Thoracic Ultrasonography, Non Cardiac

Thoracic Ultrasonography, Non Cardiac

Peter R. Morresey

Rood and Riddle Equine Hospital, Lexington, KY, USA

Thoracic Ultrasonography

Ultrasonography allows rapid, stall-side diagnosis of respiratory system disease, largely replacing the need for radiography. With ultrasonography, the pleural surfaces, pleural space, and surface of the lung can be evaluated; however, deeper pulmonary structures are not able to be visualized. In some cases of thoracic disease, pathology of the pleural space and lung may be sufficiently subtle to be unable to be detected by auscultation, radiography, or percussion, yet changes may be readily detected by ultrasonography. Severity of pulmonary consolidation is better assessed with ultrasonography than other diagnostic modalities, and consolidation can be readily identified in horses with pleural effusion. The musculature of the thoracic wall, bony and cartilaginous continuity of the ribs, and the integrity of the diaphragm can also be evaluated.

Ultrasonographic Technique

Regardless of the direction chosen, examination should be systematic and complete, allowing the operator to develop a technique that is highly repeatable between examinations. The entire thorax can be examined in a dorsal to ventral direction from the third to seventeenth intercostal space (ICS). While clipping this area provides the best image, removal of hair is best reserved for discrete areas of pathology, if at all necessary, to maintain acceptable cosmesis. Acoustic gel can be used to provide coupling; however. adequate contact can be achieved by thoroughly wetting the hair with isopropyl alcohol before slicking down the hair to remove trapped air.

The probe in placed centrally in the ICS aligned with the ribs to maximize the acoustic window. Beginning at the most dorsal aspect of the visualized lung, the probe is traversed down the ICS past the level of the diaphragm to ensure the entire lung field and pleural space are imaged. The probe is generally placed perpendicular to the thoracic wall; however, the areas shielded by the ribs can be assessed during horizontal oscillations applied to the probe to sweep the ultrasound beam along this plane. Assessment of the cranial thorax (cranial to the third ICS) presents some difficulty as the triceps musculature covers this area. The probe may be applied to the triceps muscle with depth adjusted to allow pleural and pulmonary evaluation. Alternatively, the probe can be placed under the right triceps musculature with this limb protracted cranially, and the probe angled appropriately to image the cranial mediastinum.

Normal Anatomy


Within the cranial thorax, the heart lies deep to the triceps musculature [1]. The cardiac notch of the lung allows visualization of the heart, extending from the third rib to the fourth ICS on the right side, and the third to sixth ribs on the left side [1]. Assessment of the heart by echocardiography is detailed in Chapter 21: Cardiology: Introduction.


The diaphragm extends cranially to contact the heart, with the most ventral aspect reflecting caudally. Visual separation between the lung fields and the abdominal viscera is demarcated by the hyperechoic diaphragmatic musculature which reflects on to the thoracic wall (Figure 20.1). If diaphragmatic musculature is absent, contact is occurring between the viscera and the lungs resulting from discontinuity of the diaphragm. Pleural and peritoneal fluid may be noted on either side of the diaphragm, sharply delineating it free from direct contact with viscera.

Figure 20.1 Normal diaphragm. The diaphragm (a) separates the thoracic and abdominal cavities. It inserts on the thoracic wall at approximately the eighth to ninth costal cartilage and curves caudodorsally to the eighteenth rib. It then reflects craniomedially and ends at the seventeenth ICS. Lung tip (b), spleen (c), liver (d), and the curved surface of the stomach (e). This sonogram was obtained from the left sixth ICS with a curvilinear probe operating at 4.0 MHz at a depth of 16 cm.

Figure 20.4A  Abnormal lung, inflammation. Multiple radiating hyperechoic lines (“comet-tail artifact”) radiate from the pleural surface (p) parallel to the main ultrasound beam into the pulmonary parenchyma perpendicular to the A lines (A). These are known as B lines (B) and represent areas of non-aerated lung surface. They may be few and discrete (a and b), multiple (c), or coalesce with more severe inflammation such as interstitial pneumonia resulting in shadowing of widespread areas of the pulmonary parenchyma (d). This sonogram was obtained from the right fourth ICS using a curvilinear probe operating at 4 MHz at a depth of 13 cm.

Figure 20.4B  Abnormal lung, widespread inflammation. Similar changes are present to Figure 20.6a; however, B lines are more densely located indicating more severe pulmonary inflammatory changes are present. This sonogram was obtained from the right twelfth ICS using a curvilinear probe operating at 8 MHz at a depth of 8 cm.

Figure 20.4C  Interstitial pneumonia. Severe inflammatory changes involving the pulmonary surface and parenchyma lead to coalescing of B lines resulting in widespread shadowing of deeper structures. This sonogram was obtained from the left seventh ICS using a curvilinear probe operating at 8 MHz at a depth of 8 cm.

