The normal thorax is well suited to radiographic evaluation because there is marked inherent contrast between the air-filled,
fluid-filled, soft tissue, and bony structures that comprise the thoracic viscera and thoracic wall. As has been stated before,
at least 2 orthogonal views of the thorax are required for complete and accurate interpretation. For routine evaluation of
the thorax, either a right or left lateral projection, and a dorsoventral or ventrodorsal projection of the thorax are required.
Because the thoracic viscera are asymmetrically positioned, the appearance of the diaphragm changes with patient position.
The appearance of the thorax will vary slightly when comparing left and right lateral thoracic radiographs, and dorsoventral
and ventrodorsal radiographs because of variable magnification artifacts associated with patient position. In general, the
position of the cardiac silhouette is most consistent on a right lateral projection. When attempting to characterize a pulmonary
lesion, the lesion is best defined when surrounded by well-aerated lung. The down side, or dependent lung decreases in volume
due to atelectasis when a patient is in recumbency for even short periods of time. Therefore, when attempting to assess a
pulmonary lesion, a view which places the pulmonary region of concern in the most nondependent position is best for evaluation.
In addition, thoracic radiographic images should be obtained while the animal is in the peak inspiration phase of respiration.
The dorsoventral projection provides better consistency in cardiac position and usually provides a better evaluation of the
caudal lung lobe vessels.
Trachea and Bronchial Tree
The trachea can be identified on a lateral radiographic view as a well-defined radiolucent tube extending from the larynx
to the central region of the thoracic cavity. The trachea should have well-defined, thin, radiopaque margins, and a uniform
tracheal lumen diameter. The trachea terminates at the carina, which represents the bifurcation of the major airway into
the two main stem bronchi. The trachea can be seen on a dorsoventral or ventrodorsal view positioned centrally within the
cervical and cranial thoracic regions ending at the carina in the central thorax. Typically, the intrathoracic trachea is
positioned slightly to the right of midline on the DV or VD views. In the dog, the tracheal rings, which provide some rigidity
to the tracheal wall, are incomplete dorsally. The dorsal tracheal membrane often protrudes into the tracheal lumen, particularly
in obese, small breed animals. This can produce an apparent decrease in tracheal diameter on lateral projections, but can
and should be differentiated from the clinically more important abnormalities, the hypoplastic trachea and the true collapsing
trachea.
The main stem bronchi can be seen arising from the carina, both on the lateral and the DV or VD projections. In addition,
in many instances, the major lobar bronchi can be identified coursing through their respective lung lobes. Frequently, the
lobar bronchi are not well visualized, but when seen follow the path of the adjacent lobar arteries and veins. Often the definition
of the bronchi is accentuated by mineralization of bronchial walls (often a clinically insignificant aging change) or due
to peribronchial cellular or fluid accumulations associated with a variety of disease processes.
Pulmonary Parenchyma
In normal animals, the pulmonary parenchyma should be radiolucent but normal lung parenchyma and pulmonary vascular anatomy
should still be appreciated. As mentioned before, thoracic radiographs obtained to evaluate the lungs should be obtained
with the animal in full end inspiration, which provides maximal aeration of the lungs. In general, the nondependent portions
of the lung will be better aerated than the dependent regions, which is in turn dependent on the position of the patient at
the time the radiograph is taken. In general, the major lobar airways and the secondary and tertiary pulmonary blood vessels
to the lobes should be identifiable within the lung parenchyma. The parenchyma itself should have a fine mesh-like cross-hatch
radiolucent appearance. The visceral pleural margins in the normal animal should be seen. Keep in mind however that, although
often the pulmonary lobar margins cannot be defined, lobar divisions are present within the thorax and may become apparent
with the presence of pleural fluid or pulmonary lesions that involve a single lung lobe.
