Ultrasonography of the Soft Tissue Structures of the Neck
Clinica Veterinaria Spirano, Spirano (BG), Italy
Preparation and Scanning Technique
Optimal equipment for ultrasonography of structures of the neck should include a high frequency, 10–12 MHz linear probe and a 6.6 MHz micro-convex probe. These two probes allow visualization with maximal definition of both superficial and deep structures. However, many structures of the neck can be visualized using only a 5–7.5 MHz linear probe typically used for equine obstetrics. Best images are obtained by clipping the hair with a surgical clipper blade and applying coupling gel; however, alcohol will also serve as an adequate contact medium. Ideally, place an examination glove or similar over the probe because continued exposure to coupling gel, particularly alcohol, can cause degradation of the probe and potentially affect image quality and life span of the probe (Figure 26.1).
Orientation of the probe must be kept constant over time to obtain comparable images. When scanning in the longitudinal axis (Figure 26.2A), orientate the probe so that distal structures are on the left side of the screen and proximal structures are on the right (Figure 26.2B). When scanning in the cross (short) axis (Figure 26.2C), orientate the probe so that ventral structures are on the left side of the screen and dorsal structures are on the right (Figure 26.2D).
Structures visualized when scanning the neck include major blood vessels (jugular veins and carotid arteries), parotid salivary glands, guttural pouches, thyroid glands, lymph nodes, esophagus, trachea, and muscle (Figure 26.3). All structures are bilateral, except for the esophagus, trachea, sternohyoideus muscle, and sternothyroideus muscle, so suspect images can be compared to the opposite side assuming a lesion or abnormality is unilateral.
The jugular vein (Figures 26.4, 26.5, 26.6, 26.7, 26.8, 26.9, 26.10, 26.11) is visible in the jugular groove, from the mandibular branch to the thoracic inlet. Normally, it is thin walled and easily compressible with the ultrasound probe. The blood inside the jugular vein has a hyperechoic, swirling appearance when compared to blood in the adjacent carotid artery. The distal part of the vein should be compressed to dilate the proximal portion of the vessel being scanned to aid in correctly visualizing all the structures, including the valves of the vein. Valves are normal structures that can easily be confused with a small thrombus. Valves appear as thin structures adjacent to the walls of the vessel and move in synchrony with blood flow.
Ultrasound of the jugular vein can confirm correct placement of an intravenous catheter in the vessel lumen and may aid in detecting early signs of thrombus formation associated with the catheter, although thrombi may also form in the jugular vein in the absence of an intravenous catheter or any other known trauma to the vessel. Thrombi appear as hyperechoic structures in the vessel lumen or attached to the vessel wall. Margins may be smooth and well demarcated or pedunculated in appearance. If an intravenous catheter is in the vein, a fibrin envelope or sleeve may be visualized around the catheter before a thrombus forms . Initially, blood flow is maintained, but the thrombus may increase in size to cause inflammation of the vessel (thrombophlebitis) and, potentially, complete occlusion of the vessel. Other changes associated with thrombophlebitis include perivasculitis of the surrounding soft tissue and thickening of the vessel wall .
The carotid artery (Figures 26.4, 26.5, 26.6, 26.8) is visible in the jugular groove medial to the jugular vein and lateral to the trachea. The wall is thicker than that of the jugular vein and the vessel is not compressible. The blood is hypoechoic compared to blood in the jugular vein. In the proximal neck, the carotid artery is separated from the jugular vein by the omohyoideus muscle, but in the middle and distal neck, the muscle thins and the carotid artery and jugular vein are in close proximity to each other.
The guttural pouches are ventral diverticula of the eustachian tube and extend from the nasopharynx to the middle ear. They contain air and have a capacity of 300–500 ml in adults. These structures are usually not detectable by ultrasound examination except in the diseased state.
Guttural pouch tympany (Figure 26.12) occurs in young animals, usually less than 1 year of age, when the plica salpingopharyngea, the flap of tissue covering the pharyngeal opening, fails to let air escape the guttural pouch. With the flap functioning as a one-way valve, the pouch fills with air. Some cases are mild, while others progress to respiratory distress from occlusion of the pharyngeal area from the distended guttural pouch .
Guttural pouches may contain exudate or chondroids (inspissated pus) with guttural pouch empyema (Figures 26.13, 26.14). Empyema may occur secondary to any respiratory disease, but infection with Streptococcus equi is a common finding. The pouches may contain blood if trauma occurs to any structure associated with the guttural pouch. A common finding in guttural pouch trauma and hemorrhage is fracture of the stylohyoid bone. Another cause of blood in the pouch is guttural pouch mycosis. A fungal plaque forms in the pouch and causes erosion of a blood vessel, usually the internal carotid artery, resulting in hemorrhage into the guttural pouch.