Ultrasound of the Salivary Glands

The parotid gland is the largest salivary gland, measuring on average 46mm in the vertical dimension by 37mm in the horizontal dimension. It lies below the zygomatic arch, between the posterior ramus of the mandible anteriorly and the mastoid process and the sternocleidomastoid muscle posteriorly. The gland extends a short distance over the superficial aspect of the masseter muscle (figure 1). The gland is homogeneous with a similar echogenicity to the thyroid gland. In older patients the gland is often more hyperechoic due to the increase in fat deposition within the gland. The gland and capsule contain lymph nodes. The main intraglandular duct may be seen as two parallel hyperechoic lines within the gland. The main extraglandular duct is not often seen unless dilated, but passes over the superficial surface of the masseter muscle, curving around the anterior border of the muscle before piercing the buccinator muscle. It opens into the oral cavity at the parotid papilla, located opposite the upper second molar tooth. The duct passes about 1cm below the zygomatic arch and it can be palpated clinically if the patient clenches their teeth together. In 20% of patients there is also an accessory parotid gland which is normally found overlying the masseter muscle just superior to the duct. The superficial and deep lobes of the gland are separated from each other by the retromandibular vein which can be identified on ultrasound. The facial nerve is not identified on US but passes superficial to the retromandibular vein. The gland should be scanned in both the longitudinal and transverse planes and the probe angled forward behind the ramus of the mandible to ensure as much of the deep lobe is visualised as possible.