Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

M A S S E S
Cranial mediastinal masses produce an increased thoracic density, because they displace the
air-filled lung (Figs. 2-70 and 2-71). Usually located in the ventral thorax, they also may
obscure the cardiac silhouette. Although esophageal or periesophageal masses usually will
displace the trachea ventrally, most mediastinal masses, if large enough, will displace the trachea
dorsally and away from the midline, usually to the right. The tracheal lumen may be
compressed and the cardiac silhouette and tracheal bifurcation, normally located at the fifth
or sixth intercostal space in the dog and almost always in the sixth intercostal space in the
cat, often will be displaced caudally and dorsally (Fig. 2-72). The mass, if large enough or if
locally invasive, may interfere with esophageal peristalsis and a gas-, fluid-, or food-filled
esophagus will be evident at the thoracic inlet. Sternal lymphadenopathy or masses in the
area of the sternal lymph nodes will produce a soft-tissue density in the ventral cranial
mediastinum. These masses typically have a convex dorsal margin and are located over the
second to fourth sternebrae. This aids in distinguishing them from fat accumulation, which
often occurs in the same area. Mediastinal margin irregularity and a change in contour are
much more specific signs of a mediastinal mass than is widening alone. In the ventrodorsal
radiograph, the widened mediastinum may extend on both sides of the vertebral column,
blending with the margins of the cardiac silhouette. The trachea may be displaced to the
right or left depending on the origin of the mass. The cranial margins of the cranial lung
lobes will be displaced laterally, and the entire lobes may be displaced caudally. Masses in the
tip of either cranial lung lobe may contact the mediastinum and mimic a cranial mediastinal
mass. Distinction between these may be impossible without the use of computed tomography.
Irregularity of the mediastinal margin is most often the result of a mediastinal mass
rather than fat or fluid accumulation. There are several causes of mediastinal masses including
neoplasia, cyst, trapped fluid, abscess, or granuloma (Fig. 2-73). 151,164-183
Sonography will reveal most cranial mediastinal masses. Very large masses are imaged
readily from anywhere on the cranial thoracic wall. Smaller masses may require using the
heart as a sonographic window, but some will not be positioned to be imaged transthoracically.
Sternal lymph node enlargement, which may not contact the heart, may require
the use of a parasternal window. Regardless of the imaging portal, evaluation will reveal the
presence of a mass and allow for the determination of whether it is cystic, solid, or
predominately solid with some fluid elements. The relationship of the mass to various vascular
structures may be apparent (Figs. 2-74 and 2-75). Exact identification of the tissue of
origin of the mass usually cannot be made with ultrasonography. However, sonography can
be used to guide a needle for aspiration or biopsy.
Other than esophageal masses (cardiac, aortic, or pericardial masses will be discussed
separately), those in the midportion of the mediastinum usually involve the mediastinal or
tracheobronchial lymph nodes or both (Figs. 2-76 and 2-77). Tracheal or bronchial compression
or deviation may be the only visible radiographic change, because the lymph node
borders often are obscured by concurrent increased pulmonary density or by contact with
the heart. The trachea may be elevated cranially to its bifurcation and the main caudal lobe
bronchi depressed ventrally caudal to this point. Narrowing of the tracheal and bronchial
lumina also may be evident. These changes help distinguish tracheobronchial masses from
hilar pulmonary infiltrates, which do not displace or compress the airways; left atrial
enlargement, which may elevate the left main caudal lobe bronchus; and esophageal
masses, which may depress the entire trachea, both the bronchi, and the cardiac silhouette.
Sonography occasionally can be used to evaluate masses in the midportion of the mediastinum.
Tracheobronchial lymphadenopathy sometimes can be imaged as masses adjacent
to the cardiac base, using the heart as a sonographic window. Fine-needle aspiration of
such masses only rarely can be performed safely using sonographic guidance.
Caudal mediastinal masses most often arise from or involve the esophagus and will be
discussed in that section. A contrast esophagram is helpful in distinguishing caudal mediastinal
abscesses and tumors from esophageal masses or foreign bodies. Diaphragmatic
masses and hernias may occupy the caudal mediastinum and should be considered whenever
a soft-tissue density is present in this area.
Chapter Two The Thorax 97