Thoracic ultrasound (US) is an imaging technique complementary to radiology and computed tomography in patients with thoracic diseases. Ultrasonography has many advantages (wide availability, noninvasive, quick and real-time exam, guided biopsy procedures), and it is routinely used in veterinary medicine in abdominal diseases. Many applications have been described for thoracic pathologies as well (cardiac and extra-cardiac). Among the extra-cardiac pathologies, common applications are thoracocentesis and fine-needle aspiration biopsy of pleural/subpleural, thoracic wall and mediastinal lesions.
Recently, ultrasonography of the lungs has been shown to have clinical application in small animal patients affected both by respiratory and cardiac diseases, particularly in emergency situations.
The main limitations of thoracic ultrasonography are the narrow operational window and experience of the operator. This technique should be considered as a complementary procedure in the imaging screening of the thorax and should not exclude conventional radiology.
Thoracic Wall and Pleural Space
Thoracic wall lesions are routinely assessed with conventional radiography and, when possible, are evaluated with advanced imaging modality such as computed tomography (CT). In situations in which anesthesia of the patient or economical reasons wouldn't allow the screening of a thoracic lesion with CT, a quick and safe procedure is certainly ultrasonography of the thoracic wall/cage. Soft tissue tumors of the thoracic wall or rib neoplasia (primary or metastatic) can be evaluated with ultrasound. This technique allows evaluation of, up to a certain point, the extent of the lesion, its margins and relation with the adjacent structures and the performance of fine-needle aspiration biopsy under sonographic guidance, helpful in achieving the final diagnosis. Surely ultrasonography won't have the general diagnostic power of an advanced imaging modality such as CT, but it could be enough to further characterize the disease process evident on conventional radiography.
A second, very important application of thoracic ultrasonography is the possibility of performing thoracocentesis under ultrasonographic guidance and evaluating lesions possibly masked on the thoracic radiography. Dogs and cats affected by pleural effusion are not easily manageable, dyspneic and possibly presented in life-threatening conditions. In these cases, stabilization of the clinical condition of the patient is mandatory. Thoracic ultrasonography has then a double application: first, it allows drainage of the pleural effusion under ultrasonographic guidance, avoiding stress for the patient and possible damage of the heart and large vessels; second, it can allow detection of primary thoracic lesions responsible for the formation of the pleural effusion, which could have been not visible on conventional radiography. In cases of chylothorax, US can also be used for injection of iodinated contrast medium in the sternal or popliteal lymph nodes in order to perform a lymphangiography and evaluate possible thoracic duct/s pathology.
Mediastinal masses can have a solid or fluid component (cyst, necrosis). In conventional radiology mediastinal enlargement is suggestive of fluid accumulation or space-occupying lesions. The opacity of a fluid or solid structure will be identical on conventional radiology (i.e., soft tissue opacity). Thoracic ultrasonography is important in these cases to evaluate the composition of the lesion, its extension, margins and relationship with adjacent anatomical structures. Also, suspected sternal lymphadenopathy can be confirmed with thoracic US, and fine-needle aspirates can be performed.
Another possible application of mediastinal US is the evaluation of the cardias and caudal thoracic esophagus via transabdominal-retroxiphoid approach in patients suspected of esophageal space-occupying lesions (foreign bodies, neoplasia).
Air contained in the gastrointestinal tract is well known as "ultrasonographic enemy" since, secondary to reverberation artifact, it reduces dramatically the view of underlying structures. Normal lung with homogeneously filled air parenchyma causes a total reflection of ultrasounds, with multiple clear parallel hyperechoic lines (A-lines). In these situations, the lung parenchyma is not visible, and the only detectable structure is the pleura, a smooth hyperechoic line which is synchronously moving with respiration. In pathological infiltrative conditions (edema, pneumonia, neoplasia) affecting the periphery of the lung parenchyma, different artifacts are ultrasonographically visible (B-lines). In emergency care departments, US can be potentially used to monitor lung lesions previously detected on thoracic radiography.
Moreover, a decrease in air content of the lung tissue allows performance of US-guided fine-needle aspiration, useful in achieving a fast cytological diagnosis (e.g., pneumonia versus neoplasia).