Infectious and inflammatory disorders are the commonest pathologies to affect the major salivary glands however frequently overlap in clinical presentation. Imaging plays an important role in diagnosis, usually initially performed by CT or ultrasound. MRI, with its superior soft-tissue characterization compared with CT, provides a better evaluation of tumors and tumor-like conditions. Imaging features may suggest that a mass is more likely to be benign versus malignant, however, biopsy is often needed to establish a definitive histopathologic diagnosis. Imaging plays a key role in the staging of neoplastic disease.
Key points
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Computed tomography with contrast provides excellent anatomic resolution for the diagnosis of sialolithiasis and infectious or inflammatory disorders.
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MRI with contrast is needed to distinguish cystic from solid T2 hyperintense masses.
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Although some neoplasms have distinctive imaging features, imaging is not usually reliable to provide a specific histopathologic diagnosis of salivary gland tumors, and biopsy is usually required.
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Certain imaging features may be useful to suggest a mass is more likely benign or low-grade malignant versus a high-grade malignancy.
Introduction
Salivary glands are exocrine glands in the head and neck that produce saliva, which play a significant role in normal digestive function. A variety of pathologies can affect the salivary glands including infection, autoimmune disease, trauma, obstruction and inflammation, and neoplasia, as well as iatrogenic causes. Imaging of salivary glands plays a key role in diagnosing salivary gland disorders. Contrast-enhanced CT (CECT) is the most common initial diagnostic modality for salivary pathology in adults and is the imaging modality of choice for inflammatory and obstructive etiologies which represent the most commonly encountered pathologies. MRI has better soft-tissue resolution compared with CT and is the preferred imaging modality when a salivary mass is present. Ultrasound (US) is often the initial imaging modality of choice in children and can also complement CT and MRI in certain cases.
Salivary gland anatomy
Parotid Glands
The parotid glands are the largest of the salivary glands and are located in the parotid space, which is enclosed by fascia from the superficial layer of the deep cervical fascia and situated anterior to the mastoid tip and external auditory canal (EAC), posterior to the masticator space, anterolateral to the carotid space and lateral to the parapharyngeal space. The anterior superficial aspect of the gland overlies the masseter muscle and mandible. The main parotid duct (Stensen’s duct) drains into the oral cavity through the buccal mucosa opposite the upper second molar after coursing over the masseter and piercing the buccinator muscle. , Although classically divided into superficial and deep lobes based on the plane of the facial nerve, there is no actual anatomic division within the gland. As the facial nerve branches within the parotid gland are not well depicted on routine imaging, the retromandibular vein can be used to infer its location. The parotid tail is the tissue at the inferior aspect of the superficial lobe, located between the platysma and posterior belly of the digastric muscle and anterolateral to the sternocleidomastoid muscle. The deep lobe reflects a smaller portion of the gland that extends through the stylomandibular tunnel into the prestyloid parapharyngeal space. Pathology in this location is less accessible for clinical assessment. ,
Location of a mass within superficial or deep lobes of the parotid gland has surgical implications. Tumors limited to the superficial lobe can usually be treated by partial parotidectomy whereas total parotidectomy is often needed for deep lobe lesions. The goals of parotid tumor surgery are ideally complete tumor removal and facial nerve preservation, and imaging is crucial for accurate preoperative localization and staging. ,
It is estimated that more than 20% of the normal population has accessory parotid glands(APGs), which are situated superficial to the masseter muscles. APGs are distinct from the main parotid gland and are located 6 mm anterior to it, as opposed to a facial process of the parotid which is a contiguous extension of tissue anteriorly over the masseter. APGs drain by a single accessory duct into Stensen’s duct. APGs are susceptible to pathologies such as neoplasia and inflammation that occur in the parotid gland, however, sometimes the anterior location may be clinically confusing for a salivary pathology.
A unique feature of parotid glands compared with other major salivary glands is that they contain lymph nodes, due to the parotid gland’s relatively late encapsulation. The majority of lymph nodes are in the superficial lobe. The parotid nodes receive drainage from superficial soft tissues of the face and scalp and external ear, with efferent drainage into IIA and IIB nodes. The presence of intraparotid lymph nodes explains the unique occurrence of Warthin tumor (WT) in the parotid gland as well as harboring regional and distant metastatic disease. ,
Parotid glands tend to be mildly hypoattenuating with respect to muscle on non-contrast CT (NCCT) in adults and iso attenuating in children, due to differences in intraglandular fat. On MRI, normal parotid glands tend to be mildly hyperintense on T1-weighted imaging and T2-weighted imaging (T1WI and T2WI) with respect to muscle in adults and isointense in children. Parotid glands demonstrate diffuse enhancement after contrast administration. Fat content of the parotids tends to increase with age and also tends to be higher in men ( Fig. 1 ).