Left Hemithorax

Caudal to the heart, the left liver lobe can be visualized deep to the diaphragm. Traversing caudally, the spleen will come into view, adjacent and medial to the left lobe of the liver. The stomach may be noted in cross-section deep to this interface alongside the splenic vein. The spleen is visible between the seventh and seventeenth ICS [2].

Right Hemithorax

Ventral and caudal to the heart, the diaphragm and right lobe of the liver can be visualized extending from the ninth to sixteenth ICS, with the large colon medial to the liver or in apposition with the diaphragm [2]. Dorsally, as on the left side, the lung fields are easily visualized moving across the screen in time with inspiration and expiration during breathing. Mid-thorax, deep to the diaphragm, the liver and right dorsal colon may be seen in close apposition. Caudally, between ICS sixteen and seventeen, the cranial aspect of the right kidney is visible [2].

The apposition of the parietal and visceral pleura may be noted as a hyperechoic interface (Figure 20.2). Motion of the pulmonary parenchyma can be seen synchronously with thoracic excursions during respiration.

Figure 20.2 Normal lung. Ultrasound is used to image the superficial areas of lung contacting the thoracic body wall. This lung field is triangular in shape and extends caudally to the 16th ICS. In these images, the hyperechoic line (arrow) is the pleural surface of normal lung. (a) This sonogram was obtained from the left, 8th ICS using a linear probe operating at 5.0 MHz at a depth of 8 cm. (b) This sonogram was obtained from the left, 8th ICS using a micro-convex probe operating at 6.6 MHz at a depth of 6 cm.

An anechoic space from a small amount of fluid (up to 3.5 cm reported) [1] is present in the majority of horses over the right cranioventral lung field. This may also be present in a lesser amount on the left side in these horses. This fluid creates separation of the parietal and visceral pleura. This is not an indicator of a disease state, rather this is thought necessary for lubrication of the pleural surfaces allowing unimpeded movement between the parietal and visceral pleura.

Pulmonary Appearance

Normal aerated lung surface appears as a smooth hyperechoic interface at the pleura of the thoracic wall sliding perpendicularly to the long-axis of the ultrasound beam. Parallel, hyperechoic lines beneath the pleural line and at even spaces of the distance between the ultrasound probe and the visceral-parietal pleural interface, may appear within the pulmonary parenchyma (Figure 20.3). These are known as A-lines and result from multiple acoustic reflections between the pleura and the probe. Loss of aeration is represented by B-lines, traveling perpendicular to A-lines, which radiate from the pleural surface into the pulmonary parenchyma (Figure 20.4). These are colloquially known as the comet-tail artifact. They range from solitary, multiple discrete, to coalescing, which indicate a more widespread loss of aeration and shadow deeper pulmonary structures.

Figure 20.3  Normal lung. The pleural interface appears as a hyperechoic line (p). Multiple parallel hyperechoic lines are evenly spaced through the pulmonary parenchyma, known as A lines (A). This sonogram was obtained from the right sixth ICS using a curvilinear probe operating at 4 MHz at a depth of 15 cm.

Figure 20.5A  Fractured rib. Neonatal rib fracture with minimal displacement of distal segment making diagnosis by palpation potentially difficult (F). This sonogram was obtained from the right thorax using a micro-convex probe operating at 6.0 MHz at a depth of 10 cm.

Figure 20.5B  Fractured rib. Neonatal rib fracture with overriding fracture ends (F). An acute fractured rib in a neonatal foal; the proximal (arrowhead) overrides the distal fragments (small arrowhead) which is directed medially. Pleural hemorrhage and thoracic wall hemorrhage result. This sonogram was taken from the left thorax, ninth rib using a micro-convex probe operating at 8.0 MHz at a depth of 8 cm.

Pathology of the Thoracic Wall

Rib fractures are more readily detected by ultrasonography than radiography in neonatal foals [3]. Changes in continuity of the hyperechoic border of the ribs, overriding of fracture ends, as well as the size and location of rib fragments, are visualized (Figure 20.5). Disruption of the costochondral junctions may also be noted, with often no indicative clinical signs [4]. Associated subcutaneous and intramural hematomas can also be detected and their extent assessed (Figure 20.6, 20.7). Pulmonary damage can also be assessed (Figure 20.8).

Figure 20.7  Fractured rib with mural hematoma and callus formation. This image shows a fractured rib with an organizing hematoma and the initial stage of fracture stabilization (callus formation). This sonogram of the right fifth rib was obtained with a probe operating at 8.0 MHz at a depth of 8 cm.

Figure 20.8  Fractured rib with distal fragment and lung trauma. The proximal rib (arrowhead) contains a callus (C) connecting to a fragment (F) and is overriding the distal portion (D). The adjacent lung displays increased echodensity consistent with pulmonary contusion. This sonogram was taken from the left thorax, fifth rib using a micro-convex probe operating at 8.0 MHz at a depth of 8 cm.

Pathology of the Pleural Space


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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Thoracic Ultrasonography, Non Cardiac

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