Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

56 Small Animal Radiolo g y and Ultrasono graphy
Fig. 2-25 A right parasternal short-axis view of the heart at the level
of the papillary muscles reveals the anterior papillary muscle (apm),
posterior papillary muscle (ppm), left ventricle (lv), and right ventricle
(rv). Diagnosis: Normal heart.
Fig. 2-26. A right parasternal short-axis view of the heart at the
level of the chordae tendineae reveals the chordae tendineae (ch), left
ventricle (lv), and right ventricle (rv). Diagnosis: Normal heart.
dorsal aspect) is obscured before reaching the diaphragm. The aorta tapers only slightly as
it transits the caudal thorax.
In some older cats and dogs the aorta has an S-shaped or question mark–shaped deformity
across and dorsal to the trachea (Fig. 2-36). This deformity is without any known clinical
significance.
In the ventrodorsal radiograph, the aortic arch can be detected as it crosses the left cranial
aspect of the cardiac silhouette at about the one o’clock position. Proximal to this, the
aorta is obscured by the cranial mediastinal density. The aorta usually can be followed caudally,
but only its left margin is visible. This margin gradually approaches the midline and
is lost at about the level of the cardiac apex or slightly caudal to the diaphragmatic cupula.
A prominent cranial bulge may be observed on the lateral radiograph in older dogs and
cats. This is related to the progressive shifting of the cardiac axis into a plane more parallel
to the sternum than routinely is observed. Otherwise, the aortic margin should be smooth,
tapering gradually as it progresses caudally.
Radiographic guidelines have not been established for evaluation of aortic size; however
the aorta should be roughly equal to the caudal vena cava in width. The size of the caudal
vena cava varies considerably with respiration and cardiac cycle.