(Radiology. 2000;216:19-29.)
© RSNA, 2000
Major Salivary Gland Imaging1
David M. Yousem, MD,
Michael A. Kraut, MD, PhD and
Ara A. Chalian, MD
1 From the Department of Radiology, Johns Hopkins Hospital, 600 N Wolfe St, Houck B-112, Baltimore, MD 21287 (D.M.Y., M.A.K.), and the Department of Otorhinolaryngology: Head and Neck Surgery, University of Pennsylvania Medical Center, Philadelphia (A.A.C.). Received January 22, 1999; revision requested March 30; final revision received August 6; accepted August 18. Address correspondence to D.M.Y. (e-mail: yousem@rad.jhu.edu).
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ABSTRACT
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The algorithm for imaging the salivary glands depends on the clinical scenario with which the patient presents to the clinician. Because of the importance of identifying small calculi in the gland or salivary duct as the cause of the symptom complex, nonenhanced computed tomography is often the best initial study for the evaluation of the painful gland. If an infiltrative neoplasm is highly suspected, nonenhanced and enhanced magnetic resonance (MR) imaging may be superior in demonstrating perineural, meningeal, and skull base invasion. Sialography is reserved for the evaluation of chronic sialadenitides unrelated to sialolithiasis. Thin-section MR techniques for MR sialography may soon replace conventional sialography.
Index terms: Salivary glands, calculi, 264.818 • Salivary glands, CT, 264.12111, 264.12112 • Salivary glands, diseases, 264.247 • Salivary glands, MR, 264.12141, 264.12143 • Salivary glands, neoplasms, 264.363, 264.37
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INTRODUCTION
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While neoplasms of the major salivary glands constitute less than 3% of all tumors in the body, many people have an illness related to the salivary glands at some time in their lives. The process may be self-limited, such as with viral parotitis or mumps, but in others it may be a relapsing and remitting illness, such as chronic sialadenitis secondary to sialolithiasis. Still others may be evaluated for what is thought to be a salivary gland mass but is actually a mass peripheral to the glands, such as a lymph node. Very rarely, a nonnodal extraglandular lesion such as a schwannoma, masseteric hypertrophy, or other pseudomass may be mistaken for a glandular process, even by experienced otorhinolaryngologists (1). The purpose of this review is to illustrate the role of imaging in evaluating diseases of the major salivary glands.
The range of studies needed to assess salivary gland lesions spans a wide gamut. Many of the disease processes described above may not require imaging of any kind. Still others may be readily evaluated with palpation and direct visualization either endoscopically or transorally. At the other end of the spectrum are infiltrative deep lobe parotid masses such as adenoid cystic carcinomas, which may require computed tomography (CT), magnetic resonance (MR) imaging, and/or conventional angiography as part of a preoperative evaluation for perineural, vascular, or skull base infiltration. Ultrasonography (US), underutilized in most North American sites, may supplant the role of CT and MR imaging for superficial salivary gland lesions when experienced sonographers use the technique.
Patients with major salivary gland lesions may present to the otorhinolaryngologist or oral and maxillofacial surgeon with a suspected mass, with a suspected obstruction or inflammation, or with diffuse glandular enlargement (2). Although there are overlapping entities in this simple stratification, the imaging studies ordered may change with each presentation. For the purposes of this review, major salivary gland imaging will be divided into neoplastic lesions, obstructive or inflammatory lesions, and systemic diseases.
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NEOPLASTIC LESIONS
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The usual clinical manifestation of a salivary gland neoplasm is that of an enlarging mass. The first critical step in evaluating a mass is to determine whether or not it is painful. While painful masses may be produced by obstructive or inflammatory disease, the classic painless mass in the salivary glands is usually due to a neoplasm, cyst, or lymph node. A dull, gnawing pain may be produced by some neoplasms in the glands. Other signs that a parotid neoplasm is malignant are infiltration of the overlying skin, regional adenopathy, or facial nerve palsy. Regional lymph node spread is relatively infrequent with salivary gland primary malignancies.
There are some general rules that apply to salivary gland neoplasms. The smaller the salivary gland, the higher the rate of malignancy. Thus, the rate of malignancy increases from 20%–25% in the parotid gland to 40%–50% in the submandibular gland and to 50%–81% in the sublingual glands and minor salivary glands (3–6). Nearly 80% of benign parotid neoplasms are pleomorphic adenomas (3,4). Pleomorphic adenomas, also known as benign mixed tumors, occur most commonly in middle-aged women. Monomorphic adenomas and myoepitheliomas are the other common benign tumors and may arise in both parotid and submandibular glands. Oncocytomas and Warthin tumors are rare outside the parotid region.
Multiple parotid masses are usually due to lymphadenopathy or Warthin tumors, the latter appearing almost exclusively in the parotid gland, usually in the tail of the gland in older men (Fig 1). The most common malignancy of the parotid gland is mucoepidermoid carcinoma (2,3). In the submandibular gland, pleomorphic adenomas remain the most common benign tumor (Fig 2), but adenoid cystic carcinoma is the most common malignant histologic diagnosis (5). The same diagnoses prevail in the sublingual and minor salivary glands (7), where malignancies outnumber benign tumors.
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Figure 1. Bilateral Warthin tumors. Bilateral parotid masses (arrows) are seen on this transverse, contrast material-enhanced, fat-saturated T1-weighted spin-echo (SE) (750/30 [repetition time msec/echo time msec]) MR image. The multiplicity and location at the tail of the parotid gland (near the lower mandible) are typical features of this tumor.
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Figure 2. Submandibular pleomorphic adenoma. Transverse, contrast-enhanced CT scan shows that the pleomorphic adenoma (A) arises in the right submandibular gland. The attenuation characteristics leave little indication as to whether the lesion is benign or malignant.
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Lymph nodes may manifest as intraparotid masses. Since the parotid gland encapsulates late in the 2nd trimester, it incorporates lymphatic tissue, whereas the other salivary glands do not. This accounts for the potential for malignant adenopathy involving the parotid glands. This is usually seen in the setting of a dermatologic malignancy (basal cell carcinoma, squamous cell carcinoma, and melanoma), but occasionally an upper aerodigestive system squamous cell carcinoma may result in malignant parotid adenopathy. The lymph nodes must be determined to be intraparotid or extraparotid at imaging. By the same token, lymphoma may occur primarily in the parotid gland as an infiltrative process or adenopathy, or as a manifestation of systemic disease.
