Imaging of the salivary glands is a diagnostic challenge because

Transcription

1 Imaging of the Salivary Glands Asif Abdullah, MD, Francisco F.R. Rivas, MD, and Ashok Srinivasan, MD Imaging of the salivary glands is a diagnostic challenge because of the variety of disease processes that may show similar imaging findings. This article provides an overview of anatomy, epidemiology, clinical presentation, and imaging features of salivary gland tumors as well as inflammatory and systemic processes involving these glands. The imaging features with differential diagnoses are discussed with emphasis on computed tomography (CT) and magnetic resonance imaging (MRI). A variety of disease processes, including inflammatory, infectious, obstructive, systemic, and neoplastic processes can inflict the salivary glands. The diversity of benign and malignant salivary neoplasms is probably greater than that of any other organ system. 1 Salivary tumors gland comprise 3% of all head and neck tumors and cause 0.1% of all cancer deaths. 2 As part of understanding the role of imaging in diagnosis of salivary gland disorders, normal anatomy of the glands is described first in this review followed by discussion of individual disease entities. Normal Anatomy The parotid, submandibular, and sublingual glands are paired and are referred to as the major salivary glands based on their anatomic, histological, and functional characteristics. The minor salivary glands (MSGs) are submucosal congregation of salivary tissue present in the oral cavity, paranasal sinuses, pharynx, and upper respiratory tract. It has been estimated that 750 MSG clusters are present. 1 Parotid Glands The parotid gland is the largest salivary gland, and the palpable superficial portion lies beneath the skin over the mandibular ramus. Superficial layer of the deep cervical fascia gives rise to the parotid capsule, which surrounds the parotid gland and sends septa into the parotid parenchyma dividing Division of Neuroradiology, Department of Radiology, University of Michigan Health System, Ann Arbor, MI. Address reprint requests to Ashok Srinivasan, MD, Division of Neuroradiology, Department of Radiology, University of Michigan Health System, 1500 East Medical Center Drive, Ann Arbor, MI ashoks@med.umich.edu it into 5 parotid lobules, 3 superficial and 2 deep. 3 The attachments of the parotid fascia include the mandible anteriorly, stylomandibular ligament inferiorly, and styloid process posteriorly. Occasionally, parotid tissue can herniate through the stylomandibular membrane. Thus, deep parotid tumors can present as parapharyngeal masses. Majority of the parotid gland overlies the masseter muscle and the mandible. The remaining gland extends medially through the stylomandibular tunnel formed by the posterior edge of the mandibular ramus, sternocleidomastoid muscle and posterior belly of the digastric, and the stylomandibular ligament. The stylomandibular ligament separates the parotid from the submandibular gland. The deep portion of the gland lies in the prestyloid compartment of the parapharyngeal space. As a result, a deep parotid mass can push the tonsillar fossa and soft palate anteromedially. The isthmus of the parotid gland runs between the mandibular ramus and the posterior belly of the digastric to join the retromandibular portion to the remainder of the gland. The main trunk of the facial nerve exits the skull base through the stylomastoid foramen coursing lateral to the styloid process and is immediately superficial to the posterior belly of the digastric muscle and enters the posterior capsule of the parotid gland slightly distal to this. The nerve then traverses the parotid gland lateral to the retromandibular vein. The facial nerve is usually not seen on standard imaging; however, a line drawn from the lateral border of the posterior belly of the digastric muscle and the retromandibular vein to the lateral edge of the mandible can be used as an anatomical marker. The use of the retromandibular vein as a marker for identifying facial nerve is considered a sensitive method for identifying the location of parotid gland lesions. The parotid ductal system has a tree-like branching pattern composed of multiple smaller ducts, which empty into the main parotid duct Stensen s duct which is approximately 7 cm in length. Stensen s duct emerges from the anterior aspect of the parotid gland and travels superficial to the masseter muscle and takes an abrupt medial course piercing the buccinator muscle and finally terminating in a papilla of buccal mucosa opposite the maxillary second molar tooth. Accessory parotid glandular tissue can be seen in approximately 20% of the population, usually anterior to the main parotid gland and adjacent to Stensen s duct because it courses superficial to the masseter muscle. 3 The parotid X/13/$-see front matter 2013 Elsevier Inc. All rights reserved. 65

