tympanic bulla (single black arrow), caudal to the lower, right, dependent tympanic bulla (double white arrows). Compared with Figure 7-7, the contralateral, nondependent, left tympanic bulla (black arrow) is positioned caudal to the one of interest rather than being dorsal. Note that the nondependent, left tympanic bulla is larger because of radiographic magnification. In this view, the nondependent, left temporomandibular joint is superimposed on the region where the tympanic bullae overlap.

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Fig. 7-9 Rostral 10-degree ventral–caudodorsal view of the tympanic bullae (white arrows) in a cat (see Table 7-1, View No. 9). The patient is in dorsal recumbency with the nose pointing directly at the x-ray tube. The head and neck are extended 10 degrees, and the x-ray beam, which is perpendicular to the tabletop, is centered at the base of the skull. L, left; R, right.

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.


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Fig. 7-10 Example of an external marking system for oblique skull radiographs. This is the same projection as described for Figure 7-6. In this system, the (L) at the top of the image and the (R) at the bottom of the image means that with bilaterally symmetric structures, such as maxilla and mandibles, the left side will be dorsal and the right side ventral. Thus, the positioning used in this dog is such that the right maxillary dental arcade (arrow) and the left mandibular dental arcade (arrowhead) are projected for unobstructed viewing.


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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May 27, 2016 | Posted by in ANIMAL RADIOLOGY | Comments Off on Principles of Radiographic Interpretation of the Axial Skeleton

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