Page 468 - Atlas of Small Animal CT and MRI

4.6

Small airways and parenchyma

Introduction characterize CT patterns in small animal veterinary
medicine. Specific subgross anatomic differences include
Dogs and cats have six well‐defined lung lobes that cor- more limited interlobular septal connective tissue and
respond to the anatomic organization of the principal, or much more collateral ventilation in exchange regions of
lobar, bronchi. Bronchovascular bundles are composed the lung in dogs and cats as compared to people.
of the bronchi, corresponding pulmonary vessels, and
the adjacent interstitial framework. Some pulmonary Atelectasis
disorders arise from or are distributed along the bron-
chovascular bundle. Because normal aerated lung volume is comprised pre-
Tertiary, or segmental, bronchi arise from the lobar dominantly of gas, atelectasis can result in profound vol-
bronchi, and the region of lung ventilated by each seg- ume loss with associated visceral shift and redistribution
mental bronchus is referred to as a bronchopulmonary of remaining aerated lung. Positional atelectasis fre-
segment. Bronchopulmonary segments are further sub- quently occurs as a consequence of recumbency and
divided into secondary pulmonary lobules that are small may be exacerbated by general anesthesia when assisted
subdivisions of the lung encompassed by interlobular ventilation is not used. For this reason, patients sched-
connective tissue septa and composed of a central bron- uled to undergo thoracic CT should be maintained in
chiole, accompanying arteriole and lymphatic vessel, sternal recumbency following anesthetic induction and
and a few pulmonary acini. prior to imaging. Atelectasis may also occur because of
The acinus is the largest anatomic unit within which lung compression by masses and pleural fluid or from
gas exchange occurs and includes one or more first order airway obstruction. CT features of atelectasis include
respiratory bronchioles and two to three generations of volume loss and increased lung attenuation that is
smaller respiratory bronchioles in the dog. Each branch- inversely proportional to the degree of lung collapse.
ing respiratory bronchiole and its associated alveolar Peripheral lung parenchyma is most often affected when
ducts, alveolar sacs, alveoli, and capillaries form a pri- lobar atelectasis is incomplete. Complete collapse of one
mary pulmonary lobule. Therefore, secondary pulmo- or more lobes can occur as a sequela to underlying pul-
nary lobules are composed of multiple acini, and acini monary disease or pleural effusion (Figure 4.6.1). In
are composed of multiple primary pulmonary lobules. 1 uncomplicated atelectasis, the lobar and segmental
Although there are a number of computed tomo- bronchi remain aerated, producing an air bronchogram
graphic descriptive terms characterizing regional or sign. A mediastinal shift toward the affected lung is
diffuse pulmonary patterns in people (crazy paving, characteristic and helpful in differentiating atelectasis
mosaic, etc.) based on the origin or distribution of pathol- from pulmonary consolidation. The remaining aerated
ogy in relation to the subgross anatomic architecture of lung may be hyperinflated and can redistribute to com-
the lung, care should be used in adopting these terms to pensate for atelectatic volume loss.

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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