Some would advocate the use of MR imaging as the first (and only) technique to evaluate a neoplasm of the major salivary glands. Implicit in such a decision is that the clinicians are highly confident that the process in the gland is neoplastic and not obstructive or inflammatory. If there is even a slight chance that the mass may in some way be related to sialolithiasis, CT should be recommended first, since MR imaging is not as reliable in detecting small calculi, and "pseudomasses" may accompany sialolithiasis (see Obstructive or Inflammatory Lesions).
If there is very strong suspicion that a lesion is neoplastic, there are some compelling reasons why MR imaging may be preferred over other modalities. Virtually all parotid lesions are well visualized on T1-weighted MR images because of the hyperintense (fatty) background of the gland (Fig 3) (8). The T1-weighted image gives an excellent assessment of the margin of the tumor, its deep extent, and its pattern of infiltration. This sequence, coupled with fat-saturated, contrast material–enhanced T1-weighted imaging, used primarily to address perineural spread (9,10), bone invasion, or meningeal infiltration, is the best means for "mapping" the tumor. If there is a superimposed diffuse inflammatory process, a focal mass could possibly be obscured with this sequence. On fat-saturated images, both the bone marrow and cortex of the mandible, maxilla, and skull base will be hypointense. Enhancing (hyperintense) tissue extending into this hypointense background is indicative of bone invasion. At the skull base, where the abundant fat and bony foramina will also have suppressed signal (low signal intensity), the fat-saturated, contrast-enhanced image will show spread of hyperintense enhancing tumors up the stylomastoid foramen (cranial nerve VII) (Fig 4), foramen ovale (cranial nerve V-3), or foramen rotundum (cranial nerve V-2). Therefore, if a facial or trigeminal nerve palsy accompanies a mass, MR imaging is usually the study of choice. If meningeal symptoms coexist, MR imaging may also be preferred since the leptomeninges are better assessed with MR than with CT (11).
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Figure 3a. Pleomorphic adenomas. (a) Transverse T1-weighted SE (600/11) MR image shows the mass (P) to be well highlighted against the normal hyperintensity of the parotid gland. The margination is not particularly sharp, yet the diagnosis was pleomorphic adenoma. (b) The mass (P) is hyperintense on this transverse, long repetition time (4,000 msec), T2-weighted fast SE MR image. (c) The mass (P) enhances on this contrast-enhanced, T1-weighted SE (600/30) MR image, though it has a central nonenhancing component.
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Figure 3b. Pleomorphic adenomas. (a) Transverse T1-weighted SE (600/11) MR image shows the mass (P) to be well highlighted against the normal hyperintensity of the parotid gland. The margination is not particularly sharp, yet the diagnosis was pleomorphic adenoma. (b) The mass (P) is hyperintense on this transverse, long repetition time (4,000 msec), T2-weighted fast SE MR image. (c) The mass (P) enhances on this contrast-enhanced, T1-weighted SE (600/30) MR image, though it has a central nonenhancing component.
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Figure 3c. Pleomorphic adenomas. (a) Transverse T1-weighted SE (600/11) MR image shows the mass (P) to be well highlighted against the normal hyperintensity of the parotid gland. The margination is not particularly sharp, yet the diagnosis was pleomorphic adenoma. (b) The mass (P) is hyperintense on this transverse, long repetition time (4,000 msec), T2-weighted fast SE MR image. (c) The mass (P) enhances on this contrast-enhanced, T1-weighted SE (600/30) MR image, though it has a central nonenhancing component.
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Figure 4a. Perineural infiltration of cranial nerve VII by adenoid cystic carcinoma. (a) Transverse T1-weighted SE (600/11, two signals acquired) MR image shows that the right parotid mass (m) enters the notch of the stylomastoid foramen (arrow). (b) Coronal, gadolinium-enhanced T1-weighted SE (600/17, one signal acquired) MR image shows vertical extension of the enhancing tumor (arrows) through the stylomastoid foramen (line is at the plane of transverse image in a).
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Figure 4b. Perineural infiltration of cranial nerve VII by adenoid cystic carcinoma. (a) Transverse T1-weighted SE (600/11, two signals acquired) MR image shows that the right parotid mass (m) enters the notch of the stylomastoid foramen (arrow). (b) Coronal, gadolinium-enhanced T1-weighted SE (600/17, one signal acquired) MR image shows vertical extension of the enhancing tumor (arrows) through the stylomastoid foramen (line is at the plane of transverse image in a).
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On nonenhanced and enhanced T1-weighted series, no histologic discrimination is possible since almost all masses are lower in intensity than the parent gland and almost all salivary gland neoplasms enhance. Deep infiltration into the parapharyngeal space, muscles, and bone, well seen on T1-weighted images, will strongly suggest malignancy (4) or, less frequently, rupture of the capsule of a pleomorphic adenoma (Fig 5). While benign and malignant masses exhibit considerable overlap with regard to geographic properties such as margins, shapes, and borders (Fig 6) (12,13), T2-weighted MR imaging has been shown to be a reasonably reliable (73%) predictor of whether a salivary gland tumor is benign (Fig 7) or malignant (14,15). The conventional wisdom is that a hyperintense mass on T2-weighted images is benign and a mass of low to intermediate signal intensity is malignant. Others have implied that signal intensity on T2-weighted images is of little help (4,13). In general, however, the most common benign tumor of the salivary glands, the pleomorphic adenoma, has very high signal intensity on T2-weighted images (Fig 7) (13). Benign cysts (mucous retention cyst, lymphoepithelial cyst, first branchial cleft cyst, ranula, sialocele, and pseudocysts) may also be hyperintense on T2-weighted images, and, depending on the presence of hemorrhage, infection, or hyperproteinaceous fluid, the T1-weighted image may show intermediate (solid-appearing) intensity. For this reason, administration of contrast material is helpful because cysts usually enhance on their periphery, whereas pleomorphic adenomas enhance solidly (Figs 3c, 7b). The value of contrast enhancement applies also to CT because the attenuation of the cyst may not be that of pure fluid, particularly when traumatized, infected, or hyperproteinaceous.