Submandibular Glands
The submandibular gland forms a C shape, wrapping around the posterior free edge of the mylohyoid muscle which divides the gland into the larger superficial lobe in the submandibular space (SMS) and the deep lobe which projects into the posterior aspect of the sublingual space (SLS). The submandibular gland is located inferior to the mandible with the superficial lobe inferolateral to the mylohyoid and bounded by the anterior and posterior bellies of the digastric. The submandibular gland is separated from the parotid gland by the stylomandibular ligament. The SMS also contains lymph nodes, the facial artery and vein, and branches of the hypoglossal nerve. , If a mass is separated from the submandibular gland by the facial vein then it must be a lymph node or other extraglandular mass. The main duct (Wharton’s duct) exits the anteromedial aspect of the gland and courses anterior superiorly between the sublingual gland and genioglossus muscle to terminate at the sublingual papilla in the anterior floor of the mouth. The duct is larger in width than the papilla so sialoliths commonly become lodged at the junction. A normal variant is a Stafne cyst, which is a well-defined radiographically lucent lesion in the posterior mandibular body from the intraosseous extension of submandibular tissue.
On CT, the normal submandibular glands tend to be symmetric in size and with a density between muscle and the parotid glands. On MRI, submandibular glands are slightly T1 hypo and T2 hyperintense compared with parotid gland, with diffuse enhancement ( Figs. 2 and 3 ).
Sublingual Glands
The sublingual glands are the smallest of the major salivary glands, lying deep to the body of the mandible and lateral to the root of the tongue musculature within the SLS. The sublingual gland is not truly encapsulated and drains by numerous small ducts into the floor of the mouth, although sometimes some of the ducts fuse to form Bartholin’s duct which drains into the main submandibular duct. The mylohyoid muscle sling separates the SLS and floor of the mouth from the submandibular and submental spaces. Posteriorly, there is free communication between the SMS and SLS, and the majority of people have a defect in the mylohyoid (boutonniere), which may contain herniated sublingual gland and provides another potential route of spread of disease between the SLS and SMS. The SLS also contains the lingual artery, nerve, and vein, part of the hyoglossus muscle, and the submandibular duct and deep lobe ( Fig. 2 ).
Minor Salivary Glands
The minor salivary glands are small aggregations of glandular tissue (∼800–1000) scattered throughout the submucosa of the oral cavity, paranasal sinuses, pharynx, larynx, trachea, and bronchi. The minor salivary glands are beyond the scope of this article. Large case reviews of intraoral minor salivary gland neoplasms yield a 40% to 50% malignancy rate, with the junction of the hard and soft palate being a relatively common location.
Imaging technique
CT is the most common initial imaging modality performed for suspected salivary gland pathology. It is widely available and easily interpreted, provides excellent cross-sectional anatomic resolution, and evaluates for cervical lymphadenopathy. CT is also superior to MRI in assessing osseous changes such as bone destruction and foraminal enlargement. CECT is the preferred imaging modality for inflammatory disease and suspected calcifications. Contrast does not generally interfere with the ability to detect ductal calcifications yet provides excellent delineation of ductal dilatation and has excellent spatial resolution to define parotid and periparotid inflammatory changes and distinguish abscess from phlegmon. Standard CT neck protocol with venous phase acquisition is preferred. A face protocol can also be used but will not screen the whole neck for lymphadenopathy. In patients with significant beam hardening artifacts from dental amalgam or facial hardware, angled gantry acquisition may be needed to provide adequate image quality.
