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Use of Thoracic Ultrasonography in the Ambulatory and Referral Setting
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1. Introduction
Thoracic ultrasonography is a widely used diagnostic technique for the evaluation of equine thoracic diseases in both the ambulatory and referral settings, yielding information about the lung and pleural cavity that can be attained noninvasively and stallside.1,2 The side(s) 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 dorsal displacement of the peripheral lung). The character of pleural fluid can be determined ultrasonographically, as can the type and severity of many types of pulmonary parenchymal disease. The cranial mediastinum can also be evaluated ultrasonographically.
2. Examination Technique
Ideally, the hair over the portion of the thorax under examination should be removed with a No. 40 surgical clipper blade. The size of the clipped area should initially be based on the auscultatory findings and enlarged as needed to include the entire abnormal lung. In many horses with fine hair coats, an adequate image can be obtained by spraying and saturating the hair and underlying skin with alcohol or cleaning the hair and skin and then applying an ultrasound coupling gel in the direction of hair growth.
In a normal horse, the lung can be imaged on both sides of the thorax from just below the dorsal para spinous musculature to the ventral part of the tho rax where the lung crosses the diaphragm. The initial scanning of the thorax should be performed with the highest frequency transducer that pene trates to the area of interest to obtain the best image quality. A 7.5- to 15.0-MHz tendon transducer and a depth setting of 5 to 6 cm and a 6.0- to 10.0-MHz microconvex transducer and a depth setting of 6 to 10 cm are both good for scanning the thorax of foals and horses with superficial pathology. If extensive pulmonary or pleural disease is detected in an adult horse or the horse is obese, a lower-frequency trans ducer (5.0, 3.5 or 2.5 MHz) and/or an increased dis played depth (25 to 30 cm in adult horses with severe pleural or pulmonary disease) may be needed to penetrate and successfully image the abnormality in its entirety. The scan should proceed slowly in a dorsal to ventral direction in each intercostal space (ICS) with the transducer held parallel to the ribs so that an entire respiratory cycle is imaged before moving ventrally to a different area. The right apical lung lobe and cranial mediastinum are imaged by placing the transducer in the right 3rd ICS just above the level of the point of the elbow and angling the transducer cranially across the thorax towards the point of the left shoulder.1,2 The cranial medi astinum can also be imaged by placing a low fre quency transducer over the triceps in the 3rd ICS and scanning through the musculature into the cra nial portion of the thorax.
3. Normal Structures
There is a large difference between the acoustic im pedance of air and soft tissue resulting in air being a near perfect reflector of ultrasound. Therefore, the normal visceral pleural surface of the lung appears as a straight hyperechoic line with charac teristic equidistant reverberation air artifacts indicating normal aeration of the pulmonary periphery (Fig. 1). Watching the lung as the horse breathes, the visceral pleural surface of the lung is imaged gliding over the diaphragm and moves ventrally with inhalation and dorsally with exhalation, “the gliding sign.”1,2 In most normal horses there is no pleural fluid visualized, although small accumulations (up to 3.5 cm) of anechoic pleural fluid in the most ventral portions of the thorax have been detected in clinically normal horses. The diaphragm is curvilinear and appears thick and muscular in the more ventral locations and thin and tendinous dorsally and caudally. The lung covers the cranial and caudal mediastinum in most horses, although a hypoechoic soft tissue mass (thymus) may be visualized in young horses in the cranial mediastinum.
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