2 66 A. Abdullah, F.F.R. Rivas, and A. Srinivasan gland is the only major salivary gland that contains intraglandular lymphoid tissue and may demonstrate lymph nodes, which drain the ipsilateral upper and midfacial skin, and may also receive drainage from the palatine tonsil. The parotid lymph nodes then drain primarily into level IIA lymph nodes and to a lesser extent into level IIB. Submandibular Gland The submandibular gland is the second largest salivary gland and lies in the submandibular triangle formed by the anterior and posterior bellies of the digastric muscle and the inferior margin of the mandible. 3 The gland forms a C around the anterior margin of the mylohyoid muscle, which divides the submandibular gland into a superficial and deep lobe. The marginal mandibular branch of seventh cranial nerve courses superficial to the submandibular gland and deep to the platysma. The submandibular gland is invested in its own capsule, which is also continuous with the superficial layer of deep cervical fascia. The submandibular duct, also known as Wharton s duct, exits the medial surface of the gland and courses between the mylohyoid muscle laterally and hyoglossus muscle medially, and over the genioglossus muscle and finally empties into the anterior aspect of the floor of the mouth. The hypoglossal nerve runs parallel and slightly inferior to the submandibular duct. The anterior facial vein has been suggested as a useful imaging landmark to help determine if a mass arises within or adjacent to the submandibular gland. 4 A lesion arising within the gland will never be separated from the gland by this vein. If the vein separates the mass and the gland, the lesion may have originated from submandibular lymph nodes (level IB) or the adjacent soft tissues. There is no intrinsic lymphatic tissue within the submandibular gland. The major lymphatic drainage of the submandibular gland is the deep cervical and jugular chain nodes, most commonly level II lymph nodes. Sublingual Gland The almond-shaped sublingual gland is the smallest major salivary gland that lies just deep to the floor of mouth, between the mandible and genioglossus muscle, and bounded inferiorly by the mylohyoid muscle. Wharton s duct and the lingual nerve pass between the sublingual gland and genioglossus muscle. Unlike the parotid and submandibular glands, the sublingual gland has no true fascial capsule. The sublingual gland lacks a single dominant duct. Instead, it is drained by approximately 10 small ducts (the ducts of Rivinus), which exit the superior aspect of the gland and open along the sublingual fold on the floor of mouth. Occasionally, several of the more anterior ducts may join to form a common duct (Bartholin s duct), which typically empties into Wharton s duct. Of note, the ducts of the sublingual glands are too small for the injection of contrast, making a sialogram of this gland virtually impossible. The sublingual gland s lymphatic drainage is into the submental and submandibular lymph nodes (level IA and IB). Minor Salivary Glands The MSGs lie within the submucosa of the oral cavity, palate, paranasal sinuses, pharynx, larynx, trachea, and bronchi. They are particularly concentrated in the buccal, labial, palatal, and lingual regions. The gingivae, anterior hard palate, and true vocal cords have relatively sparse concentrations of MSG. These glands are not usually visualized on imaging unless associated with masses. Imaging Techniques Salivary imaging has changed from predominant use of plain films and sialograms to nearly complete usage of CT, MRI, and ultrasonography. 1 CT is the modality of choice for patients with a clinical history and findings suggestive of inflammatory disease, such as acute, painful, diffuse parotid, or submandibular swelling. Similarly, recurrent subacute episodes of mildly painful and tender parotid or submandibular swelling indicate an inflammatory process, either infectious or noninfectious. Because the imaging identification of calcifications (sialoliths) is important in the diagnosis of these inflammatory processes, CT is the preferred modality. Generally, CT in this setting is best performed without administration of contrast material, as small opacified blood vessels may simulate small sialoliths. 5 If an abscess or inflammatory process is suspected, adding enhanced scans after identifying stones on nonenhanced scans may be useful. In children and pregnant women, ultrasonography is usually considered the first step, especially for lesions in the superficial lobe of the parotid gland. It can be used to distinguish focal from diffuse disease, assess adjacent vascular structures and vascularity, distinguish solid from cystic lesions, guide fine-needle aspiration biopsy, and perform nodal staging. For further characterization of tumors, crosssectional imaging, such as CT or MRI must be performed. Although CT or MRI can be performed for assessing salivary gland masses, MRI may be preferred because of its ability to not only localize a lesion and assess its extracranial extent but also its higher accuracy in assessing perineural and intracranial spread of malignancies. Recently, new MRI techniques, such as dynamic contrastenhanced MRI (DCE-MRI), diffusion-weighted MRI (DW- MRI), and proton MRI spectroscopy, have shown promising results in the differentiation between benign and malignant salivary gland tumors. However, these techniques are not routinely used in daily clinical practice and will only be briefly mentioned in this review. Obstructive or Inflammatory Processes Clinical Presentation Sialolithiasis usually presents with clinical signs of salivary gland swelling and tenderness. Sialolithiasis is the second

