Salivary gland imaging RAD Magazine, 38, 450, 37-38

by Stefanie C ThustNeuroradiology fellow

Richard J P SmithSpR Radiology

Polly S RichardsConsultant radiologist

Barts Health NHS Trust, London

The salivary glands are exocrine glands with twomain functions: Firstly, saliva production aids thepassage of food via the oesophagus into the stom-ach; secondly, salivary enzymes initiate thebreakdown of nutrients, thus forming the firststep of digestion. Salivary glands can be dividedinto major and minor glands. There are threepaired major glands, namely the parotid, sub-mandibular and sublingual glands, and numer-ous minor salivary glands scattered throughoutthe upper aerodigestive tract including the nasaland oral cavity, paranasal sinuses, soft palate,larynx and pharynx. Various diseases may affectthe salivary glands, either in isolation or as partof an underlying systemic condition. Broadly,these can be grouped into benign non-neoplasticconditions, benign neoplasms and malignant neo-plasms. As an approximate rule, the risk ofmalignancy in a lesion increases with decreasinggland size, being highest in the minor glands. Salivary gland imaging Different imaging modalities can be valuable in assessingsalivary gland disease, whereby the choice of modality willdepend on local protocol, clinical features and, importantly,the site of suspected pathology. Owing to technical advances,in many centres ultrasound is nowadays the investigation ofchoice for major salivary gland disease. It allows a quick,cheap and thorough assessment without the use of ionisingradiation and can additionally be employed as imaging guid-ance for fine-needle aspiration (FNA) and core biopsy.

Ultrasound is able to simultaneously evaluate glandparenchyma and large ducts as well as demonstrate ductdilatation. Stimulation of saliva with a sialogogue oftenreveals the cause of sialectasis. Digital subtraction sialog-raphy (DSS) remains useful in the detection of calculus dis-ease, particularly where ultrasound is inconclusive ornegative despite a convincing history. DSS can also be usedto guide endoluminal procedures such as basket extraction ofsmall mobile stones and balloon dilatation of salivary ductstrictures.

Magnetic resonance sialography (MRS) may be used toassess sialectasis when DSS is contraindicated, with theadditional advantage of allowing simultaneous parenchymalassessment. MRS employs a T2-weighted non-contrastsequence and the use of a sialogogue can improve duct visu-alisation. Due to its excellent soft tissue contrast and abilityto assess deep structures, MRI is the modality of choice to

evaluate minor salivary gland disease, perineural spread orskull base involvement of malignant tumour of any salivarygland, and deep lobe of parotid lesions for surgical assess-ment. CT imaging is highly sensitive in detecting radio-dense salivary calculi and is particularly useful in assessingmultiple clustered calculi, which may be difficult to distin-guish on ultrasound. Although the soft tissue contrast of CTis usually inferior compared to MRI, contrast-enhanced CTplays a role in the imaging of acute inflammatory pathology,facilitating quick assessment of sick patients requiringurgent intervention, for example in a suspected abscess orwhere there is concern regarding airway compromise. Benign non-neoplastic diseaseSalivary calculus disease (sialolithiasis) is a common dis-ease affecting the major glands, particularly the sub-mandibular gland due to its relatively increased mucuscontent and narrow duct orifice. Approximately 80% ofstones develop in the submandibular gland compared to 19%in the parotid. Calculi are multiple in 25% of cases and canreach several centimetres in size. On ultrasound calculi areseen as hyperechoic foci with post-accoustic shadowing. Thesensitivity of ultrasound for the detection of calculi has beenreported around 90% and depends on calculus size and loca-tion, whereby small calculi near duct orifices tend to be mostdifficult to identify. The use of palpation and a sialogogue aspart of an ultrasound examination may help localise stones.Sialolithiasis typically presents with ‘meal-time syndrome,’ iegland swelling induced by eating. It is the commonest causeof duct enlargement (sialectasis) and may be a source of(recurrent) infection.

