Positioning: Dog and Cat

Following the general principles of radiographic geometry that were discussed in Chapter 5, radiographic examination of structures in the appendicular skeleton in dogs and cats should consist of a minimum of two orthogonal projections, a lateral view and a craniocaudal (dorsopalmar, dorsoplantar) view. If a bone lesion is suspected, it is important that the field of view include the joint proximal and distal to the bone of interest. If a joint is being investigated, the primary x-ray beam should be centered on the joint.

Also discussed in Chapter 5, the terminology used for radiographs of an extremity made with the x-ray beam striking the front or back of a limb depends on whether it is a front limb or hind limb and whether the center of the x-ray beam strikes the limb proximal or distal to the antebrachiocarpal or tarsocrural joint. In spite of being named differently, whether the x-ray beam strikes the front or back of a limb for any given projection, dorsoplantar versus plantarodorsal for example, does not affect the appearance of the radiograph in most instances because the radiograph is a two-dimensional representation of a three-dimensional object.

For certain regions of the appendicular skeleton of dogs and cats, there are ancillary projections in addition to the standard orthogonal views that can add important information by allowing evaluation of the edge of surfaces that cannot be assessed in the conventional lateral and craniocaudal (dorsopalmar, dorsoplantar) projections. Examples of such ancillary views would be the following:

1. Cranioproximal-craniodistal view of canine proximal humerus for evaluation of bicipital groove for changes associated with biceps or supraspinatus tendinopathy (Fig. 14-1, A)

2. Flexed dorsoplantar view of canine tarsus for evaluation of trochlea of talus for changes associated with osteochondrosis, without superimposition of calcaneus (Fig. 14-1, B)

3. Dorsal acetabular rim view of canine pelvis for evaluation of lateral aspect of acetabulum for remodeling (Fig. 14-1, C)

The views in Figure 14-1 do not encompass all ancillary views that have value in appendicular radiography and are provided only as examples of how changing the orientation of the x-ray beam and skeleton can lead to projection of clinically important surfaces that cannot be assessed adequately in the standard projections. These ancillary views, and others, are discussed throughout the textbook where appropriate, and examples of representative abnormalities are provided.

Positioning: Horse

The principles described for radiographic terminology, the minimum number of projections, and field of view described for the appendicular skeleton in dogs and cats also apply to the horse. In equine long bones, it may not be possible to include the proximal and distal joint in the same image because of the length of the bone. If this is the case, the proximal and distal end of the bone, and the respective joints, should be examined in different images.

There are very few instances in the horse where lateral and craniocaudal (dorsopalmar, dorsoplantar) views of a limb will suffice. These two views might be adequate for examining the radius or tibia, but even in those sites oblique views are often needed to assess all aspects of the surface of these large bones. These oblique views are obtained from a point of entrance different from that used for lateral or craniocaudal (dorsopalmar, dorsoplantar) projections. The principles of obtaining oblique projections and the associated terminology are discussed in detail later. In addition to oblique views, many other ancillary projections have been developed for equine skeletal radiography, and these vary according to the site being radiographed. These ancillary views will be covered in other chapters where specific equine joints and regions are discussed.

One aspect of equine radiography that deserves special mention is the common practice of holding the cassette by hand. Equine-extremity radiographs are almost always acquired with the horse in a standing position, and handholding the cassette is convenient. However, handholding of cassettes increases the chance that occupational radiation exposure limits will be exceeded or radiation safety practices will be compromised. Individuals handholding a cassette for equine radiography should wear protective lead aprons and lead gloves, as well as a thyroid shield and protective lead glasses. It must be remembered that this equipment protects only against scattered photons, and the gloved hand must never be placed in the primary x-ray beam; this was addressed in detail in Chapter 1. Also noted in Chapter 1, rotation of technical personnel in and out of the equine radiology service will distribute the occupational radiation dose over a larger number of people and minimize individual exposure doses.

The use of a cassette holder device for equine extremity radiography is encouraged because it will minimize occupational radiation exposure. A cassette holder secures the cassette and has an extension handle so that the hands are distant from the primary x-ray beam. A disadvantage of a cassette holder is that as the distance from the hands to the cassette increases, the force needed to hold the cassette in position also increases, making motion artifact more likely than when holding the cassette directly. However, this is not an excuse to abandon cassette holders. With practice, the technique can be mastered.

Oblique Projections

For many anatomic regions in dogs, cats, and horses, especially complex regions such as the carpal and tarsal joints, the occasional ancillary projection is not adequate, and oblique projections should be obtained routinely. Oblique views are generally intended to project different edges of a joint or region. More than the edge of a structure can be assessed in any radiograph, but important edge lesions can be overlooked unless they are projected tangentially where they will be visible at the periphery of the part. Oblique views maximize the chances of projecting an edge lesion tangentially in a complex joint. In oblique views of complex joints, the point of entrance of the primary x-ray beam is typically moved 30 to 45 degrees medial and lateral to the point of entrance used for a craniocaudal (dorsopalmar, dorsoplantar) view.

It is important to understand the geometry of oblique projections so that the location of abnormalities can be understood. The concept of oblique projections will be illustrated using the equine carpus as an example, but the principles apply to other regions and other joints. The principles of oblique projections based on radiography of a canine carpus are also available.¹ Labeled oblique radiographs of equine and canine joints are included in Chapter 13.

The edges projected in an oblique radiograph of a limb can be deduced by understanding the geometry of these projections, which is reviewed in Table 14-1 (page 258) and in Figures 14-2 through 14-5 (pages 254-257). In Figures 14-2 through 14-5, the imaging plate or cassette is oriented perpendicular to the primary x-ray beam, but the imaging plate is not shown for the sake of clarity.