3 Imaging of the salivary glands 67 Figure 1 (A) Axial contrast-enhanced computed tomography (CT) shows a hyperdense calculus in the distal left submandibular duct (black solid arrow) with resultant dilatation of submandibular duct proximally. (B) Postcontrast CT image at an inferior level shows enlargement and increased enhancement of left submandibular gland (dotted white arrow). The proximal extra glandular submandibular duct on the left is dilated (solid white arrow). The intraglandular duct is also dilated. A and B also demonstrate linear enhancement along the left submandibular duct walls. Patient had sialolithiasis with resultant sialadenitis and sialodochitis. most common disease of major salivary glands after mumps. 6 A sialolith in the proximal duct can result in diffuse or focal enlargement of the gland. Ongoing obstruction and inflammation eventually may lead to chronic sialadenitis. In rare instances, sialadenitis results from etiologies other than an obstructing stone, such as autoimmune sialadenitis. Other important causes of sialadenitis include odontogenic infections, cat scratch disease, toxoplasmosis, mycobacteria, and postradiation among other viral etiologies. 7 The MSGs rarely show inflammatory change other than mucous retention cysts from local obstruction. Sialolithiasis predominantly involves the submandibular gland because of its alkaline, more viscous, mucinous saliva in addition to the anatomic predisposition for stasis in the Wharton s duct because of its more uphill course and wider lumen. Calculi may be multiple (25%) and may occur within intraglandular ductal tributaries or within the main ducts. 8 When present in the gland itself, the symptoms may be relatively minor, whereas ductal sialoliths usually have a more precipitous presentation. Imaging On cross-sectional imaging, the parotid gland may appear enlarged; demonstrate abnormal attenuation or intensity and asymmetric enhancement. Overlying adjacent subcutaneous fat may demonstrate inflammatory stranding. Intraglandular lymph node enlargement can also be seen. Calcified sialolithiasis can be clearly identified on CT as a high-attenuation focus and may be located within the ductal system of the gland and/or within Stensen s duct (often dilated). If Stensen s duct is dilated and has thickened enhancing walls, sialodochitis is usually present. On MRI, the gland can have either higher or lower than normal signal intensity on T2- weighted scans, depending on whether edema or cellular infiltration predominates. The calculus itself is most often seen as a low signal intensity focus on both T1-weighted and T2-weighted images. In cases of acute sialoadenitis, the submandibular gland is enlarged and demonstrates avid enhancement. Submandibular lymphadenopathy is usually present along with adjacent inflammatory fat stranding. As in the parotid gland, calcified sialoliths may be present within the gland and/or in Wharton s duct (Fig. 1). Chronic inflammatory diseases of the major salivary glands may be because of recurrent infectious and noninfectious etiologies. Noninfectious conditions, such as previous irradiation, autoimmune disease, or idiopathic causes, may also lead to chronic inflammation. Chronic nonobstructive diseases involve the parotid glands more frequently than they do the submandibular glands in contrast to obstructive etiologies that usually have a predisposition for submandibular glands. 9 A ranula is a mucous retention cyst that occurs predominantly in the sublingual gland. There are 2 forms of ranula. A simple ranula is a retention cyst that remains in the sublingual space above the level of the mylohyoid muscle and lateral to the genioglossus muscle. The simple ranula is considered a true cyst because it has an epithelial lining. In contrast, a plunging ranula is a mucocele that develops from the rupture of the wall of a simple ranula and can extend below the mylohyoid muscle into the submandibular space. On CT, a ranula has well-circumscribed margins with thin walls and appears ovoid in shape with homogenous low internal attenuation (Fig. 2). Repeated hemorrhage may alter internal attenuation to high-attenuation areas and may mimic a solid mass. Superimposed infection can result in enhancement and thickening of cyst wall, mimicking an abscess cav-

4 68 A. Abdullah, F.F.R. Rivas, and A. Srinivasan Figure 2 (A) Axial contrast-enhanced CT shows a simple ranula, seen as a well-circumscribed and ovoid lesion with homogenous low internal attenuation (black solid arrow). It is confined to the left sublingual space, medial to the mylohyoid and lateral to the genioglossus muscles. (B) Coronal T2-weighted MRI demonstrates a multiloculated fluid signal/cystic lesion below the sublingual space and lateral to the mylohyoid muscle into the left submandibular space (white solid arrow). This patient had a left plunging ranula. ity. Adjacent inflammatory changes may also be seen in such cases. In some cases, fistulas or sinus tracts can be demonstrated on CT; however, these are usually better identified by sialography. 10 The plunging ranula extends inferiorly and dorsally to the submandibular gland below the mylohyoid muscle, whereas ventrally it may cross the midline to the contralateral floor of the mouth. Differential diagnoses on CT include a dermoid, epidermoid, cystic lymph node, and a thyroglossal duct cyst. 11 A dilated Wharton s duct because of obstruction anteriorly may mimic a ranula in the lateral floor of the mouth. However, a dilated and obstructed duct usually extends into the submandibular gland, and the gland may also demonstrate asymmetric enhancement pattern to the contralateral normal submandibular gland. On MRI, a ranula has a low T1-weighted and high T2- weighted signal intensity. This appearance may resemble a lymphatic malformation or a lateral thyroglossal duct cyst. However, signal intensities can vary depending on protein contents of the ranula. Ranula with high protein concentration may mimic dermoids, epidermoids, and lipomas. 