Salivary glands

Technique

Conventional radiographs can be used to localize radiopaque salivary stones. These are most common in the submandibular gland and duct. Sialography can visualize the ductal system of the parotid or submandibular glands and detect any obstruction by radiolucent stones or tumor as well as demonstrate inflammatory changes. Isotope studies using Technetium 99m pertechnetate are today used only for studying salivary gland function.

Computed tomography or magnetic resonance imaging can be us ed to assess salivary gland tumors particularly those located in the deep portion of the parotid gland as well as assess their relationship to the skull base and the facial nerve. Ultrasound can be used to differentiate between solid and cystic lesions which sometimes can be difficult by the other imaging modalities.

Anatomy

The three large paired salivary glands are responsible for the major production of saliva. Accessory salivary tissue is found throughout the mucosal membranes of the mouth and the oropharynx and the rest of the upper aerodigestive tract.

The parotid salivary glands are found inferior to the preauricular area wrapping around the posterior aspect of the ramus of the mandible. The gland rest posteriorly on the stemocleidomastoid and the posterior belly of the digastric muscles. The facial nerve exits the stylomastoid foramen and enters the gland where it branches. This leads to surgical implications and the gland is therefore divided into a superficial and deep portion. The superficial portion is lateral to the mandible resting against the masseter muscle. The deep portion extends behind the mandible in front of the styloid process. This portion therefore reaches the parapharyngeal space. Scattered lymph nodes can be found throughout the gland.

The submandibular gland is the size of a walnut wrapping around the posterior margin of the mylohyoid muscle and sits between the mandible and the hyoglossus muscle towards the sublingual space. The submandibular duct runs towards the papilla surrounded by the sublingual salivary glands.

Pathology

Sialolithiasis

Intermittent swelling of any of the major salivary glands, often related to food intake, is seen in obstruction of the major duct by a calculus. Calculi are most common in the ductal system of the submandibular glands and are frequently calcified. Chronic recurring episodes of calculi and obstruction can lead to chronic changes with strictures. Sialography is used to outline the ductal system and verify a obstruction caused by a calculus.

Figure 23. Sialoectasis Sialogram of the parotid gland; lateral projection. In the glandular parenchyma pools of contrast can be seen. The accessory paroid gland is also affected (arrow).

Obstruction of a sublingual gland can give rise to a retention cyst called a ranula. A ranula can be seen as a bluish submucosal swelling in the floor of the mouth. As a ranula grows and ruptures it can dissect beyond the sublingual space and into the submandibular space becoming what is called a diving ranula.

lnfections

Acute bacterial infections often only needs clinical assessment before treatment. In recurring infections sialography should be performed to detect any underlying cause like stones or strictures or to demonstrate chronic ductal changes with caliber variations. The sialogram should be done after the infection has been brought under control.

Systemic diseases

Symmetrically enlarged parotid glands can be seen in sarcoid and after heavy metal poisoning. Enlarged glands are also found in diabetics and alcoholics. Lymphnode enlargement can occur in the parotid glands in tuberculosis, lymphoma and in HIV positive patients. In the latter lymphoepithelial cysts can be found in the glands preceding any other symptoms of AIDS. Ultrasound as well as computed tomography and magnetic resonance imaging can be used to assess all forms of diffuse salivary gland enlargement. In Sjögren's syndrome the salivary gland tissue is replaced by periductal lymphocyte infiltrates. The salivary gland changes leading to mouth dryness are accompanied by a symptom complex consisting of keratoconjuntivitis sicca and arthritis. The sialogram will demonstrate characteristic sialoectasis in the parotid glands (Fig. 23).

Figure 24. Malignant tumor of the parotid gland -computed tomography Transverse section through the parotid glands. A well demarcated expansion is seen in the deep portion of the parotid gland (black arrows). The retromandibular blood vessels are displaced laterally (white arrow).

Tumors

Salivary gland tumors are relatively rare. About 80% of the tumors occur in the parotid glands. Among these parotid tumors another 80% are benign while in the submandibular glands 40-50% of the tumors are benign. Solid masses in the salivary glands can be assessed by ultrasound, computed tomography or magnetic resonance. Benign parotid tumors are most often round and well demarcated being either cystic or solid. They are commonly found in the superficial lobe. Ultrasound can be used to assess superficial parotid tumors while computed tomography or magnetic resonance imaging can be used to demonstrate involvement of the deep lobe. A poorly delineated tumor in the deep lobe with accompanying facial nerve paralysis is highly suggestive of a malignant lesion (Fig. 24). Fine needle aspiration cytology possibly by ultrasound guidance can be us ed to give the final diagnosis.

Sven G. Larsson and Anthony A. Mancuso