A large part of the head and neck structures and their related pathologies are located less than 5 cm beneath the skin and thus can be visualized by ultrasonography (USG) .
The sonographic appearance of muscles, ducts, and blood vessels is hypoechoic (fairly dark) relative to fat that is hyperechoic or echogenic. Mucosa appears hyperechoic as well and can be simply distinguished from the hypoechoic muscle which it normally covers. The surface of the mandible near the salivary glands presents as hyperechoic linear line and precludes visualization of the parotid gland’s deeper portions [1–3].
11.1.1 Parotid Gland
The hypoechoic masseter muscle with its pinnate reflection form is a key landmark in the transverse view of the parotid area. Ultrasound easily demonstrates whether pathology is inside or outside of the salivary gland. Typically, the parotid gland has a homogeneous echotexture on ultrasound (Fig. 11.1). The superficial lobe of the parotid is seen superficial to the well-visualized retromandibular vein which acts as a surrogate indicator for the more superficial intra-parotid facial nerve. In the axial plane, the Stensen duct (parotid duct) can be visualized as paired and parallel echogenic lines 3 mm apart, lying superficially to the masseter muscle but is generally only detectable as a hypoechoic/anechoic structure when it is obstructed (Fig. 11.2) [4, 5]. Stensen duct travels medially and anteriorly, then at the border of the masseter, makes a steep right turn to cross the buccal fat pad and hypoechoic buccinator muscle opening into the vestibule of the mouth, opposite to the second maxillary molar .
US examination can be performed reliably for some facial expression and masticatory muscles including: frontalis; orbicularis oculi; depressor labii inferioris; depressor anguli oris; orbicularis oris; mentalis; temporalis; and masseter (Figs. 11.3, 11.4, 11.5). Other facial muscles may not always be evaluated in all cases or with adequate reliability. Echo intensity is negatively related to muscle strength, particularly in advanced ages. In patients with chronic facial palsy, echo intensity in mimic muscles increases during denervation and decreases (restoration of baseline values) during reinnervation. Echo intensity and muscle size of some masticatory muscles (masseter and temporalis) appear not to be affected by the facial palsy .
Based on the USG characteristics of skin, muscle, and subcutaneous tissue, the normal upper lip tissues can be divided into five layers from superficial to deep. The first layer is a continuous dense hyperechoic line created by the epidermis of the skin and ultrasonic coupling agent. The second layer is somewhat hyperechoic and include superficial muscle fibers of the orbicularis oris muscle, the right and left philtrum columns, philtrum dimple, and skin connection. The third layer is a cord-like hypoechoic area formed by deep fibers of the orbicularis oris. The fourth layer is a slightly hyperechoic region generated by submucosa of the orbicularis oris, upper lip artery, and glandular tissue. The fifth layer is a slightly hyperechoic line created by the mucous layer of the upper lip .
11.1.4 Masseter Muscle
Masseter muscle lies on the ascending mandibular ramus (echogenic reflection with distal acoustic shadowing) and is anterior to the parotid gland with its distinctive pinnate reflection pattern. The scan plane for masseter muscle is perpendicular to the muscle’s anterior border and to the surface of the underlying ramus. It is close and approximately parallel to the occlusal plane in the thickest part of the muscle (region with the most lateral distention) . A healthy masseter muscle has a relatively smooth internal texture of moderate echogenicity on USG images and is visualized to abut right against the ramus of mandible (Fig. 11.6). It can be displayed on USG both in relaxation and maximum contraction. Generally, large white shadow on the top represents the skin echo. The masseter muscle mass appears hypoechoic beneath the skin. Normal masseter muscle has a heterogeneous speckled look in cross-sectional USG images due to hyperechoic bands that are possibly internal fascia . These bands abate or get lost in presence of inflammation; therefore, this is an essential structural index of masseteric infection . The deep part of the masseter can be at its optimum length and clearly portrayed in the USG scan when the patient is in dental intercuspal jaw position . In case of unilateral swelling of the cheek barring parotid gland pathology, masseter hypertrophy due to occlusal parafunction should be considered (Fig. 11.7a, b). USG with a narrow-surfaced probe is effective to diagnose the condition and assess whether the hypertrophic part is in the lower, middle, or upper third of the masseter muscle .