10 Neoplastic and Nonneoplastic Masses As a rule of thumb, the probability of a solid lesion being malignant in a salivary gland increases with decreasing size of the gland (ie, lowest risk in parotids and highest in MSGs). Nonneoplastic Masses First branchial cleft cyst is a rare branchial complex congenital anomaly typically located within or immediately adjacent to the parotid gland and presents with a history of swelling or recurrent parotid gland infection. Occasionally, a fistula can be observed with the inferior opening at the angle of the mandible and the superior opening on the floor of the external ear canal. 12 First branchial cleft cyst may appear solid on ultrasonography imaging because of internal hemorrhage or infection. Color Doppler usually does not show internal flow, confirming their cystic nature. On contrast-enhanced CT or MRI, the cyst wall may appear thickened and demonstrate enhancement because of superimposed infection. Increased signal intensity on T1-weighted sequences may be observed suggesting internal hemorrhage or proteinaceous debris (Fig. 3). Hemangiomas of the parotid gland are rare in adults, but they are the most common salivary gland masses in children. 13 Involvement of submandibular glands by hemangiomas is rare. Capillary hemangiomas manifest as unilateral parotid swelling or bluish discoloration of the skin shortly after birth. These lesions have a female predilection. Most capillary hemangiomas spontaneously regress by the end of first decade of life. 13 Imaging is most often not required in these patients unless treatment is being planned for unresolved lesions. Venous malformations (formerly known as cavernous hemangiomas) are rare, manifest in older children, and do not undergo resolution. On CT, these lesions demonstrate avid enhancement, lobular contour, may extend to the

5 Imaging of the salivary glands 69 Figure 3 Branchial cleft cyst. Contrast-enhanced CT shows a smooth-walled low-attenuating/cystic lesion in the superficial lobe of the left parotid gland. Pathology revealed a first branchial cleft cyst. Areas of subtle increased attenuation within the lesion suggested the presence of hemorrhage or debris. overlying skin, and have internal phleboliths. On MRI, the lesions usually demonstrate high signal intensity on T2- weighted imaging and gradual enhancement on postcontrast dynamic images. Lymphatic malformations, also known as lymphangiomas, are congenital malformations of the lymphatic system with majority presenting by 2 years of age. 13 In contrast to congenital hemangiomas, spontaneous regression is rare. Although lymphatic malformations generally arise in the posterior triangle of the neck, they are infiltrative in nature and can spread to invade the parotid and submandibular glands. Clinically, they are characterized as painless, soft, or semisolid masses. They may demonstrate sudden enlargement, which may suggest superimposed infection or hemorrhage. Facial nerve palsy can result from compression because of abrupt enlargement. On cross-sectional imaging, lymphatic malformations typically consist of nonenhancing cystic areas with thin septations. Solid nodular-enhancing areas may also be present, suggesting presence of mixed venolymphatic malformations. Hemorrhage can lead to fluid-fluid levels, with variable signal intensities depending on varying stages of blood degradation products. Benign Lesions Pleomorphic adenoma is the most common benign salivary gland tumor in adults and most commonly occurs in middleaged females as a palpable slowly growing mass. 2 Majority of pleomorphic adenomas occur in the parotid glands followed by the submandibular glands. The alternative term, benign mixed tumors (BMTs), is attributed to the histological heterogeneity that leads to a variety of imaging findings. These generally vary with the size of the lesion because small tumors appear homogeneous and well defined with avid enhancement on MRI, whereas larger tumors tend to have a lobulated contour and are more heterogeneous consisting of necrotic and hemorrhagic areas. On MRI, BMTs classically have a low T1-weighted and high T2-weighted signal intensity. However, larger tumors usually have a heterogeneous low-to-intermediate T1-weighted and an intermediate-tohigh T2-weighted signal intensity (Fig. 4). On CT, most small BMTs are well-circumscribed spherical tumors that usually have a higher attenuation than the surrounding parotid gland. Occasionally, they may have a cystic appearance on CT. Dystrophic calcifications may also be seen within the lesion, which highly favors the diagnosis of BMT. A variable rate of recurrence has been reported for pleomorphic adenoma ranging from 1% and 50% depending on the initial surgical procedure and age of follow-up. 2 Recurrences are usually multiple and clustered. There are 3 malignancies associated with pleomorphic adenoma: (1) the carcinoma ex-pleomorphic adenoma; (2) the rare variant, the true malignant mixed tumor (carcinosarcoma); and (3) the metastasizing mixed tumor. The carcinoma ex-pleomorphic adenoma is a malignant change of a BMT or a malignant tumor in a patient who previously underwent surgery for a pleomorphic adenoma. Carcinoma ex-pleomorphic adenomas have a high metastatic rate varying from 25% to 76% into regional lymph nodes, lungs, bones, and brain. 