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Figure 5. Seeding of pleomorphic adenoma. Parotid (arrow) and deep lobe or parapharyngeal (arrowheads) masses are present on the left side on this transverse T1-weighted SE (500/11) MR image. The patient underwent prior attempted resection of a deep lobe pleomorphic adenoma, but the capsule of the tumor was violated. Months later she presented with this picture of tumor infiltrating the operative bed and the surrounding tissue.
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Figure 6. Right submandibular gland adenocarcinoma. Transverse CT scan shows an exophytic, well-defined mass (M) in the right submandibular region that proved to be an adenocarcinoma. First and foremost, one should attempt to determine whether this lesion is a submandibular lymph node metastasis or a primary lesion from the submandibular gland (as in Fig 2). The anterior facial vein, which usually lies between submandibular and extraglandular masses (1), was not conspicuous in this case. The gland and mass were resected together, and the origin from the submandibular gland was uncovered. Although this lesion was well defined, it was malignant (compare with Fig 2).
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Figure 7a. Pleomorphic adenoma. (a) Transverse T2-weighted SE (3,000/80, one signal acquired) MR image shows that the lesion (*) is hyperintense. This may raise the question of a cyst versus a pleomorphic adenoma. (b) With administration of a gadolinium-containing contrast agent and fat saturation, the mass (*) is seen to enhance avidly on this coronal T1-weighted SE (600/17, one signal acquired) MR image, compatible with a solid mass.
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Figure 7b. Pleomorphic adenoma. (a) Transverse T2-weighted SE (3,000/80, one signal acquired) MR image shows that the lesion (*) is hyperintense. This may raise the question of a cyst versus a pleomorphic adenoma. (b) With administration of a gadolinium-containing contrast agent and fat saturation, the mass (*) is seen to enhance avidly on this coronal T1-weighted SE (600/17, one signal acquired) MR image, compatible with a solid mass.
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Most high-grade mucoepidermoid carcinomas, undifferentiated carcinomas, adenocarcinomas, and squamous cell carcinomas of the major salivary glands have low to intermediate signal intensity on long repetition time images (Fig 8). Some malignancies, however, exhibit elevated signal intensity on T2-weighted images. Most commonly, this is seen in low-grade mucoepidermoid carcinomas (14), in some adenoid cystic carcinomas (15), and rarely in adenocarcinomas. Among benign masses that are not hyperintense on long repetition time images, Warthin tumor, the second most common benign mass in the adult parotid gland, is often of intermediate, low, or mixed signal intensity on T2-weighted images (8). The chronic sialadenitides, such as Sjögren disease, Mikulicz disease (also known as Sjögren type 1 or sicca syndrome without a connective tissue disorder), and radiation sialadenitis, may also appear hypointense on T2-weighted images (14). In most series, these exceptions account for the 25% error rate that occurs if one relies solely on signal intensity on T2-weighted images to predict histologic diagnosis.
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Figure 8a. Mucoepidermoid carcinoma of the parotid gland. (a) Transverse T2-weighted SE (3,000/90) MR image shows an intermediate-signal-intensity mass (arrow) slightly lower in intensity than that of the native parotid tissue. (b) The ill-defined nature of the mass (arrow) was exemplified by the fuzzy margins on this transverse, contrast-enhanced, fat-saturated, T1-weighted SE (600/30) MR image. The diagnosis was high-grade mucoepidermoid carcinoma.
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Figure 8b. Mucoepidermoid carcinoma of the parotid gland. (a) Transverse T2-weighted SE (3,000/90) MR image shows an intermediate-signal-intensity mass (arrow) slightly lower in intensity than that of the native parotid tissue. (b) The ill-defined nature of the mass (arrow) was exemplified by the fuzzy margins on this transverse, contrast-enhanced, fat-saturated, T1-weighted SE (600/30) MR image. The diagnosis was high-grade mucoepidermoid carcinoma.
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The CT attenuation of masses, other than for differentiating benign cysts from solid masses, and lipomas from other neoplasms (Fig 9), does not help in predicting histologic diagnosis since most malignant and nonmalignant solid masses have similar CT attenuation (Fig 10). Although high-grade malignancies tend to have irregular infiltration into the glandular parenchyma (13,16), which can be detected with both MR imaging and CT, there are enough exceptions to this rule that CT has only fair accuracy (60%–70%) in predicting the histologic diagnosis of a lesion (17). Because CT appearance is not a good predictor of histologic diagnosis and since CT is less accurate than MR imaging for determining the extent of disease (16), there is little reason to advocate CT over MR imaging in the setting of a suspected salivary gland mass. However, neither study can be relied upon wholly to predict histologic diagnosis; hence, fine-needle aspiration or biopsy is usually required to establish the diagnosis in the major salivary glands.
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Figure 9a. Lipomas. (a) Although the attenuation of this patient's parotid glands is low due to fatty infiltration on this transverse CT scan, a palpable (see marker superficially) lipoma (L) in the left parotid gland was appreciated. (b) This lipoma (arrow), hyperintense on this transverse T1-weighted SE (600/11) MR image, was extraglandular but still contained by the deep cervical fascia surrounding the gland.
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Figure 9b. Lipomas. (a) Although the attenuation of this patient's parotid glands is low due to fatty infiltration on this transverse CT scan, a palpable (see marker superficially) lipoma (L) in the left parotid gland was appreciated. (b) This lipoma (arrow), hyperintense on this transverse T1-weighted SE (600/11) MR image, was extraglandular but still contained by the deep cervical fascia surrounding the gland.
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Figure 10. Mucoepidermoid carcinoma of the parotid gland. Transverse CT scan shows an ill-defined mass (C) that has less attenuation than that of enhancing parotid tissue in the right parotid gland. The attenuation of this mass is the same as that of the pleomorphic adenoma in Figure 2 and less than that of the carcinoma in Figure 6.