CECT is also sensitive for detection of salivary gland masses, however, MRI with its improved soft-tissue contrast provides the imaging modality of choice for assessment of salivary gland tumors (SGTs). MRI adequately defines local extension of tumor, including extraglandular extension and perineural spread. MRI techniques vary by institution but generally include axial and coronal non-contrast T1WI without fat saturation (FS) and T1FS + C as well as some combination of axial and coronal T2WI without and with FS. T1 precontrast images are performed without FS because the bright signal of the fat provides good contrast with the low signal of the mass. Postcontrast imaging is necessary to distinguish whether a T2 hyperintense mass is cystic or solid, and to delineate perineural disease. Diffusion-weighted imaging (DWI) is also recommended to allow apparent diffusion coefficient (ADC) analysis of salivary masses. Advanced imaging techniques, such as MRI perfusion and spectroscopy imaging, are also being used in combination with DWI/ADC analysis to help distinguish benign from malignant salivary neoplasms. ,
US is able to distinguish solid from cystic salivary masses, detect sialolithiasis, diagnose and follow-up acute and chronic inflammatory disorders as well as autoimmune disease (Sjogren’s syndrome) of the major salivary glands, and evaluate lymph nodes. Given its lack of ionizing radiation, US is often the first imaging study performed for salivary disorders in children. US is useful to guide fine needle aspiration biopsies of salivary gland masses and cervical lymphadenopathy. US is limited compared with CT and MRI in assessing deeper spaces of the neck, skull base, and floor of the mouth. Additionally, US is operator-dependent, and artifacts may obscure evaluation or confuse the diagnosis.
Fluorodeoxyglucose (FDG)-PET detects areas of increased metabolic activity and is used for staging and restaging many types of cancers and monitoring response to therapy. As both benign and malignant salivary gland neoplasms as well as non-neoplastic entities such as inflammatory diseases may be FDG avid, FDG-PET has limited value in the initial diagnosis of SGTs. FDG-PET can however be extremely useful for evaluating metastatic disease and in post-treatment cases, both for defining the extent of residual or recurrent disease and providing prognostic information.
Sialography is generally not a first-line imaging study. MR sialography, obtained with heavily T2-weighted sequences provides a noninvasive alternative to conventional or digital subtraction sialography in the detection of sialolithiasis, ductal stenoses, and ectasia, which helps evaluate disorders such as chronic recurrent sialoadenitis, Sjogren syndrome (SjS), juvenile recurrent parotitis, and post-trauma.
Salivary gland pathology
Non-neoplastic Etiologies
Sialadenitis
Inflammatory pathologies are the most common disorders to affect the salivary glands. The choice of imaging modality varies with the suspected cause and location but generally CT or US is employed initially for suspected obstructive or inflammatory disease.
Sialolithiasis
The majority of salivary gland stones occur in the submandibular gland and are most commonly solitary. Most submandibular calculi are located in the main duct, most commonly at the ostium or along the bend at the posterior margin of the mylohyoid. The opening of Wharton’s duct is narrower than the caliber of the duct, resulting in stone impaction at this point. Submandibular duct stones are more likely to be chronic than parotid duct stones and may present with intermittent symptoms. Chronic sialolithiasis may demonstrate glandular atrophy. Tiny asymptomatic calculi within the parotid glands are sometimes present on CT scans.
CT has increased sensitivity over plain films for detecting focal calcifications. It has been estimated that 20% of submandibular stones and 40% of parotid stones may not be detected on plain films. Suspected cases of uncomplicated sialolithiasis have traditionally been imaged with NCCT due to concerns that contrast might interfere with the ability to detect calcified stones. CECT, however, has not been shown to have significantly decreased sensitivity compared with NCCT for the detection of obstructive sialolithiasis and allows for evaluation of potential complications like glandular inflammation or abscess or alternative diagnosis, such as tumor. NCCT images are not indicated due to unnecessary radiation. Dual-energy CT techniques allow for virtual unenhanced CT images. US is effective at diagnosing sialolithiasis, especially stones > 2 mm, and is often the first-line modality in children. MRI is less sensitive than CT at detecting calcifications but MR sialography can identify radiolucent stones as hypointense foci in a dilated duct ( Fig. 4 ).
Infectious sialadenitis
Acute suppurative (bacterial) sialadenitis most commonly presents as acute painful unilateral salivary gland swelling, more commonly affecting the parotid than the submandibular glands. Acutely on CT and MRI, there is unilateral glandular enlargement with increased enhancement, ductal dilatation, edema in the adjacent fat, reactive lymphadenopathy, and potential spread of infectious/inflammatory changes into adjacent muscles and soft tissues. T2 signal can be high or low depending on whether more edema or cellular infiltrate is present. US most commonly shows an enlarged, hypoechoic gland. Imaging is helpful to detect findings such as a drainable abscess or large obstructing calculus that indicate the process is unlikely to respond to conservative treatment alone ( Fig. 5 ).