1 The carcinosarcoma is rare and has a bad prognosis. The metastasizing benign mixed tumor is a rare variant. Metastases may be multiple and occur in the lung, bone, and soft tissue, often over decades. On MRI, a carcinoma ex-pleomorphic adenoma can have a smooth wall with focally invasive or totally invasive margins. 2 Warthin tumor, also known as cystadenoma lymphomatosum, is the second most frequent benign parotid mass in adults and commonly presents in older males, usually smokers. It presents as bilateral and multiple masses in approximately 10% of patients. 1 Rarely, it may arise in extraparotid locations, most frequently in periparotid lymph nodes in the area of the parotid tail or in the upper neck, where it can mimic a lymph node. Malignant degeneration of Warthin tumor is extremely rare, representing 1% of Warthin tumors. 1 Imaging features include a well-circumscribed cystic and solid mass with heterogeneous enhancement usually located in the tail of the parotid gland. The cyst wall is usually thin and fairly smooth with the exception of a focal peripheral nodule, which suggests a cystic tumor rather than a branchial cleft or lymphoepithelial cyst (Fig. 5). Rare benign tumors of the parotid gland, including oncocytomas, myoepitheliomas, monomorphic adenomas, and basal cell adenomas, lack typical imaging patterns. However, diagnosis of a parotid lipoma can easily be made on CT or MRI because of its fat content, with fat attenuation seen on CT and signal intensities isointense to fat on all pulse sequences on MRI. Rarely, facial nerve schwannoma and neurofibroma can also occur in the parotid glands. Malignant Lesions Mucoepidermoid carcinoma (MEC) is the most common malignancy of the salivary gland and comprises approximately

6 70 A. Abdullah, F.F.R. Rivas, and A. Srinivasan Figure 4 Benign mixed tumor. Well-defined rounded mass with preserve fat cleavage planes is seen in the left parapharyngeal space. The mass shows low signal in T1 (white arrow in A) and high signal on T2-weighted sequences (dotted black arrow in C) with avid enhancement on postcontrast T1-weighted sequence (black arrow on B). 2.8%-15.5% of all salivary gland tumors. 14 Approximately 50% occur in the major salivary glands; 80% of these occur in the parotid gland, 8%-13% in the submandibular gland, and 2%-4% involve the sublingual gland. 14 In the MSGs, MEC most commonly arises on the palate, but a significant number may also be found in the retromolar area, floor of the mouth, buccal mucosa, lip, and tongue. Although MEC occurs most frequently in the year age range, it may be found at any age. 1 In children, it is the most common malignant salivary gland tumor. 14 Previous radiation exposure has been cited as a risk factor in children. Histologically, MEC is composed of varying proportions of epidermoid cells, mucus-secreting cells, and intermediate cells. Prognosis varies with histological grade, adequacy of excision, and clinical staging. Complete surgical excision is the treatment of choice for MEC. On imaging, histologically low-grade MECs are well circumscribed, whereas high-grade lesions tend to have poor margins and invade adjacent tissues. On CT, low-attenuation cystic areas may be present. Occasionally, they may have associated calcifications and resemble BMTs. 14 On MRI, they frequently demonstrate low-to-intermediate signal intensity on T1- and T2-weighted images (Fig. 6). MEC tend to metastasize primarily to lymph nodes, bone, and lung. Perineural tumor spread along the course of the facial nerve into the mastoid segment of the temporal bone should also be assessed. Adenoid cystic carcinoma (ACC) accounts for 2%-6% of parotid gland tumors and is the most common malignancy in the submandibular gland after MEC. 15 ACC is a slow-growing neoplasm with patients presenting with long-standing pain associated with an indolent mass. Patients may also have associated mucosal ulcerations, commonly in the region of the palate. Although ACCs occur mainly in the parotid gland, the submandibular gland, and the palate, the minor salivary sites are more frequently involved than all the major salivary glands combined. MSGs ACCs portend a worse prognosis, with the ACC overall having a high propensity for perineural tumor invasion. On imaging, ACCs may have a variable appearance from aggressive to a fairly benign appearance. The major salivary gland ACCs generally has a well-delineated appearance, whereas minor gland tumors generally appear more aggressive with infiltrative characteristics. 15 Tumors may demonstrate skull base extension via perineural extension, involving branches of the trigeminal and/or the facial nerves. On CT, widened skull base foramina may be seen along with obliteration of normal fat present at the extracranial opening. However, MRI is far more sensitive than CT in detecting subtle perineural invasion, and on contrast-enhanced MRI, abnormal nerve enhancement can often be seen in a mildly enlarged nerve. MRI has also proven to be beneficial for evaluation of postoperative recurrences. Primary squamous cell carcinoma rarely arises in the salivary glands because of squamous metaplasia occurring in patients with chronic inflammation; however, metastatic squamous cell carcinoma is far more common. 16 The typical imaging features are areas of necrosis in solid tumors mass best characterized on fat-suppressed T1-weighted MRI with intravenous contrast (Fig. 7). Primary lymphoma of the salivary glands is rare and can only be diagnosed if there is lack of intra- or extraglandular

