Page 323 - Small Animal Internal Medicine, 6th Edition

CHAPTER 20 Diagnostic Tests for the Lower Respiratory Tract 295

ULTRASONOGRAPHY

VetBooks.ir In the emergency setting, thoracic ultrasonography is used
for the rapid identification of pleural effusion. Its applica­
tion in the rapid diagnosis of pulmonary edema and other
parenchymal disease is growing. In this setting the acronyms
TFAST (Thoracic Focused Assessment with Sonography for
Trauma) and VetBLUE (Veterinary Bedside Lung Ultra­
sound Examination) are used (Lisciandro, 2011; Lisciandro
et al., 2014).
In the nonemergent setting, ultrasonography is used to
evaluate pulmonary mass lesions adjacent to the body wall,
diaphragm, or heart and also consolidated lung lobes (Fig.
20.11). Because air interferes with sound waves, aerated
lungs and structures surrounded by aerated lungs cannot be
examined. However, some patients with a reticular intersti­
tial pattern on thoracic radiographs have sufficient infiltrates
to be visualized where they abut the body wall. The consis­
tency of lesions often can be determined to be solid, cystic,
or vascularized. Some solid masses are hypolucent and
appear to be cystic on ultrasonograms. Vascular structures
may be visible, particularly with Doppler ultrasound, and
this can be helpful in identifying lung lobe torsion. Ultraso­
nography can also be used to guide needles or biopsy instru­
ments into solid masses for specimen collection. It is used in
evaluating the heart of animals with clinical signs that cannot
be readily localized to the cardiac or the respiratory system.
Ultrasonographic evaluation of patients with pleural disor­
ders is discussed in Chapter 23.

FIG 20.10
Ventrodorsal view of the thorax in a cat showing a cystic COMPUTED TOMOGRAPHY AND
lesion (arrowheads) in the left caudal lung lobe. Differential MAGNETIC RESONANCE IMAGING
diagnoses included neoplasia and Paragonimus infection.
CT and magnetic resonance imaging (MRI) are used rou­
tinely in human medicine for the diagnostic evaluation of
lung disease. The accessibility of CT in particular has led
or pneumonectomy in dogs and cats. The right middle and to its increased use in dogs and cats and it is now used
left cranial lobes are most commonly involved. The lobe routinely in the diagnostic evaluation of challenging respi­
usually twists at the hilus, obstructing the flow of blood ratory cases. The resultant three­dimensional images are
into and out of the lung lobe. Venous drainage is obstructed more sensitive and specific for the identification of certain
before arterial flow, causing the lung lobe to become airway, vascular, and parenchymal diseases as compared
congested with blood. Inflammation and necrosis ensue. with thoracic radiography. In one study of dogs with meta­
Over time, air is absorbed from the alveoli and atelectasis static neoplasia, only 9% of nodules detected by CT were
can occur. identified by thoracic radiography (Nemanic et al., 2006).
Lung lobe torsion is difficult to identify radiographi­ Images are routinely obtained before and after the intrave­
cally. Severe bacterial or aspiration pneumonia resulting in nous injection of a contrast agent, which further enhances
consolidation of these same lobes is far more common and the characterization of lesions and allows for the identifica­
produces similar radiographic changes. The finding of pul­ tion of macrothrombi and emboli. Thoracic CT has also
monary vessels or bronchi traveling in an abnormal direc­ become routine for the planning of thoracic surgery. Com­
tion is strongly suggestive of torsion. Unfortunately, pleural pared with standard radiography, the extent of mass lesions
fluid, if not present initially, often develops and obscures and their relationship to major vessels and other critical
the radiographic image of the affected lobe. Ultrasonogra­ structures is better defined, and multifocal disease is more
phy is often useful in detecting a torsed lung lobe. Bron­ likely to be identified (e.g., metastatic lesions or multiple
choscopy, bronchography, computed tomography (CT), or cavitary lesions). The risks associated with CT scanning are
thoracotomy is necessary to confirm the diagnosis in some minimal except that light, general anesthesia is required to
animals. eliminate patient motion and to allow for breath­holding