Ultrasonography of the Head
University of Liverpool, Wirral, UK
Ultrasonographic assessment of the head has most frequently been utilized to image the ocular and periocular structures (see Chapter 25). Imaging modalities such as radiography, endoscopy, and, increasingly, computed tomography are more commonly utilized to image other structures of the head. This is largely due to the fact that a number of anatomic areas of interest, such as the nasal passages, paranasal sinuses, and cranium, are encased within bone, precluding ultrasonographic assessment of these structures. In addition, soft tissue structures, such as the guttural pouch, are air filled in the normal horse, which limits ultrasonographic assessment of them. Bony structures such as the rami of the mandible may impede access when attempting to image soft tissue structures located in the caudal and more ventral regions of the head. However, ultrasonography has been shown to be a useful imaging modality in assessment of the temporomandibular joint and as an adjunctive technique for assessing the larynx and adjacent structures. Superficially located and other accessible soft tissue structures, such as salivary glands, masseter muscle, and tongue, can also be assessed ultrasonographically, as can the surface of the thin bones of the skull. Therefore, whilst its applications may be relatively limited in imaging of the non-ocular structures of the head, ultrasonography can provide valuable adjunctive information when assessing a variety of structures.
Sedation may not be required in all horses but may improve image acquisition, particularly where the larynx is being assessed, to avoid movement artifact and improve patient compliance.
Normal Anatomy and Scanning Technique
Scanning the temporomandibular joint requires a 7.5–10 MHz linear or convex transducer and a standoff may be required in thin horses with little periarticular fat. The standard views are caudolateral, lateral, and rostrolateral. Structures that can be visualized include all or part of the temporal bone, including the retro-articular process (on the caudolateral view), the mandibular fossa (on the lateral view), and the articular tubercle (on the rostrolateral view). Additionally, the condylar process of the mandible and the articular disc, cartilage, and fluid, as well as the joint capsule can be seen, along with the parotid salivary gland, parotidoauricularis muscle, and the transverse facial vein (on the rostrolateral view).
For the caudolateral view, place the transducer (with or without a standoff) over the dorsal aspect of the vertical ramus of the mandible centered over the temporomandibular joint (TMJ) in a transverse position (Figure 10.1). The long axis of the transducer should be positioned in a rostroventral to dorsocaudal direction so that it is oriented parallel to the frontal and nasal bones. The condylar process of the mandible and retro-articular process of the temporal bone can be visualized as thin hyperechoic lines (Figure 10.2). Sandwiched between these two bones is the fibrocartilaginous intra-articular disc, which has a homogeneous, moderate echogenicity, similar to that of the menisci in the stifle joint (i.e. midway between muscle and fascial echogenicity). The base of this triangular-shaped structure is abaxial and approximately 2 cm wide in the adult horse; the disc then narrows axially. The disc also narrows towards the rostral aspect of the joint. Articular cartilage can be visualized as a hypoechoic layer between the bone and the intra-articular disc; this may be up to 3 mm thick in foals and is often barely visible in adult horses. It is uncommon to visualize any articular fluid. The parotid salivary gland lies superficial to these structures and the more hyperechoic TMJ capsule fibers can be seen to merge with this structure. The parotidoauricularis muscle can also be imaged on this view.
For the lateral view, rotate the dorsal aspect of the probe by 90° (Figure 10.1). The same structures can be visualized, with the mandibular fossa of the temporal bone now being visualized dorsally and the intra-articular disc again having a triangular shape (Figure 10.3). The parotidoauricularis muscle can sometimes be imaged on this view.
For the rostrolateral view, the whole probe should be moved rostrally by 1–2 cm and the dorsal tip rotated a further 30° in a rostral direction (Figure 10.1). The articular tubercle of the temporal bone is visible dorsally and the mandibular condyle ventrally. The intra-articular disc now appears as a thin wedge sandwiched between these two structures, and there is little in the way of soft tissue structures overlying this aspect of the joint (Figure 10.4). The transverse facial vein can often be visualized ventrally on this view.
Any disruption to the normal, smooth periarticular outline of the mandibular condyle or temporal bone or disruption to the substance of the intra-articular disc, for example tears or focal hypoechogenicity, is considered to be highly indicative of pathology within the TMJ (Figure 10.5).
Normal Anatomy and Scanning Technique
Ultrasonography of the larynx requires an 8.5–12.5 MHz linear array or convex transducer. The standard views are rostrovental, midventral, caudoventral, and caudolateral. Structures that can be visualized include parts of the basihyoid bone including the lingual process, portions of the ceratohyoid and thyrohyoid bones, the base of the tongue, the insertion of the thyrohyoid muscles, the ventral and abaxial aspects of the thyroid and cricoid cartilages, portions of the cricoarytenoideus lateralis, cricoarytenoideus dorsalis, and vocalis muscles, the vocal cords, and the abaxial aspects of the arytenoid cartilages.
To image the larynx, the horse’s head should be held in a slightly extended position and the transducer initially positioned over the ventral aspect of the larynx. For the rostrovental view, the transducer should be placed in a transverse position immediately rostral to the base of the basihyoid bone between the rami of the mandible (Figure 10.6). The lingual process of the basihyoid bone can be identified (Figure 10.7). In some horses, narrow width of the intermandibular space may prevent this view from being obtained if a linear transducer is used. The lingual process can be followed caudally to the body of the basihyoid bone and, by rotating the transducer in a more rostral orientation, the ceratohyoid bones may be visualized as two, flat hyperechoic structures that course in a dorsal direction (Figure 10.8). The base of the tongue can be visualized but may require use of a lower-frequency transducer (2–5 MHz) to assess its full depth. The same structures can be visualized on the ventral midline (median) and just off the midline (paramedian) in a longitudinal view.