7 Imaging of the salivary glands 71 Figure 5 Warthin tumor. (A and B) Contrast-enhanced CT neck images from the same patient demonstrate solid and homogenously enhancing masses in bilateral parotid lobes (solid white arrows in A and B). (C and D) Contrast enhanced CT neck images from a different patient demonstrate a cystic mass with a thin enhancing wall (dotted white arrow on C.) and a rounded enhancing wall nodule (solid black arrow in D). nodal involvement. Parotid lymphomas are considered extranodal marginal zone B-cell lymphomas. 1 Other rare malignancies of the parotid glands include adenocarcinoma, salivary duct carcinoma, and acinic cell carcinoma. Metastatic disease to the parotid gland is not uncommon, especially from head and neck malignancies. Systemic Diseases A number of systemic diseases affect the salivary gland, including sarcoidosis, HIV, and Sjogren syndrome (SS) among the most common. Other rare diseases include Kikuchi and Kimura diseases. Sarcoidosis is a systemic disease characterized by noncaseating granulomas afflicting multiple organ systems. The parotid glands are affected in approximately 10%-30% of patients. In majority of sarcoidosis patients, bilateral parotid gland enlargement is commonly present, with stasis of salivary flow. 17 There is nontender chronic enlargement of the parotid glands, with some patients also demonstrating involvement of MSGs, resulting in xerostomia. The parotid glands can also be involved in the classic triad of Heerfordt syndrome along with uveitis and facial nerve palsy in patients with sarcoidosis. On cross-sectional imaging, sarcoidosis manifests as multiple benign-appearing noncavitating intraparotid lymph nodes, which have been described radiographically as being foamy in appearance. Associated adjacent cervical adenopathy is often seen, which is highly suggestive of sarcoidosis. The primary radiologic differential diagnosis includes lymphoma. Occasionally, a sarcoid granuloma may present as a solitary parotid mass, which makes differentiation from other solitary parotid lesions difficult. Rarely, small sarcoid granulomas may occur within the parotid gland. These are often subclinical and are found incidentally if imaging is performed for other reasons and cannot be diagnosed reliably on imaging findings alone. 18 SS is a systemic autoimmune condition affecting the exocrine glands. The diagnosis is usually made on clinical grounds when symptoms and signs of keratoconjunctivitis sicca, xerostomia, and/or a connective tissue disease, usually in conjunction with rheumatoid arthritis, are present. The adult form of SS is most commonly seen between the ages of 40 and 60 years, with majority of cases seen in females. 19 The

8 72 A. Abdullah, F.F.R. Rivas, and A. Srinivasan Figure 6 High-resolution postcontrast T1-weighted images of a parotid/seventh cranial nerve MRI (A and B). Enhancing mass with infiltrative margins is seen in the superficial lobe of the left parotid gland (solid arrow in A). There is abnormal linear and nodular enhancement in geniculate, labyrinthine, and canalicular portions of left facial nerve (solid arrow in A). This patient had a mucoepidermoid carcinoma of the left parotid gland with retrograde perineural spread of neoplasia to the facial nerve. parotid disease tends to be progressive, occasionally requiring a parotidectomy to relieve symptoms of recurrent infection. These patients have a 44 times greater risk of developing non-hodgkin lymphoma than the general population. 20 The involved glands may appear normal on initial imaging in early disease but demonstrate glandular enlargement with progressive disease. 21 The imaging appearance may mimic findings of chronic sialoadenitis or sialosis. Eventually, a honeycomb appearance of the parotid gland is seen in late stages, with imaging findings resembling other granulomatous diseases (Fig. 8). On MRI, discrete collections of low signal intensity, reflecting pooling of watery saliva can be seen on T1-weighted imaging. Furthermore, the severity of fat deposition within the salivary glands correlates well with the impaired rates of salivary flow in these patients. Therefore, monitoring of fat deposition might be useful marker in assessing disease progression in patients with typical clinical and serologic findings. 22 Other manifestations of SS include diffuse bilateral macroscopic cystic changes termed benign lymphoepithelial cysts (BLEL), which vary from a few milli- Figure 7 Contrast-enhanced (A) and precontrast (B) T1-weighted MRI demonstrate a solid and infiltrative right parotidenhancing mass with ill-defined spiculated margins (solid arrow in A and B). The mass infiltrates the sternocleidomastoid muscle laterally as well as the right carotid sheath, encasing both internal and external carotid arteries. This patient had a diagnosis of primary squamous cell cancer of the parotid gland.

