Thoracic ultrasonography yields information about the lung and pleural cavity heretofore unavailable to the veterinarian. Almost the entire thorax can be evaluated ultrasonographically, including the cranial mediastinal region. The side or sides of the thorax affected, as well as the precise location of lesions, can be determined in most horses because the involved lung segment is usually pleural based (exceptions are lesions located in the axial portion of the lung with no peripheral lung involvement, or a hernia in the axial portion of the diaphragm with no gastrointestinal viscera against the thoracic wall or displacing the lung dorsally). The character of pleural fluid can be determined ultrasonographically, as can the type and severity of underlying pulmonary parenchymal disease in horses with pleural effusion.

Normal structures

There is a large difference between the acoustic impedance of air and soft tissue resulting in air being a near perfect reflector of ultrasound. Therefore, the normal visceral pleural edge of the lung appears as a straight hyperechoic line with characteristic equidistant reverberation air artifacts indicating normal aeration of the pulmonary periphery. Watching the lung as the horse breathes; the visceral pleural edge of the lung is imaged gliding ventrally across the diaphragm with respiration. In most normal horses there is no pleural fluid visualized. However, small accumulations (up to 3.5 cm) of anechoic pleural fluid have been detected in clinically normal horses.

Pleural effusion

The most common pleural abnormality (pleural effusion) appears as an anechoic to hypoechoic space between the lung, thoracic wall, diaphragm, and heart. This fluid causes compression of normal healthy lung parenchyma (compression atelectasis), retraction of the lung towards the pulmonary hilus, and a ventral lung tip that floats in the surrounding fluid. The amount of pleural fluid in the thorax can be roughly estimated by the fluid level and the amount of pulmonary parenchymal consolidation or abscesses present. With pleural effusions, another normal structure, the pericardial-diaphragmatic ligament, a normal pleural reflection of the parietal pleura over the diaphragm and heart is imaged as a thick membrane floating in pleural fluid. This membrane runs from the thoracic side of the diaphragm over the heart and appears as a 3-6 mm thick, undulating sheet of homogeneous tissue. Similarly, scanning in the cranial mediastinum in horses with pleural effusion usually reveals sonolucent fluid with dorsal displacement of the lung. A thick echogenic band of tissue is imageable dividing the mediastinum into right and left sides.

Pleural fluid character

The sonographic pattern of pleural effusions includes anechoic, complex non-septated, and complex septated fluid. Composite fluids are complex and more echogenic than normal, containing fibrin, cellular debris, a higher cell count and total protein concentration, and gas. Anechoic sonolucent fluid represents a transudate or modified transudate with a relatively low cell count and total protein concentration. Increased echogenicity of the fluid indicates an increased cell count or total protein concentration. Blood within the pleural cavity (hemothorax), or within any body cavity often has a hypoechoic to echogenic swirling pattern, may be septated, and can usually be differentiated from a more purulent exudate by the latter's more homogeneous echogenic appearance, and the tendency for layering to occur with pyothorax. Clotting may be imaged in pleural fluid as soft, echogenic masses.

Hemangiosarcoma should always be considered in the differential diagnosis of hemothorax in adult horses, as this is one of the more common thoracic neoplasms in horses. Thoracic trauma should be considered in neonatal foals with hemothorax and the ribs carefully evaluated for fractures. An ultrasonographic evaluation of the thorax is indicated in all neonatal foals with known or suspected thoracic trauma to look for pericardial, cardiac, pulmonary or diaphragmatic injury and hemothorax or pneumothorax, as any of the preceding conditions can be life threatening. The cells and cellular debris in pyothorax are more echogenic, heavier, and in the most ventral location while the less cellular fluid or gas cap is detected dorsally. Free gas within the fluid (polymicrobullous fluid) is imaged as small, very bright hyperechoic echoes within pleural fluid with more free gas echoes imaged dorsally in the pleural fluid. The free gas echoes often adhere to the fibrinous pleural surfaces and may be detected here initially without being mixed into the pleural fluid. Free gas echoes may also be compartmentalized in only one portion of the thorax when initially imaged, but usually spread rapidly to all portions of the thorax. Free gas echoes are usually caused by an anaerobic infection within the pleural cavity.

Fibrin has a filmy to filamentous or frond-like appearance and is usually hypoechoic. Fibrin is deposited in layers or in web-like filamentous strands on the parietal and visceral pleural surfaces. Loculations between the parietal and visceral pleural surfaces of the lung, diaphragm, pericardium, and inner thoracic wall limit pleural fluid drainage. As these fibrin strands become more organized and fibrous they become more rigid and echogenic, often distorting the structures to which they are attached during one phase of respiration and possibly restricting pulmonary mechanics. This fibrin may eventually organize in the cranial mediastinum and wall this area off from the rest of the thorax, resulting in a cranial mediastinal abscess.