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Nuclear scintigraphy may help diagnose Warthin tumors (cystadenoma lymphomatosum) and oncocytomas. These tumors are unique in that they show increased radiotracer uptake at technetium pertechnetate imaging. Since neither of these lesions has substantial malignant potential, a course of observation may be advocated in the elderly patient with surgical contraindications. This is in contrast to the treatment of pleomorphic adenomas. Since pleomorphic adenomas have the potential for malignant degeneration and/or coexistent adenocarcinoma, they are treated more aggressively with surgical extirpation (18). Positron emission tomography (PET) has been studied to see if it can reliably predict histologic diagnosis (19). At an accuracy rate of 69%, PET compared unfavorably with accuracy rates reported for MR imaging (14,15,19). Therefore, the role of scintigraphy is usually limited to confirming the clinical diagnosis of Warthin tumors in those patients with multiple parotid masses.
US is a useful technique to assess superficial parotid, submandibular, and sublingual masses (20,21). US is not as readily used in North America, and the expertise in interpreting sonograms of the head and neck is not as widespread in the United States as it is in Europe and Japan. In appropriate hands, however, US can help analyze superficial salivary gland lesions with the same precision as CT and MR imaging. Gritzmann (22) demonstrated in a blinded retrospective review that 287 (95%) of 302 space-occupying lesions of the major salivary glands (285 in the parotid, 13 in the submandibular, and four in the sublingual glands) could be completely delineated at US. All 302 neoplasms were hypoechoic to normal glandular tissue. US enabled correct assessment of whether a lesion was benign or malignant in 272 cases (90%) on the basis of definition of the margins of the tumor, but 28% of malignant lesions (16 of 58 lesions) were misinterpreted as being benign (22). US differentiated extraglandular from intraglandular lesions with an accuracy of 98% (all mistakes were periparotid lymph nodes). Other reports have confirmed that US also does an excellent job in enabling distinction of glandular from extraglandular masses (23). This differentiation is helpful to the otorhinolaryngologist who may not be able to discern the planes as well by means of palpation.
Malignant salivary tumors show a higher grade of vascularity at color Doppler US than do benign tumors (24). Peripheral vascularity with a hypovascular center implies a pleomorphic adenoma (24). The echotexture of lymphoma is finer than that in benign adenopathy (25).
The limitations of US (even in the best of hands) include its inability to evaluate deep parotid masses, lesions obscured by the mandible, parapharyngeal extension, retropharyngeal and deep neck adenopathy, and the intracranial or skull base extent of a mass (22). US shows a lack of specificity for cystic lesions, and the relationship of a tumor to the facial nerve is hard for surgeons to appreciate on US images.
To the surgeon, it is the facial nerve that is the critical structure when operating on the parotid gland. By contrast, submandibular and sublingual glandular tumors are removed with relative impunity. While submandibular and sublingual gland resections can injure the lingual nerve or hypoglossal nerve and possibly the marginal branch of the facial nerve, these deficits are easily compensated for by the patient. However, the consequences of facial nerve injury are so substantial functionally and cosmetically that much effort is directed toward teasing out the trunk and peripheral branches of the facial nerve before even addressing the mass. Though some parotid malignancies may manifest with facial nerve impairment (24%) (2), a new postoperative facial paralysis is the bane of the surgeon and the patient.
Even with thin-section MR imaging, there is controversy as to the reliable identification of the facial nerve below the skull base (26–29). Its course is surmised by its egress from the stylomastoid foramen; its passage anterior to the posterior belly of the digastric muscle, lateral to the plane of the stylomandibular tunnel, and around the retromandibular vein; and its branching pattern to the muscles of facial expression (30–32) (Fig 11). The line connecting the lateral surface of the posterior belly of the digastric muscle and the lateral surface of the mandibular ascending ramus has also been used to separate superficial (lateral to the facial nerve) and deep (medial to the facial nerve) parotid masses (33). The differentiation of deep or superficial parotid masses is critical from the standpoint of the extent of dissection needed to separate the nerve from the tumor or to gain access to the tumor, the attendant risk to the facial nerve, and, in the case of tumors extending into the parapharyngeal space, the need for a cervical approach with or without mandibulotomy (6). Demonstration or suspicion of direct invasion of the nerve at the stylomastoid foramen (or above) prods the surgeon to plan for transmastoid identification of the facial nerve to control disease and prevent tumor spillage. The superficial parotidectomy thereby becomes skull base surgery with its attendant risks (to the other cranial nerves, venous sinuses, carotid artery, and temporomandibular joint function) and morbidity. If the nerve at the skull base is invaded, the cartilaginous auditory canal may have to be addressed and possibly resected. A radical mastoidectomy is contemplated and even the ascending ramus of the mandible may be removed (6). MR imaging does well in demonstrating the perineural, vascular, and dural invasion that may be present with parotid malignancies (34).
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Figure 11. Markers for the seventh cranial nerve. Transverse T1-weighted SE (500/11) MR image shows that the seventh cranial nerve emerges from the stylomastoid foramen to course through the fat (straight arrow) immediately below this exit. In its course through the parotid gland (p), it runs lateral to the retromandibular vein (curved arrow). This is the best sequence for defining anatomic landmarks.
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Adenoid cystic carcinoma has a very high rate of perineural spread (50%–60%) (35). It is the most common malignancy to affect the minor salivary glands and the sublingual gland, and it is one of the three most common tumors of the parotid and submandibular glands. Visualization of the cranial nerves and tumoral infiltration around these nerves to and through the skull base is probably best evaluated by using nonenhanced and gadolinium-enhanced, fat-saturated, T1-weighted MR sequences (9,10,34).
Multiple painless masses in the parotid gland invoke a differential diagnosis of Warthin tumors (Fig 1) (36), multiple pleomorphic adenomas (18), oncocytomas, lymph nodes, lymphoma, acinic cell carcinoma, metastases (37), human immunodeficiency virus (HIV)–related cysts and nodules (38), and Sjögren syndrome (39,40). Lymphoma, associated with Sjögren syndrome or not, may infiltrate the entire parotid gland or present as multiple lymph nodes in the gland. Cystic degeneration of a benign-appearing neoplasm suggests a Warthin tumor or pleomorphic adenoma (41).