9 Imaging of the salivary glands 73 Figure 8 Axial T2-weighted (A) and postcontrast T1-weighted (B) images of a magnetic resonance imaging cervical spine requested for evaluation of transverse myelitis. In addition to enhancing signal intensity changes within the spinal cord (incomplete arrows in A and B), there are multiple small fluid containing/microcytic lesions within bilateral parotid glands (solid arrows in A and B). These imaging findings favored the possibility of transverse myelitis secondary to Sjogren disease, which was eventually confirmed clinically. meters to several centimeters in diameter. The imaging findings of BLEL can mimic the cysts seen in HIV patients. However, SS generally do not present with diffuse cervical adenopathy, which is commonly seen in untreated HIV patients. 1 HIV infection may have a variety of manifestations in the head and neck, including the salivary glands. 23 On imaging, patients with AIDS may demonstrate bilateral cystic and solid intraparotid tumors that have been termed AIDS-related parotid cysts. Clinically, patients classically present with unilateral or bilateral painless enlarging parotid masses. 24 Cervical lymphadenopathy and nasopharyngeal lymphoid enlargement commonly accompany AIDS-related parotid cysts and may help distinguish this entity from BLEL of Sjogren disease (Fig. 9). Advanced Imaging Recently, new MRI technologies, such as DCE-MRI, DW- MRI, 25 and proton MR spectroscopy, have shown promising results in the differentiation between benign and malignant salivary gland tumors. Malignant salivary gland tumors can be differentiated from pleomorphic adenomas but not from Warthin tumors using DCE-MRI. A washout ratio of 30% enables the differentiation between malignant and Warthin tumors. 26 Apparent diffusion coefficient (ADC) values have also been used to differentiate malignant from benign tumors of the parotid glands. In a study of 33 pleomorphic adenomas and 13 malignant salivary gland tumors, an overlap in ADC values was observed in those pleomorphic adenomas that showed hypercellularity with less myxoid stroma. 25 However, when comparing the ADC values of different DW-MRI studies, attention has to be paid to the underlying b-values. The lower the applied b-values, the higher the influence of perfusion, whereas when applying high b-values only, the ADC reflects mainly diffusion properties of the tissue. 27 Therefore, the choice of various b-values is crucial in the differentiation of benign and malignant tumors. Figure 9 Contrast-enhanced CT demonstrates enlarged parotid glands with multiple intraparotid cysts of variable size (white solid arrows). Additionally, multiple prominent lymph nodes are seen bilaterally at levels II (black solid arrows). This HIV positive patient had multiple AIDS-related parotid cysts.

