Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

182 Small Animal Radiolo g y and Ultrasono graphy
Interstitial patterns may be classified as chronic or active according to their radiographic
appearance. An active interstitial infiltrate is characterized by poorly
defined, wide linear interstitial densities with extensive blurring of vascular margins.
As the condition becomes chronic, or resolves with fibrosis, the interstitial densities
become thinner and better defined and the vascular borders become more clearly
visible.
Recognition of an interstitial pattern is important, because it identifies the abnormal
density as being within the supporting tissues of the lung rather than within the end–air
spaces, pleural space, or mediastinum, or outside the thoracic cavity. It also strongly suggests
a cellular nature of the infiltrate. Finally, it suggests that an attempt to aspirate material
via the airways will not likely yield diagnostic material. Rather, direct sampling (e.g.,
fine-needle aspirate, biopsy) is more likely to be successful.
B R O N C H I A L PAT T E R N
The bronchial pattern results from fluid or cellular infiltrate within the bronchial wall and
peribronchial and perivascular connective tissue of the lung. 503,509-511 It results in
bronchial wall thickening and outlining of many bronchial structures not normally identified
(Fig. 2-165). These bronchial structures may be seen as paired, somewhat parallel lines
that converge slightly and branch in pairs. These paired lines are thinner and do not taper,
as do pulmonary vessels, maintaining a fairly uniform width throughout their course. They
have been described as “railroad tracks” or “tram tracks” because they converge slightly as
they extend toward the lung periphery in a fashion similar to the way railroad tracks appear
to converge as they get farther away from an observer. When these structures branch perpendicular
to the x-ray beam they produce a tissue-dense circle or “doughnut” with a radiolucent
center. This pattern suggests that an attempt to aspirate material via the airways
should be rewarding.
Another form of the bronchial pattern is seen with bronchiectasis. 510-512 Two forms of
bronchiectasis have been described: tubular and saccular. Tubular bronchiectasis consists
of bronchial dilation and loss of the normal taper of the bronchial lumen as it progresses
distally into the lung. 513 Saccular bronchiectasis consists of focal bronchial dilations. Both
forms are associated frequently with bronchial thickening due to chronic inflammation.
Fig. 2-164 A 7-year-old male
German Shepherd dog with a mass
in the caudal abdomen. Thoracic
radiographs were obtained to evaluate
the dog for pulmonary metastasis.
A photographic enlargement of
the caudal lung lobes is illustrated.
The pulmonary vessels are obscured
by a pattern of linear soft-tissue densities.
This represents pulmonary
interstitial fibrosis and is a common
finding among the normal features
of the lungs of an aged dog that has
no clinical signs. Diagnosis:
Pulmonary fibrosis.