Inflammatory parenchymal diseases can be divided intouniglandular and diffuse multiglandular pathology. Bacterialsialadenitis tends to be uniglandular as sequelae of sialec-tasis but often occurs on a background of a severe underly-ing critical illness, for example severe dehydration,immunocompromise or in the neonatal period. Ultrasounddemonstrates an enlarged hypoechoic gland with hypervas-cularity and associated reactive lymphadenopathy near thegland. Of note, the parotid gland is the only salivary glandwhich may also contain intraglandular lymph nodes.Imaging features of benign reactive lymphadenopathy areregular enlargement with a preserved nodal shape,echogenic hilum and vascular architecture. Complicationssuch as abscess formation or airway compromise may ensuefrom bacterial sialadenitis. In severe cases cross-sectionalCT or MR imaging may be needed to assess for deep spread.

Chronic sclerosing sialadenitis is a rare benign inflam-matory condition of the submandibular gland of uncertainaetiology, possibly related to underlying occult duct stric-tures and/or calculus disease. It eventually results in glandatrophy, fatty parenchymal replacement and fibrosis thatcan present as a hard mass and mimic a malignant neo-plasm. An uncommon juvenile form, ‘recurrent parotitis ofchildhood,’ is recognised in which recurring episodes clini-cally resembling mumps begin in early childhood with reso-lution of symptoms by the mid teens.

In contrast to bacterial sialadenitis, viral sialadenitis typ-ically affects the parotid glands in a symmetrical fashion,and can be caused by a variety of agents such as mumps,coxsackie A virus, Ebstein-Barr virus (EBV) andcytomegalovirus (CMV). Ultrasound demonstrates diffuselyenlarged, hypoechoic and hypervascular glands, an appear-ance, that on imaging grounds alone, may be difficult to dis-tinguish from other diffuse inflammatory diseases.

Sjogren’s syndrome is an autoimmune inflammatory con-dition that results in diffuse multiglandular parenchymalabnormality. Clinically it causes dry eyes (keratoconjunc-tivitis sicca) and a dry mouth (xerostomia) and can occureither in isolation as ‘Sicca syndrome’ or in combination withan underlying collagen vascular disorder such as rheumatoidarthritis or systemic lupus. The disease process involveslymphocytic infiltration of salivary glands with destruction ofthe normal gland architecture, resulting in formation ofpseudocystic acini initially and larger cysts in advanced dis-ease. These can be seen as hypoechoic foci on ultrasound oras multiple T2 hyperintense foci within the glands on MRI.In advanced disease, lymphocyte aggregates can also leadto formation of mass-like components, which may be con-fused with a neoplasm. Bacterial superinfection in diffuseglandular disease may also complicate imaging findings witha mixed picture.

Sarcoidosis is another autoimmune condition to affectmultiple salivary glands. Its glandular appearances may beindistinguishable on imaging from Sjogren’s as a result ofnon-caseating granulomatous infiltration, although it is moretypically accompanied by systemic lymphadenopathy.Sialadenosis is characterised by diffuse enlargement fattyinfiltration of the major salivary glands, which can be seenas generalised hyperechogenicity on ultrasound. It is areversible non-inflammatory condition seen in diabetes mel-litus, alcoholism and hypothyroidism.

There are a number of causes of cystic lesions of the sali-vary glands. Sialoceles are cystic dilatations of salivary ductsand occur secondary to obstruction. Cyst formation in thesalivary glands presents as anechoic areas on ultrasoundand fluid signal on MR/CT respectively. Retention cysts con-taining salivary content are called ranulas and most oftenoccur in the sublingual gland. If a ranula ruptures orextends into the submental or submandibular triangle dueto dehiscence of the myelohyoid muscle, it is called ‘plunging’or ‘diving’. First branchial cleft cysts are congenital abnor-malities and present as a pretragal intraparotid swelling,which can be simple cystic on ultrasound, but frequentlycontains echogenic debris. Benign cysts may also occur aspart of HIV sialopathy affecting up to 5% of patients prior tothe onset of AIDS. Lymphovascular malformations are slow-growing multicystic lesions, which may intermittentlyenlarge as the patient bends over. They often contain bloodflow, but this may be too slow to detect on Doppler ultra-sound. MRI may reveal blood product in the form of T1hyperintensity, blood-fluid levels or haemosiderin staining.The presence of phleboliths (characteristic rounded calcificdeposits, which occur in veins) is pathognomonic.Benign neoplasmsPleomorphic adenoma is the most common benign salivarygland tumour, usually arising in the parotid. On ultrasound,these typically appear as solitary homogenous lobulatedlesions with only mild vascularity and acoustic enhance-ment. On MRI, their T2 signal is similar to water, but theyusually demonstrate homogenous contrast enhancement.Larger lesions can demonstrate calcification and necrosis,which may lead to misdiagnosis as malignancy. Importantly,approximately 10% of pleomorphic adenomas show malig-