10 74 A. Abdullah, F.F.R. Rivas, and A. Srinivasan Role of Positron-Emission Tomography (PET) Imaging The role of PET in the diagnosis and management of nonsquamous cell carcinoma of the head and neck, including salivary gland tumors, remains largely unclear. PET also has limited role for differentiating benign from malignant salivary gland tumors, primarily because of high fluorodeoxyglucose uptake by some benign salivary gland tumors, especially Warthin tumors and occasionally oncocytomas. Hence, PET and PET/CT may not be reliable for classifying salivary masses as benign or malignant; 28 both false-positive and false-negative results may be seen and care must be taken when interpreting these studies. Conclusions In conclusion, it is important for the radiologist to be aware of normal salivary gland anatomy and the imaging appearances of the common disease processes affecting these glands to provide a comprehensive and thorough evaluation with imaging. Although there may be some overlap in imaging features among the various disease processes, imaging still plays an important role in assessing presence of clinically silent multiglandular disease and provides a baseline study for tumors that may undergo therapeutic interventions. References 1. Som PM, Curtin HD: Head and Neck Imaging (Vol. 2, 3rd ed). St. Louis, MO, Mosby, 1996, pp Som PM, Shugar JM, Sacher M, et al: Benign and malignant parotid pleomorphic adenomas: CT and MR studies. J Comput Assist Tomogr 12:65-69, Johns ME: The salivary glands: Anatomy and embryology. Otolaryngol Clin North Am 10: , Weissman JL, Carrau RL: Anterior facial vein and submandibular gland together: Predicting the histology of submandibular masses with CT or MR imaging. Radiology 208: , Yousem DM, Kraut MA, Chalian AA: Major salivary gland imaging. Radiology 216:19-29, Avrahami E, Englender M, Chen E, et al: CT of submandibular gland sialolithiasis. Neuroradiology 38: , McQuone SJ: Acute viral and bacterial infections of the salivary glands. Otolaryngol Clin North Am 32: , Levy DM, ReMine WH, Devine KD: Salivary gland calculi. Pain, swelling associated with eating. JAMA 181: , Travis LW, Hecht DW: Acute and chronic inflammatory diseases of the salivary glands: Diagnosis and management. Otolaryngol Clin North Am 10: , Coit WE, Harnsberger HR, Osborn AG, et al: Ranulas and their mimics: CT evaluation. Radiology 163: , Sumi M, Izumi M, Yonetsu K, et al: Sublingual gland: MR features of normal and diseased states. AJR Am J Roentgenol 172: , Lowe LH, Stokes LS, Johnson JE, et al: Swelling at the angle of the mandible: Imaging of the pediatric parotid gland and periparotid region. Radiographics 21: , Mulliken JB, Glowacki J: Hemangiomas and vascular malformation in infants and children: A classification based on endothelial characterization. Plast Reconstr Surg 69: , Harnsberger HR: Handbook of Head and Neck Imaging (2nd ed). St. Louis, MO, Mosby, 1990, pp Sigal R, Monnet O, de Baere T, et al: Adenoid cystic carcinoma of the head and neck: Evaluation with MR imaging and clinical-pathologic correlation in 27 patients. Radiology 184:95-101, Gaughan RK, Olsen KD, Lewis JE: Primary squamous cell carcinoma of the parotid gland. Arch Otolaryngol Head Neck Surg 118: , James DG, Sharma OP: Parotid gland sarcoidosis. Sarcoidosis Vasc Diffuse Lung Dis 17:27-32, Kurdziel KA: The panda sign. Radiology 215: , Vogl TJ, Dresel SH, Grevers G, et al: Sjoegren s syndrome: MR imaging of the parotid gland. Eur Radiol 6:46-51, Rice DH: Noninflammatory, non-neoplastic disorders of the salivary glands. Otolaryngol Clin North Am 32: , Selva O Callaghan A, Bosch Gil JA, Solans Laque R, et al: Primary sjogren s syndrome: Clinical and immunological characteristics of 114 patients [in Spanish]. Med Clin (Barc) 116: , Izumi M, Eguchi K, Nakamura H, et al: Premature fat deposition in the salivary glands associated with Sjögren syndrome: MR and CT evidence. AJNR Am J Neuroradiol 18: , Sperling NM, Lin PT, Lucente FE: Cystic parotid masses in HIV infection. Head Neck 12: , Som PM, Brandwein MS, Silvers A: Nodal inclusion cysts of the parotid gland and parapharyngeal space: A discussion of lymphoepithelial, AIDS-related parotid cysts, and branchial cysts, cystic Warthin s tumors, and cyst in Sjogren s syndrome. Laryngoscope 105: , Yabuuchi H, Fukuya T, Tajima T, et al: Salivary gland tumors: Diagnostic value of gadolinium-enhanced dynamic MR imaging with histopathologic correlation. Radiology 226: , Wang J, Takashima S, Takayama F, et al: Head and neck lesions: Characterization with diffusion-weighted echo-planar MR imaging. Radiology 220: , Thoeny HC, De Keyzer F, Boesch C, et al: Diffusion-weighted imaging of the parotid gland: Influence of the choice of b-values on the apparent diffusion coefficient value. J Magn Reson Imaging 20: , Uchida Y, Minoshima S, Kawata T, et al: Diagnostic value of FDG PET and salivary gland scintigraphy for parotid tumors. Clin Nucl Med 30: , 2005