Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 87
Fig. 2-61 A 5-year-old male cat with dyspnea and pale mucous
membranes. A diaphragmatic hernia had been repaired 4 months
earlier. In the ventrodorsal radiograph there is a soft-tissue density
that obliterates the entire left thoracic cavity. The cardiac silhouette
and trachea are displaced to the right. There is a small amount of
fluid present in the pleural space on the right side. This fluid outlines
the lung lobes and separates them from the lateral thoracic wall. The
radiographic changes are indicative of a soft-tissue density within the
left hemithorax. This could be a mass or trapped pleural fluid. A thoracocentesis
was performed and purulent material was removed.
Diagnosis: Loculated pyothorax.
or costal cartilage, although some inflammatory lesions may be encountered. When an
intrapulmonary lesion is located at the edge of a lung lobe and contacts the parietal pleura,
it can appear similar to a pleural or extrapleural mass. The shape of the soft-tissue density
and evaluation of the point of junction between chest wall and lung lesion may distinguish
between pulmonary and extrapleural masses. 121,122 Pulmonary masses are usually round
and lack the shoulder observed with extrapleural masses. Obtaining opposite lateral views
will be helpful in distinguishing among pulmonary and pleural or extrapleural masses.
Horizontal-beam radiography may be helpful in demonstrating a mass by using gravity to
displace the pleural fluid away from the mass. In some instances biopsy, via needle aspirate
or surgery, is the only way to make a definitive diagnosis. Fluoroscopic examination, computed
tomography, ultrasonography, and pleurography have been recommended; however,
they rarely are required.
Herniation of abdominal viscera through a diaphragmatic tear may mimic the appearance
of a pleural or extrapleural mass. Positional radiographs, evaluation of the abdomen,
and careful evaluation of the diaphragm usually will identify the hernia.
Encapsulated or loculated pleural fluid, granulomatous masses, or pleural neoplasms may
produce radiographic evidence suggestive of pleural masses (Fig. 2-61). Primary and metastatic
pleural neoplasms are rare. They often are hidden by pleural fluid and are not identified
until the fluid is removed. 123,124 A fluid collection that does not conform to the normal linear
or triangular shape of the pleural space as defined by the lung lobes, or a fluid density that
maintains its position and shape despite alteration in the animal’s position during radiography,
is indicative of a pleural mass. Rib involvement is unusual. Uneven lung compression or
displacement of the heart or other mediastinal structures also can be observed.
As a screening technique for pleural lesions, ultrasonography is usually impractical
because of the large area that must be evaluated. The transducer must be placed immediately
over the lesion to be identified. However, once an area of interest is identified radiographically,
the area may be evaluated readily to determine the tissues involved and the