Perhaps the most useful study that one can perform to ascertain the nature of a painless mass in the salivary glands is image-guided aspiration or biopsy (Fig 12). The request for image-guided aspirations most often arises when a deep lobe parotid mass or parapharyngeal space ectopic minor salivary gland lesion is incidentally discovered. Since the lesions are nonpalpable and difficult to approach endoscopically, the radiologist may be called on to obtain a tissue sample. Whether or not one chooses to puncture the lesion from the anterior face via a retromaxillary (transbuccal) or a transparotid approach depends on the comfort level of the radiologist and the course of the carotid vessels with respect to the mass. Cytologic aspirates with 22-gauge spinal needles are usually sufficient, but occasionally core biopsies of the gland with 18–20-gauge needles may be required for a definitive diagnosis. Because of the precarious position of the facial nerve, a core biopsy is generally shunned in sampling parotid masses. It should be noted that inflammatory masses of the salivary glands may mimic epithelial neoplasms at cytology because desquamated cells frequently populate the former. This is a known potential pitfall of head and neck cytology and accounts for many inaccurate aspiration results (42,43).
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Figure 12. CT-guided aspiration of deep lobe parotid mass. Via a transbuccal approach, the needle passed between the ascending ramus of the mandible and the maxilla, with its tip (arrow) at the edge of the deep lobe parotid mass (P). Several 2-cm thrusts into the mass under syringe suction and transverse CT guidance yielded cells compatible with mucoepidermoid carcinoma. The well-defined margins belied the malignant nature of the mass.
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In those cases in which the diagnosis is known from a biopsy or aspiration performed under palpation guidance, the emphasis in imaging should shift toward delineation of the size and extent of the mass rather than histologic discrimination. Once again, symptoms may suggest one modality over another since the presence of cranial nerve palsies, meningeal signs, venous obstruction, and fixation of the mass to vital structures such as the carotid artery may lead to a preference for MR imaging, but bone pain and adenopathy might suggest that CT is the favored modality.
Some head and neck surgeons do not perform any imaging or aspirations prior to operating on a salivary gland mass. This is usually in the setting of a superficial painless mobile parotid mass with normal facial nerve function (presumed to be a pleomorphic adenoma).
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OBSTRUCTIVE OR INFLAMMATORY LESIONS
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Most patients with sialolithiasis (salivary gland calculi) will have the cardinal signs of pain and swelling of the gland (44–47). Sialolithiasis is the second most common disease of the salivary glands after mumps (44). The gland may be diffusely or focally enlarged with a sialolith in the proximal duct.
Sialolithiasis is predominantly a disease of the submandibular gland since 80%–82% (143 of 174) of sialoliths occur here (22,44). Sialoliths form more frequently in the submandibular gland because of the more alkaline, thicker, and viscous saliva the submandibular gland produces. Other factors that predispose to stasis in the Wharton duct (eg, an uphill course, a dependent gland, a wider lumen, and a tighter orifice) may play a role as well. Calculi may be multiple (25%) and may occur within intraglandular ductal tributaries or within the main ducts. When in the gland itself, the symptoms may be relatively minor, whereas ductal sialoliths usually have a more precipitous presentation (44).
Ductal calculi may initially be treated with secretagogues and/or dilation of the duct. Transoral resection of sialoliths and sialodochoplasty can be performed for isolated distal duct (close to the ampulla) sialoliths. Imaging may help define the location of isolated nonpalpable or multiple sialoliths. For proximal or glandular sialoliths, the surgeon may decide to treat the patient with resection of the gland. This is often the preferred treatment for patients with recurrent bouts of sialolithiasis with sialadenitis, since this region of the duct is not well accessed transorally. A cervical (submandibular) surgical approach may be taken with sialoliths that extend beyond the mylohyoid (in the proximal duct).
Most of the imaging modalities that are sensitive to calculi, such as conventional radiography, CT (48), and US (49,50), can demonstrate sialoliths with high accuracy. Of 185 patients with major salivary gland calculi, calculi were identified and localized correctly (intraductal vs intraglandular) with US in 174 patients (94%) reported in Gritzmann's review (22). Fourteen of the 174 cases detected at US were not evident at conventional radiography. However, US is less accurate than CT in distinguishing multiple clusters of stones from single large stones (48,50). Generally, CT in this setting is best performed without administration of contrast material, since small opacified blood vessels may simulate small sialoliths (Fig 13). If an abscess or inflammatory process is suspected, adding enhanced scans after identifying stones on nonenhanced scans may be useful.
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Figure 13a. (a-d) Transverse CT scans of ductal and glandular calcifications. (a) There is a large solitary sialolith (arrow) in the right submandibular duct. (b) These glandular calcifications (arrows) could easily be mistaken for vessels on this contrast-enhanced CT scan. (c) Are these vessels or calcifications (arrows) along this ill-defined monomorphic adenoma of the submandibular gland? (d) Image obtained with bone window settings is equivocal due to the small size of the opacities (arrows), but the nonenhanced scans showed that the posterolateral one was a calcification and the anteromedial structure was a vessel, hence the value of a nonenhanced scan.
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Figure 13b. (a-d) Transverse CT scans of ductal and glandular calcifications. (a) There is a large solitary sialolith (arrow) in the right submandibular duct. (b) These glandular calcifications (arrows) could easily be mistaken for vessels on this contrast-enhanced CT scan. (c) Are these vessels or calcifications (arrows) along this ill-defined monomorphic adenoma of the submandibular gland? (d) Image obtained with bone window settings is equivocal due to the small size of the opacities (arrows), but the nonenhanced scans showed that the posterolateral one was a calcification and the anteromedial structure was a vessel, hence the value of a nonenhanced scan.
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Figure 13c. (a-d) Transverse CT scans of ductal and glandular calcifications. (a) There is a large solitary sialolith (arrow) in the right submandibular duct. (b) These glandular calcifications (arrows) could easily be mistaken for vessels on this contrast-enhanced CT scan. (c) Are these vessels or calcifications (arrows) along this ill-defined monomorphic adenoma of the submandibular gland? (d) Image obtained with bone window settings is equivocal due to the small size of the opacities (arrows), but the nonenhanced scans showed that the posterolateral one was a calcification and the anteromedial structure was a vessel, hence the value of a nonenhanced scan.