11.1.5 Temporomandibular Joint (TMJ)
USG imaging of the temporomandibular joint (TMJ) is also possible with the availability of higher resolution devices and higher frequency probes. With ultra-higher resolution devices (>15 MHz), contrary to probes of 12–15 MHz, TMJ structures, lower and upper joint compartments, some temporomandibular disorders (i.e., dislocation) can be demonstrated reliably with higher sensitivity and accuracy .
11.2 Diseases of Facial Soft Tissues and Muscles: Brief Review of Typical USG Aspect of the Most Frequently Encountered Pathologies
11.2.1 Inflammatory Changes
A furuncle is a localized infection of the hair follicle with reaction of the immediate tissue. It can arise in any hair-bearing area of the skin. Patients generally exhibit painful swelling in the affected skin which is red and tense, and a boil may be visible. If not treated appropriately, complications such as facial cellulitis and cavernous sinus thrombosis, which presents with chemosis, headache, fever, proptosis, and cranial nerve III, IV, V, and VI palsies might develop [14, 15]. USG typically displays a star-shaped, hypoechoic, loosely organized lesion on the skin (Fig. 11.8). The extension of the inflammation into the deeper structures may be evaluated without difficulty.
On USG, abscess formation appears as heterogeneously hypoechoic or anechoic areas with distal acoustic enhancement (Fig. 11.9a). A thick and irregular border indicates hyperperfusion (increased peripheral blood flow) on color Doppler USG (Fig. 11.9b) [16, 17]. Pus within the abscess might initially seem hyperechoic or isoechoic with the adjacent lymph node and becomes anechoic or hypoechoic when antibiotic therapy is administrated . Gas within the tissues such as an abscess leads to ultrasonic scatter that produces a “white out” appearance . Movement of the infected fluid can be detected with a real-time USG examination .
Image Interpretation Pearls
Doppler sonography or color duplex evaluation of the angular vein is needed for furuncles superior to the oral fissure. The presence of venous thrombosis indicates a risk for intracranial spread .
Myositis is an immune-mediated inflammatory muscle disease with clinical signs of a palpable mass, localized pain, and tenderness. Typical US features of myositis involve intramuscular spindle-shaped mass, thickened perimysium with a honeycomb pattern (in the axial plane), hypervascularity, and loss of normal fibrillar echotexture. In the acute phase, myositis shows heterogeneous higher echogenicity (hyperechoic) and increased muscle thickness. In the chronic stage of myositis where there are fibrosis and fatty replacement, USG shows higher echogenicity (hyperechoic) with decreased muscle thickness (Fig. 11.10) .
Subcutaneous painful retro-auricular swelling with clinical signs of inflammation is suggestive of mastoiditis. USG or color duplex (duplex or Doppler?) sonography of the temporal bone may reveal subcutaneous, inhomogeneous, poorly demarcated, irregular, poorly (or not at all) perfused, hypoechoic lesion, defect in the cortex of the mastoid process, elevation of the outer periosteum by a hypoechoic underlying layer of purulent material between the periosteum and cortex, a vascular periphery in the retro-auricular area, and signs of intracranial extension through this external cortical defect. Utilization of color duplex sonography might identify inflammation-related hyperemia around periosteum and nearby soft tissue abscesses. Exclusion of temporal bone erosion by USG can be beneficial to differentiate between an inflammatory mastoid process and retro-auricular lymph node, retro-auricular erysipelas or dermoid cyst with presence of inflammation .
18.104.22.168 Preauricular Sinus
Preauricular sinus (PAS) is a congenital malformation which is seen as small opening in the external ear, frequently adjacent to the anterior limb of the ascending helix, and may have a fistula aimed towards the external auditory canal. Assessment of preauricular sinus and fistula can be hard with a US transducer, so a fair amount of gel should be used to have a clear USG image. On a USG image, the sinus or fistula seems to emerge from the skin orifice and fistula can reach the external auditory canal. The sinus is generally short and narrow; hence, USG might be able to demonstrate the entire sinus and tract .
Image Interpretation Pearls
If the fistula may be probed, a contrast material application, i.e., hydrogen peroxide, can be tried to demarcate the pathology more clearly in the deeper tissues .
Sialadenitis is inflammation of the salivary glands, the most common being the parotid gland, followed by submandibular and sublingual glands (Figs. 11.11a, b, 11.12). It may be acute or chronic. In the acute form, the affected gland is enlarged, hyperemic, and hypoechoic . In chronic sialadenitis, the affected gland may be atrophic and diffusely hypoechoic with irregular borders resembling the sonographic appearances of a cirrhotic liver . In recurrent sialadenitis there may be sialectasis.