Page 155 - Feline diagnostic imaging

156 9 Normal Radiographic Anatomy
shift toward the normal hemithorax when the opposite
side has increased in volume, secondary to unilateral pleu-
ral effusion, or a very large pulmonary mass.

9.5 Thoracic Lymph Nodes

Three sets of thoracic lymph nodes are located within the
mediastinum (Figure 9.8). The cranial sternal lymph nodes
lie along the internal thoracic artery and vein. These lymph
nodes are located more caudally on the sternum than the
dog, typically dorsal to the cranial aspect of the third stern-
ebra. They receive afferents from the diaphragm, pericar-
dium, ventral thoracic and abdominal walls, and peritoneal
cavity [3,4]. The abdomen should be closely evaluated if
these lymph nodes are enlarged. The cranial mediastinal
lymph nodes are located ventral and/or medial to the tra-
chea in the cranial mediastinum at the level of the first and
second ribs. These nodes receive afferent lymphatics from
the trachea, esophagus, heart, pericardium, and pleura, as
well as muscles of the neck, thorax, and abdomen, scapula,
last six cervical vertebrae, thoracic vertebrae, ribs, thyroid,
thymus, and mediastinum [4]. The tracheobronchial (hilar)
lymph nodes are paired, with one pair located inside the
bifurcation of the mainstem bronchi (carina), just dorsal to
the left atrium. They are located slightly more cranial than
the same nodes in the dog. Additional tracheobronchial
nodes are located lateral to the carina. These nodes receive
afferent lymphatics from the lungs and bronchi [4].
Figure 9.5 Ventrodorsal thoracic image of a normal adult cat. Enlargement typically results in ventral deviation of the
The caudoventral mediastinal reflection is noted as a thin mainstem bronchi and carina.
radiopaque line extending caudally (arrow). It defines the left
lateral extension of the accessory lung lobe, but is not visualized
well in all cats. A metallic microchip is present, superimposed
over the left cranial aspect of the heart. 9.6 Thymus

cava, left subclavian artery, brachycephalic trunk, and cra- The thymus may be visualized in kittens as a soft tissue
nial mediastinal lymph nodes. These structures are seen opacity along the cranial border of the heart on the lateral
individually only when air is present in the mediastinum. view. The thymus reaches maximum size when the cat
While large amounts of fat can be deposited in the medi- reaches sexual maturity, then progressively involutes,
astinum, it is insufficient to contrast the soft tissue struc- decreasing visualization until about 1 year of age [4,5].
tures. On the VD/DV view, the mediastinum is
superimposed over the spine, and should be similar in
thickness, although fat deposits within the mediastinum 9.7 Trachea
will cause nonpathologic thickening (Figure 9.3).
A mediastinal shift occurs with a change in size of one or The feline trachea is composed of approximately 40 carti-
more lung lobes in one hemithorax. A loss of volume due laginous rings that are completed dorsally by the trachealis
to atelectasis or lobe removal will result in a medastinal muscle. It extends caudally from the cricoid cartilage,
shift toward the affected side. The heart is the largest organ traveling parallel to the spine as it passes through the cervi-
in the mediastinum, and a deviation to one side (on a prop- cal soft tissues. At the thoracic inlet, the trachea deviates
erly positioned VD or DV view) is indicative of a medasti- slightly ventrally from the thoracic spine, ending at the
nal shift (Figure 9.7). Alternatively, the mediastinum will carina, where it bifurcates into the right and left principal