Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 45
A
Fig. 2-14 A 4-year-old spayed Doberman Pinscher with a mass in the oral cavity. The thorax was
radiographed to evaluate the dog for the possibility of pulmonary metastasis. A, In the initial lateral
radiograph the trachea appeared to be deviated dorsally, cranial to the cardiac silhouette. B, A second
lateral thoracic radiograph was obtained with the head and neck extended. The tracheal deviation is
no longer identified. There was no evidence of pulmonary metastasis. Diagnosis: Normal thorax.
B
normal esophagus can be seen on the lateral radiograph as a fluid-dense stripe dorsal to
the caudal vena cava and extending caudally from the level the heart to the esophageal
hiatus of the diaphragm. In other individuals, visualization of this structure may represent
an abnormal finding.
The esophagus is located to the left of the trachea at the thoracic inlet, becomes dorsal
to the trachea in the cranial mediastinum, and continues at that level to the diaphragm.
The esophagus can become dilated when an animal is anesthetized or exhibit marked
aerophagia associated with struggling during restraint. This dilation should not be misinterpreted
as esophageal disease.
C A R D I AC S I L H O U E T T E
Radiographic evaluation of the cardiac silhouette is complicated by the size and shape variations
in different species, breeds, and individuals, and by the effect of malpositioning on
its appearance. The cardiac silhouette is not a profile of the heart itself but is composed of
the heart, pericardium, pericardial and mediastinal fat, and other structures at the hilus of
the lung.
Cardiac size varies slightly with respiration. A minimal increase in size at expiration is
enhanced by a decreased lung volume and increased sternal contact.
The cardiac size and shape vary during the cardiac cycle. 47 This change is most noticeable
in larger dogs and when short exposure times are used. Ventrodorsal radiographs of
the heart during ventricular systole are characterized by a widened cranial portion, narrowed
apex, and bulging or accentuation of the main pulmonary artery.
Because the angulation and position of the heart within the thorax vary depending
on the animal’s thoracic conformation, the cardiac shape also varies. Dogs with deep and
narrow thoracic conformation have an elongated, thin, oval, or egg-shaped heart, which
often seems relatively small when compared with the thoracic volume. On the lateral
views of these dogs the apex–base axis of the heart is almost perpendicular to the spine
with minimal sternal contact (see Figs. 2-11 and 2-14). The normal variation in cardiac
shape, observed when comparing ventrodorsal and dorsoventral radiographs, is most