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Figure 13d. (a-d) Transverse CT scans of ductal and glandular calcifications. (a) There is a large solitary sialolith (arrow) in the right submandibular duct. (b) These glandular calcifications (arrows) could easily be mistaken for vessels on this contrast-enhanced CT scan. (c) Are these vessels or calcifications (arrows) along this ill-defined monomorphic adenoma of the submandibular gland? (d) Image obtained with bone window settings is equivocal due to the small size of the opacities (arrows), but the nonenhanced scans showed that the posterolateral one was a calcification and the anteromedial structure was a vessel, hence the value of a nonenhanced scan.
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Recently, some investigators have advocated fast T2-weighted MR imaging with thin sections to noninvasively evaluate the ductal architecture of the salivary glands and to identify stones (Fig 14). Although beautiful images of the salivary ductal system can be generated with thin-section, T2-weighted MR imaging (51–53), tiny calculi within the gland and even in the larger ductal system can be overlooked on MR images because of the signal void associated with the calcified stone. Clearly, CT, US, and simple conventional radiography will enable identification of the calculi, but MR imaging affords the opportunity to visualize the effect the sialolith has on the ductal system. Is this necessary? In most instances, the answer is no. However, in those cases of a painful gland associated with chronic sialadenitis without a sialolith seen at CT or conventional radiography, the irregularity of the ductal system may give a clue as to the cause of the painful gland. Strictures may be present where a sialolith has resided. The poor outflow of saliva leads to chronic sialadenitis. In other cases, the sialadenitis may not be from calculi but may be due to autoimmune inflammatory conditions that have a signature sialographic appearance—pruned, truncated main ducts with punctate or globular collections peripherally in the glandular parenchyma (54). The larger ducts are spared as the disease affects the periphery first and spares the central ductal system. Tiny abscesses in the glands will show similar punctate cystic regions.
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Figure 14a. Submandibular calculi visualized at MR imaging. (a) Transverse T1-weighted SE (500/11) MR image shows two areas of low signal intensity (arrows) in the floor of the mouth on the left. (b) These calculi (arrows) are confirmed on this transverse T2-weighted SE (4,000/80) MR image, and the obstructed duct (arrowhead) is evident. Inflammation around the stones accounts for some peripheral high signal intensity. (c) The duct (arrowhead) can also be seen on this contrast-enhanced, T1-weighted, fat-saturated, spoiled gradient-echo (35/2.1, 30° flip angle) MR image. The walls enhanced, presumably due to inflammation (sialodochitis). (d) There was coincidental sialadenitis of the left submandibular gland seen as enlargement and high signal intensity on this transverse, fat-saturated, T2-weighted fast SE (4,000/80) MR image. Note intraglandular ductal dilatation (arrows).
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Figure 14b. Submandibular calculi visualized at MR imaging. (a) Transverse T1-weighted SE (500/11) MR image shows two areas of low signal intensity (arrows) in the floor of the mouth on the left. (b) These calculi (arrows) are confirmed on this transverse T2-weighted SE (4,000/80) MR image, and the obstructed duct (arrowhead) is evident. Inflammation around the stones accounts for some peripheral high signal intensity. (c) The duct (arrowhead) can also be seen on this contrast-enhanced, T1-weighted, fat-saturated, spoiled gradient-echo (35/2.1, 30° flip angle) MR image. The walls enhanced, presumably due to inflammation (sialodochitis). (d) There was coincidental sialadenitis of the left submandibular gland seen as enlargement and high signal intensity on this transverse, fat-saturated, T2-weighted fast SE (4,000/80) MR image. Note intraglandular ductal dilatation (arrows).
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Figure 14c. Submandibular calculi visualized at MR imaging. (a) Transverse T1-weighted SE (500/11) MR image shows two areas of low signal intensity (arrows) in the floor of the mouth on the left. (b) These calculi (arrows) are confirmed on this transverse T2-weighted SE (4,000/80) MR image, and the obstructed duct (arrowhead) is evident. Inflammation around the stones accounts for some peripheral high signal intensity. (c) The duct (arrowhead) can also be seen on this contrast-enhanced, T1-weighted, fat-saturated, spoiled gradient-echo (35/2.1, 30° flip angle) MR image. The walls enhanced, presumably due to inflammation (sialodochitis). (d) There was coincidental sialadenitis of the left submandibular gland seen as enlargement and high signal intensity on this transverse, fat-saturated, T2-weighted fast SE (4,000/80) MR image. Note intraglandular ductal dilatation (arrows).
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Figure 14d. Submandibular calculi visualized at MR imaging. (a) Transverse T1-weighted SE (500/11) MR image shows two areas of low signal intensity (arrows) in the floor of the mouth on the left. (b) These calculi (arrows) are confirmed on this transverse T2-weighted SE (4,000/80) MR image, and the obstructed duct (arrowhead) is evident. Inflammation around the stones accounts for some peripheral high signal intensity. (c) The duct (arrowhead) can also be seen on this contrast-enhanced, T1-weighted, fat-saturated, spoiled gradient-echo (35/2.1, 30° flip angle) MR image. The walls enhanced, presumably due to inflammation (sialodochitis). (d) There was coincidental sialadenitis of the left submandibular gland seen as enlargement and high signal intensity on this transverse, fat-saturated, T2-weighted fast SE (4,000/80) MR image. Note intraglandular ductal dilatation (arrows).
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Sialography is contraindicated in the acute setting of sialadenitis because of the possibility of exacerbating the symptoms associated with the infection. The retrograde injection of contrast agents can force inflammatory products into the more peripheral parenchyma of the gland. Furthermore, the act of instrumenting the duct may irritate it, cause narrowing from posttraumatic edema or stricture formation, and lead to reduced drainage of the infected saliva. MR sialography has the advantage of not requiring cannulation of the duct and a heightened sensitivity to edema in the salivary gland, which may not be evident at CT.
Can MR sialography replace conventional sialography, a cumbersome procedure that is contraindicated in patients with acute sialadenitis and may result in irritation and injury to the duct if not performed properly? The answer appears to be yes, if not now then very soon (Fig 15). The caveat is that the need to perform sialography should be restricted to a very few number of cases when clinical assessment, serology (especially for the autoimmune causes), conventional radiography, and/or CT cannot facilitate diagnose of the cause of the chronic sialadenitis.
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Figure 15. MR sialography in a healthy volunteer. The main submandibular duct (arrows) and some of its tributaries (arrowheads) can be seen on this transverse, maximum intensity projection reformatted image from a three-dimensional, T2-weighted fast SE (5,000/102, one signal acquired) data set.
