Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 93
Fig. 2-66 A 5-year-old female
Keeshond with dyspnea and anemia.
Radiographs revealed a hydrothorax.
A right parasternal long-axis view
revealed the presence of pleural fluid
(f) and a heteroechoic mass (m), with
a small cystic area (arrow) arising
from the visceral pleura and lying
adjacent to the heart. Diagnosis:
Hemangiosarcoma.
dorsoventral with ventrodorsal radiographs of animals with pneumothorax; however,
the major features are the same (Fig. 2-67). 152,153 In the recumbent lateral radiograph,
these features include separation of the lung lobes from the ventral and dorsal thoracic
wall, separation of the heart from the sternum, separation of the lung lobes from the
diaphragm, and an overall increase in pulmonary density due to lung lobe atelectasis.
Air may become trapped within the mediastinum as the animal is rotated (Fig. 2-68).
This will produce soft tissue–dense well-defined lines (mediastinal folds), usually in the
caudal, postcardiac mediastinum, extending from the cardiac apex to the sternum or
diaphragm. 154
In the recumbent ventrodorsal and dorsoventral radiograph, the lungs will be separated
from the lateral thoracic wall by air density. This is identified by the lack of pulmonary
vasculature and airways traversing the air density beyond the border of the
lung. The density of the lung lobes will increase due to atelectasis. Air may be identified
between the lungs and the diaphragm. If only small amounts of air are present, it may
not be visible on the ventrodorsal view because the air is centralized below the sternum.
It may be recognized more readily on the dorsoventral view, because it may result in
minimal displacement of the lungs away from the visceral pleura. On occasion, air may
accumulate between the heart and lung lobes and may become trapped within the
mediastinum, outlining the mediastinal folds but not the individual mediastinal
structures.
A lateral radiograph obtained using a horizontal x-ray beam is helpful for detecting
small amounts of pleural air. Furthermore, small amounts of pleural air may be accentuated
by obtaining radiographs at expiration rather than at inspiration.
As with pleural fluid, air should move freely within the pleural space and distribute
evenly, rising to the highest point within the thorax. The lungs should collapse uniformly.
If this does not occur and one lung lobe or a portion of a lung lobe is denser than
the others, lung lobe disease should be suspected. Additional radiographs or repeated
evaluation after removal of the pleural air may be helpful in these instances. Unilateral
pneumothorax is rare in dogs and cats. However, if previous or coexisting pleural or
mediastinal disease has produced adhesions or pleural thickening, a unilateral pneumothorax
can occur.