nant transformation over time, therefore excision should beperformed. Lesions contain a pseudocapsule and care shouldbe employed during surgery not to rupture this, as seedingmay lead to diffuse recurrence. Warthin’s tumour is the sec-ond common salivary neoplasm, typically occurring in oldermale patients, with a propensity for smokers. It arises fromparotid intraglandular lymphoid tissue, typically in the tail,and is multiple or bilateral in approximately 15% cases.Ultrasound shows an ovoid hypoechoic mass with a vascularechotexture and this may produce blood-stained mucous-likematerial on FNA. Of note, while imaging features may besuggestive of a particular (benign) lesion, these are by nomeans specific, and it is mandatory to perform cytologicalevaluation of any solitary or dominant salivary gland mass. Malignant neoplasmsUS with FNA is the first line investigation for all majorsalivary gland masses, with MRI reserved for surgical plan-ning and staging of proven malignancies. It allows high res-olution assessment for local spread, especially to deeperstructures. The most common salivary gland malignancy ismucoepidermoid carcinoma, a tumour that typically occursin the parotid gland and can resemble pleomorphic adenomaon imaging. Adenoid cystic carcinoma is the most commonmalignancy of the minor salivary glands, submandibularand sublingual glands and has a poor prognosis, with fre-quent incidence of nodal and pulmonary metastases.Primary squamous cell carcinoma of the salivary glands isextremely rare, as there is no squamous epithelium withinnormal salivary glands. However, squamous cell metastasesfrom a non-salivary head and neck cancer may commonlymetastasise to intra or periglandular lymph nodes and cantherefore be confused with a primary salivary malignancy.Secondary lymphomatous involvement of the salivary glandsis relatively common, particularly in high grade lymphomas,and can produce diffuse and multifocal abnormalities onimaging. It is paramount that any imaging assessment ofthe salivary glands includes evaluation of the cervical lymphnodes, as these may be involved in benign and malignantconditions. Imaging features of malignant lymph nodeinvolvement include abnormal shape, loss of the normalhilar architecture and deranged or peripheral vascularity.SummaryA variety of modalities may be employed in salivary glandimaging depending on clinical features, the site of suspectedpathology and local protocol. Ultrasound has emerged as thetechnique of choice for major salivary gland disease andforms a useful aid for FNA/biopsy. Any solitary or dominantsalivary gland mass should undergo cytological evaluation.MRI is of particular value in delineating and staging sali-vary gland malignancy.

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FIGURE 1US image of sjogren’s syndrome of a salivary glanddemonstrating multiple scattered cysts and hypoe-choic solid masses.

FIGURE 3An axial T1 weighted image through the parotidglands demonstrating a carcinoma in the left parotidgland that arose from a preceding pleiomorphicadenoma.

FIGURE 2Digitally subtracted sialograph (left) and screeningradiograph (right) demonstrating a stricture whichis then balloon dilated. TABLE 1

Uniglandular vs multiglandular and systemic disease.

Uniglandular disease Multiglandular/systemicdisease

Calculi/sialolithiasis Sjorgrenʼs disease

Bacterial sialadenitis Viral sialadenitis

Epithelial tumours Benign lympho-epithelial lesions

Sarcoid

Lymphoma