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Sialolithiasis often causes sialadenitis (44). The inflamed gland will be large and painful, and the cardinal signs of warmth and redness may be present over the gland. Secondary involvement of the glands by odontogenic abscesses may also lead to infection of the gland. Purulent material may be expressed from the ductal orifice in most bacterial infections and is the best clinical sign of an infected gland.
Other causes of acute parotitis include viral (mumps), bacterial (staphylococcus and streptococcus), granulomatous (tuberculosis, candida, cat-scratch fever), and postpartum parotitis. Poor dental hygiene may contribute to the development of infections affecting the submandibular, sublingual, and parotid glands. The minor salivary glands rarely show inflammatory change other than mucous retention cysts from local obstruction.
If the patient is experiencing his or her first episode of acute sialadenitis in the setting of dehydration or a palpable sialolith, the otorhinolaryngologist may elect to forego imaging and treat empirically. Surgeons are loath to operate in the setting of acute parotitis. Surgical planes are obliterated, and the dissection of the facial nerve is very cumbersome when the surrounding gland is edematous and inflamed. Medical management is indicated. Even when an abscess is present, most head and neck surgeons prefer to wait until antibiotics have reduced the diffuse inflammation in the gland before tackling the focal mass.
On CT or MR images, one may see the inflamed gland as enlarged, of abnormal attenuation or intensity, and enhancing avidly (Fig 16). There is usually inflammatory stranding into the overlying subcutaneous tissue and thickening of the investing deep cervical fascia with acute parotitis. This is seen equally well at MR imaging and CT. The gland will have high signal intensity at T2-weighted MR imaging. The presence of adjacent lymph nodes or intraglandular lymph nodes may suggest the inflammatory nature of the lesion but can also be seen, in another clinical setting, with neoplasms.
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Figure 16. Acute right-sided parotitis. Transverse, contrast-enhanced, fat-saturated, T1-weighted SE (600/30) MR image shows marked enhancement of the right parotid gland (thick and thin arrows) compared with the left. The superficial subcutaneous tissue is also inflamed.
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CT and MR imaging in the coronal plane for inflammatory conditions of the parotid and submandibular glands may be helpful for evaluating the extent of the lesion. The relationship of the inflammatory mass to the floor of the mouth (as with a ranula) for submandibular lesions and the skull base for parotid gland masses has importance with respect to the surgical approach. A ranula has been termed a mucous escape cyst, a mucous retention cyst, and a mucocele of the sublingual gland or neighboring minor salivary glandular tissue (Fig 17). A ranula that is superficial to the mylohyoid muscle (a simple ranula, epithelial lined) is usually addressed transorally. It may be treated with resection or, in some cases, marsupialization. The lingual and hypoglossal nerves must be carefully identified during the operation. A ranula that plunges through the muscular floor (a plunging ranula or pseudocyst, not epithelial lined) may be excised through a transcervical submandibular incision with a neck dissection. This allows complete resection of the cyst and will help spare the lingual and hypoglossal nerve. Alternatively, the surgeon may excise the sublingual gland transorally and pack the cyst or place a drain in it. By treating the gland, some believe the plunging cyst will resolve on its own.
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Figure 17a. Simple ranula. (a) In the right sublingual gland, the hyperintense lesion (arrow) on this transverse T2-weighted SE (3,000/102) MR image could represent a pleomorphic adenoma or a cyst. (b) The absence of enhancement on this fat-saturated, T1-weighted SE (600/17) MR image suggests a cystic lesion, in this case a simple ranula of the sublingual gland. (c) The nonenhancing ranula (curved arrow) lies superior to the geniohyoid muscles (g) and has not perforated through the mylohyoid musculature on this coronal T1-weighted SE (600/17) MR image with fat saturation.
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Figure 17b. Simple ranula. (a) In the right sublingual gland, the hyperintense lesion (arrow) on this transverse T2-weighted SE (3,000/102) MR image could represent a pleomorphic adenoma or a cyst. (b) The absence of enhancement on this fat-saturated, T1-weighted SE (600/17) MR image suggests a cystic lesion, in this case a simple ranula of the sublingual gland. (c) The nonenhancing ranula (curved arrow) lies superior to the geniohyoid muscles (g) and has not perforated through the mylohyoid musculature on this coronal T1-weighted SE (600/17) MR image with fat saturation.
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Figure 17c. Simple ranula. (a) In the right sublingual gland, the hyperintense lesion (arrow) on this transverse T2-weighted SE (3,000/102) MR image could represent a pleomorphic adenoma or a cyst. (b) The absence of enhancement on this fat-saturated, T1-weighted SE (600/17) MR image suggests a cystic lesion, in this case a simple ranula of the sublingual gland. (c) The nonenhancing ranula (curved arrow) lies superior to the geniohyoid muscles (g) and has not perforated through the mylohyoid musculature on this coronal T1-weighted SE (600/17) MR image with fat saturation.
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Occasionally, one will identify an abscess in the gland that is associated with infiltration of the subcutaneous tissue and/or the glandular tissue. As always, one should exclude a sialolith as the cause for the abscess on a nonenhanced CT scan, US scan, or conventional radiograph. Contrast-enhanced CT and MR imaging are currently the best ways to demonstrate a glandular abscess. Because of the fatty attenuation and intensity that is intrinsic to the parotid glands, particularly as one ages, abscesses are particularly well seen on enhanced CT scans and fat-saturated, T1-weighted MR images. CT has been shown to be of benefit in evaluating the inflammatory masses when compared to MR imaging (55).
One should include superinfected HIV-related cysts, suppurative parotid lymph nodes, and cystic degeneration of neoplasms with or without superimposed infection in the differential diagnosis of abscesses of the gland.
There are a number of manifestations of chronic sialadenitis, both clinically and radiographically. Most important, the changes from chronic sialolithiasis may result in a small atrophic gland with focal intraglandular calcifications. Rarely, one may see a large ductal stone still remaining with no evidence of acute inflammation and inducing no pain. This occurs most frequently in the submandibular gland.
