Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

26 Small Animal Radiolo g y and Ultrasono graphy
A
B
Fig. 2-1 A 12-year-old male Labrador Retriever was brought in for evaluation of a large mass in the caudal
abdomen. The cardiac silhouette appears shorter and more round when the dorsoventral radiograph
(A) is compared with the ventrodorsal radiograph (B). Diagnosis: Normal thorax.
in the right lateral recumbent than in the left lateral recumbent radiograph. 1,11,12 In left lateral
recumbency, the cardiac silhouette may fall away from the sternum as the right middle
lung lobe inflates. This produces a radiolucency that separates the heart from the sternum.
In right lateral recumbency, contact between the heart and sternum usually is maintained.
In the ventrodorsal radiograph, the cardiac silhouette appears longer and narrower, the
accessory lung lobe appears larger, and the caudal vena cava appears longer than in the
dorsoventral radiograph (Fig. 2-1). 13,14
The x-ray beam should be centered over the thorax, because geometric distortion of the
thoracic structures will occur, especially when larger films are used (Fig. 2-2). The x-ray beam
should be centered just behind and between the caudal scapular borders in the dorsoventral
or ventrodorsal radiograph and at the fourth to fifth intercostal space in the lateral radiograph.
The forelimbs should be pulled cranially and fully extended on both views.
In order to obtain a properly positioned lateral thoracic radiograph, especially in dogs
with narrow and deep thoracic conformation, the sternum must be elevated slightly from
the x-ray table by a foam sponge or other radiolucent device. Proper lateral positioning can
be recognized on a radiograph when the dorsal rib arches are superimposed and the costochondral
junctions are at the same horizontal level (Fig. 2-3). In a ventrodorsal or
dorsoventral thoracic radiograph, the sternum and vertebral column should be superimposed
and the distance from the center of the vertebral bodies to the lateral thoracic wall
should be equal on both the right and left sides (Fig. 2-4).
In the lateral radiograph, malpositioning may produce artifactual tracheal elevation
and splitting of the main stem bronchi (see Fig. 2-3). Malpositioning also alters the shape
of the cardiac silhouette and can create or obscure the impression of cardiomegaly or cardiac
chamber enlargement. The cardiac apex always shifts in the same direction as the ster-