Christopher R. Lamb, MA, VetMB, DACVR, DECVDI, MRCVS, ILTM
Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London
In animals with suspected thoracic disease, radiography is normally done first because it enables assessment of many thoracic structures, particularly the lungs. Ultrasonography is a complementary technique for examining the thorax because it enables examination of structures that are not normally visible radiographically (e.g., internal structures of the heart) and is useful when structures are obscured by disease.
Animals may be examined through the intercostal spaces. Sector transducers are preferable because their small footprint fits more easily between the ribs. Keeping the patient in sternal recumbency encourages fluid to accumulate ventrally, which improves visualisation of intrathoracic structures. Alternatively, it is possible to see into the thorax using a caudal approach via the liver, which is done by placing the transducer at the xiphoid and directing it craniodorsally. This may be done with the patient standing or in lateral recumbency. I routinely have a quick look into the thorax when scanning the liver, which enables detection of unsuspected pleural or pericardial fluid.
In an animal presented with dyspnoea and dull thorax on percussion, in which pleural fluid is suspected, ultrasonography may be done before radiography because it may be less stressful to the patient if it is kept in sternal recumbency. This helps minimise the risk of precipitating a respiratory crisis. When pleural fluid is found, immediate drainage under ultrasound guidance may also be done before radiography. Thoracic radiographs are likely to be more useful when most of the fluid has been removed.
Ultrasonography can be used to detect pleural fluid, estimate its volume and evaluate its character. Fluid transmits ultrasound very well, hence the fluid improves visibility of associated pleural, diaphragmatic or pulmonary lesions. Pleural fluid appears as a hypoechoic or anechoic space between the thoracic wall and lung. Ultrasonography should be used routinely to guide diagnostic pleural aspiration. When using a transhepatic approach, the presence of a mirror-image artifact on the cranial aspect of the liver virtually rules out pleural fluid.
Variations in echogenicity of pleural fluid include:
exudates, haemorrhage--may contain internal echoes and tissue strands or septa due to fibrin or adhesions
gas bubbles in pleural fluid raises the possibility of anaerobic, gas-forming, bacterial infection
Free-flowing pleural fluid may provide acoustic windows at a number of sites; however, localised fluid or masses may make limited contact with the thoracic wall and it may be necessary to search for an adequate window in these instances. Masses in the caudal lobes or accessory lobe are sometimes most clearly visible transhepatically. When only a small volume of pleural fluid is present, it may be necessary to use a parasternal intercostal approach with the animal in ventral recumbency to obtain an adequate window or to scan transhepatically with the animal standing.
Ultrasonography may aid diagnosis of ruptured diaphragm, particularly in animals with equivocal radiographic findings or those that are too dyspnoeic to be radiographed safely. Animals with a large amount of pleural fluid and minimal herniation of viscera may not be diagnosed radiographically. Conversely, traumatised animals with pneumohaemo-thorax may sometimes have pleural gas pockets that can adopt tubular shapes and mimic intestine on radiographs. In both instances ultrasonography can provide definite evidence of the state of the diaphragm.
A transhepatic approach is recommended for ultrasonography because the position of the diaphragm in ultrasound images is normally inferred from the position of the cranial aspect of the liver, which is closely opposed to the diaphragm. The specular echo normally observed on the cranial aspect of the liver is a composite echo representing the surface of air-filled lung and diaphragm. The diaphragm is visible as a distinct, thin echogenic line in ultrasound images of animals with pulmonary consolidation or pleural fluid. A continuous curved echogenic line corresponding to the position of the diaphragm should be visible perpendicular to the ultrasound beam in transverse images in virtually all animals in which the diaphragm is intact, regardless of the presence of pleural or peritoneal fluid or hepatic parenchymal lesions. Because of this, ultrasonography using a transhepatic window with the transducer just caudal to the xiphoid process is an accurate method for detection of diaphragmatic rupture, and the principal ultrasonographic sign of diaphragmatic rupture is lack of the normal anatomical appearance. This approach is far easier than a piecemeal approach through various intercostal spaces.
Ultrasonographic signs of ruptured diaphragm include:
Asymmetrical or invisible diaphragmatic outline
Abnormal liver position
Cranial displacement of abdominal viscera
Abdominal viscera in contact with the heart
It is uncommon to observe the torn edges of the diaphragm. Other mobile tissue, such as the omentum or fibrinous strands could mimic the appearance of a torn piece of the diaphragm. In contrast, a focal, regular defect in the diaphragm may be observed ultrasonographically in animals with true (pleuroperitoneal) diaphragmatic hernia or peritoneopericardial diaphragmatic hernia.
Inability of ultrasound to adequately penetrate the air-filled lung prevents its use for examination of normal thoracic structures other than the heart. The normal lung, being air-filled, reflects virtually all the ultrasound beam, producing a high amplitude, specular echo and preventing assessment of deeper structures; however, when the lungs are collapsed, consolidated or displaced by fluid, the ultrasound beam will penetrate.
Pulmonary consolidation (due to lobar collapse, bronchopneumonia or infarction) may be recognised because it produces a liver-like appearance without a mass effect or loss of the normal shape of the lobe. In some cases of consolidation the principal bronchus is visible as an anechoic branching structure, known as a fluid bronchogram. Tiny bubbles of trapped air often remain in consolidated lung, enabling it to be distinguished from herniated liver.
Assessment of lobar volume aids diagnosis in animals with consolidated lung:
Decreased volume = collapse, e.g., bronchial obstruction, effect of pleural fluid
Normal volume suggests bronchopneumonia
Increased volume suggests infiltration (e.g., neoplasia) or congestion (e.g., torsion)
A pulmonary mass may be visible ultrasonographically if it extends to the surface of the lung (visceral pleura) and comes into contact with the thoracic wall or diaphragm (parietal pleura). Like most masses in other parts of the body, the aetiology of a pulmonary mass cannot be determined from its echogenicity, but ultrasound is a convenient method for guiding fine needle aspirate or biopsy.
In the presence of pleural fluid normal structures such as the mediastinum and caudal vena cava may be visible. The mediastinum is normally visible as an echogenic band of tissue dividing the cranial thorax; the large mediastinal blood vessels may be visible dorsally when sufficient fluid is present. In obese animals, the mediastinum may be wide, but fat normally has a uniform hyperechoic appearance that can be distinguished from masses.
A mediastinal mass may be visible if the mass comes into contact with the thoracic wall or is surrounded by pleural fluid. Once identified, ultrasound-guided aspiration or biopsy can be performed to provide a definite diagnosis. Ultrasound-guidance improves accuracy of needle placement and avoids vital structures such as the heart and great vessels.
Mediastinal masses may have variable echogenicity and cavitary components; it may not be possible to distinguish neoplasm, abscess or haematoma on the basis of ultrasonographic findings. The commonest mediastinal mass--lymphoma--is typically markedly hypoechoic. Finding a multilocular cavitary mass divided by thin septa suggests thymoma. It is worth distinguishing these neoplasms because they are treated differently.
Peritoneopericardial diaphragmatic hernia (PPDH)
This condition, which is usually unassociated with clinical signs, is found sporadically in cats and dogs. Thoracic radiographs usually show enlarged cardiac silhouette with heterogeneous opacity, broad contact between the cardiac silhouette and the line of the diaphragm, and cranial displacement of abdominal viscera. Ultrasonography enables a more detailed assessment of the contents of the pericardium.
1. Reichle JK, Wisner ER. Non-cardiac thoracic ultrasound in 75 feline and canine patients. Vet Radiol Ultrasound 2000;41(2):154-62.