A mucous plug in the duct may also cause a painful, swollen gland (Kussmaul disease). One pseudomass associated with calcifications in the gland is termed the "Kuttner tumor," a focal, masslike firmness of the submandibular gland due to chronic sialadenitis from sialolithiasis.
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SYSTEMIC DISORDERS OF THE MAJOR SALIVARY GLANDS
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In the category of systemic disorders that affect the major salivary glands, there are a number of autoimmune diseases. HIV-related cysts and nodules (Fig 18), Sjögren type 1 (previously known as Mikulicz disease) and Sjögren type 2 (Fig 19), and sarcoidosis fall within the general rubric of autoimmune disorders with salivary gland manifestations (56). CT is probably the best way to image patients with systemic disorders since calculi may be at the root of or a byproduct of the acute symptoms related to the systemic disorder. Sjögren syndrome and sarcoidosis predispose to stone formation. The attenuation of the gland may be increased with both Sjögren disease and sarcoidosis. Focal masses may be present with cysts, nodules, and lymph nodes in all of the diseases listed above.
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Figure 18. HIV-related lesions. Transverse, contrast-enhanced CT scan depicts a right-sided parotid cyst (straight arrow) and multiple small nodules in the left parotid gland (curved arrow) in this patient who was HIV positive but had not fulfilled criteria for acquired immunodeficiency syndrome.
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Figure 19. Sjögren disease. While both parotid glands (arrowheads) show cystic changes in and enlargement of the gland on this coronal T2-weighted SE (4,000/80) MR image, the left side also shows periparotid adenopathy (arrow). This pattern may be seen with Sjögren disease or HIV-related lymphoepithelial cysts and nodules.
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Sialography may be useful in staging Sjögren syndrome, since patients' symptoms may not correlate well with severity of the disease (Fig 20). The results of a prospective study comparing MR sialography (6,000/115; flip angle, 60°; pixel size, 0.29 x 0.31 x 3 mm) with the standard of reference conventional sialography showed an agreement rate of 89% (P < .001) in staging the disease (57). For identifying stage I or greater disease and stage II or greater disease, MR sialography was 100% and 91% accurate, respectively (57). Punctate, globular, and destructive patterns may be discerned with MR sialography (58).
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Figure 20. Conventional sialography. The extent of the ductal system and its tributaries is well seen on this conventional sialogram in a patient who has Sjögren disease. Note the tiny areas of cavitation (arrows) in the periphery.
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In most cases, the diagnosis of Sjögren disease is made clinically based on the sicca syndrome and the connective tissue disorder (eg, rheumatoid arthritis) combined with serology of antinuclear antibodies. Sialography, be it MR or conventional, is used primarily to stage disease (see previous section). For those cases in which the diagnosis is in question, some would favor performing a lip biopsy of the minor salivary gland tissue over sialography. Still others have suggested that a biopsy of the parotid gland has a higher yield than a labial biopsy since the sensitivity for Sjögren disease with labial minor salivary gland biopsy is 58% (21 of 36 patients) compared to 100% (36 of 36 patients) for parotid biopsy (59).
Sjögren syndrome increases the risk of parotid lymphoma by more than 4,400% (60). Therefore, any dominant mass in a Sjögren-affected parotid gland must be considered lymphoma and requires aspiration or biopsy with assessment of cellular clonality. MR imaging has been particularly helpful with identifying dominant masses within glands affected by Sjögren disease (60).
Som et al (39) have coined the term "acquired immunodeficiency syndrome–related parotid cysts" to describe the cysts associated with HIV infection and note that they are hard to distinguish from Sjögren-related benign lymphoepithelial lesions. In patients who are HIV positive, one can see both cysts or lymphoid nodules in the parotid glands. Complementary findings of cervical adenopathy, adenoidal hypertrophy, and tonsillar enlargement may also be present (61).
In addition to the autoimmune disorders in the preceding paragraphs, the salivary glands may be affected by other systemic disorders. Sialosis refers to a bilateral, painless enlargement of the salivary glands that may be caused by systemic disorders such as diabetes mellitus, alcoholism, hypothyroidism, and malnutrition. Some medications may provoke sialosis (including some antibiotics, diuretics, and psychotropic medications). The disorder is rarely imaged but usually shows enlarged parotid glands of increased attenuation and slightly increased T2 intensity.
Granulomatous diseases, including fungal infections, Wegener disease, tuberculosis, syphilis, and cat-scratch fever, may produce painless focal or generalized masses in the salivary glands. If there are calcifications within a painless mass in the parotid gland, granulomatous diseases and pleomorphic adenoma become the most likely diagnoses. Sarcoidosis predisposes to glandular calcifications (Fig 21) with or without nodes.
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Figure 21. Glandular calcifications in a patient with sarcoidosis. Transverse nonenhanced CT scan shows the presence of multiple small calcifications in the parotid glands bilaterally.
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Unusual disorders such as Kimura disease (eosinophilia, lymphoid proliferation in neck nodes and salivary glands, usually occurring in Asian patients), amyloidosis, toxoplasmosis, actinomycosis, and hematogenous metastases (usually from thyroid or renal cell carcinoma) rarely will affect the parotid gland.
The clinical history will usually offer the best means for establishing that the parotid gland is affected by systemic disorders. Imaging, if required, is used primarily to assess an atypical dominant mass. In this scenario, MR imaging is probably the most effective study.
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SUMMARY
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Most disorders of the salivary glands will manifest a few discrete clinical scenarios. Because sialolithiasis may have a variety of manifestations, CT should be the mainstay of imaging. Optimal imaging evaluation of the salivary glands may require nonenhanced (to visualize sialoliths), enhanced (to identify abscesses, cysts, and neoplasms), or both nonenhanced and enhanced (for painful masses for which one cannot exclude sialolithiasis) CT scans. On the other hand, with a subacute manifestation of a nonpainful, noninflammatory mass, where there is a high degree of suspicion for neoplasm, contrast-enhanced, fat-saturated MR imaging will best demonstrate the extent of disease and the presence of perineural seeding. Sialography, if indicated at all, may be performed with heavily T2-weighted MR techniques.
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ACKNOWLEDGMENTS
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The authors would like to acknowledge the kind support provided by Laurie A. Loevner, MD, in the analysis and collection of the images and material presented herein. 
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ABSTRACT
INTRODUCTION
