In this article, various imaging modalities are discussed for evaluation of parotid disease, from congenital to inflammatory to neoplastic etiologies. Key imaging characteristics are outlined using case examples. Introduction to biological imaging is highlighted. Additionally, image-guided biopsy techniques are illustrated for sampling parotid and parapharyngeal space lesions in a minimally invasive manner.
Key points
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Ultrasound is a readily available initial imaging modality for the workup of parotid space tumors in the pediatric population.
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Computed tomography and MRI are essential for characterization of parotid space lesions and certain features may provide clues to the underlying histology.
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Biologic imaging is a new frontier that can offer insight into the behavioral characteristics of tumors.
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Image-guided biopsies offer a minimally invasive approach for histologic diagnosis.
Introduction
Given the location and anatomy of the parotid gland, lesions in the parotid gland can often remain indolent. This is true of lesions in the deep portion of the parotid gland or exophytic lesions extending into the paraphayrngeal space. As a result, many parotid lesions are incidental findings on imaging for other causes (neck trauma, headaches). The workup of incidental parotid lesions and patients presenting with specific symptoms related to their parotid gland, that is, facial pain or cheek mass, are some of the reasons for dedicated parotid gland imaging.
Imaging modalities
Conventional Radiographs
The use of plain radiographs and sialography are nearly obsolete in the current era of modern imaging. Only 60% of parotid calculi are seen on radiographs and determining whether they are parenchymal versus ductal is very limited. Computed tomography (CT) had a nearly 10-fold increased sensitivity to sialolith detection over conventional radiographs.
Sialography
Before cross-sectional imaging, siolograms were important for identifying lesions based on displacement of the ductal system and for assessing processes resulting in ductal obstruction. After the advent of CT, sialograms played a role in differentiating subacute and chronic sialdenitis and autoimmune causes such as Sjögren syndrome. Serology has superseded the diagnostic accuracy of conventional sialography.
MR Sialography
MR sialography is a useful alternative to sialograms. It is performed with a heavily T2-weighted high-resolution fast spin echo sequence (TR 3600/TE 800) with fat suppression with a 4- to 6-cm slab thickness and a surface coil or multichannel head coil. This technique offers a noninvasive means of assessing the ductal anatomy with comparable sensitivity to conventional sialography for large stone detection. Small stones, those approximately 3 mm, are more difficult to identify on MRI. MRI is also limited by the time required for the single sequence acquisition and the susceptibility to motion degradation for this sequence. For select cases, MR sialography can be added to the MRI parotid study.
Ultrasound
Another noninvasive tool for examining the parotid gland is high-resolution ultrasonography (US). In the United States, US is underused, whereas in Europe and Asia it is often the first diagnostic test for parotid pathology workup. Given that the bulk of the parotid gland is superficial, US serves as a readily available screening test for parotid pathology. Because most clinicians are familiar with cross-sectional anatomy, a sound understanding of the ultrasonographic appearance of the parotid space and spatial relationships is necessary for successful imaging of the parotid gland. For pediatric patients, it is the preferred diagnostic test given the concerns for radiation exposure and the low incidence of primary tumors in this population.
Wide-band linear transducers of 5 to 12 MHz are used for examination. Most superficial lesions are best evaluated with frequency probes greater than 7.5 MHz with evaluation of the deeper portions of the gland best evaluated with lower linear frequency probes (5-7 MHz) given the need for deeper penetration. This also forms a significant limitation for parotid US because lesions in the deep portion of the parotid gland, at the stylomandibular notch, or parapharyngeal space are obscured by the mandible. For these lesions, cross-sectional imaging is recommended ( Fig. 1 ).
Conventional Angiography
Catheter-based angiography has limited role in workup of parotid region tumors. Aside from the rare instances of a large arteriovenous malformation or hemangiomas that may require preoperative embolization, most parotid space tumors are not excessively hypervascular. With the advent of high resolution multislice CT scans, preoperative planning can be performed adequately with cross-sectional imaging. Even for the aforementioned hypervascular lesions, a CT angiography is adequate for preoperative planning and adds the benefit of providing better spatial resolution than conventional angiography.
Computed Tomographic Imaging of the Parotid Gland
In the normal adult parotid gland, the gland is fatty with Hounsfield units of 15 to 25. On CT imaging, this translates into attenuation that is slightly darker than muscle but brighter than subcutaneous fat ( Fig. 2 ). In the pediatric population, the fat content is smaller and hence the gland is often brighter. A denser gland can mask small intraglandular lesions. For inflammatory disease or sialadenitis, CT is the preferred imaging modality given its spatial resolution and sensitivity to calcification. Limitations of CT include extensive dental artifacts that cannot be avoided with angled axial imaging, contrast contraindications in densely glandular tissue where an intraglandular neoplasm is suspected, assessment of perineural spread of disease, and radiation concerns in the pediatric population.
With the advent of multidetector CT scans, most parotid space imaging would include thin section acquisition (≤2.5 mm) from the inferior orbits through the mandible. If the entire neck is scanned (for workup of malignant lymphadenopathy), scanning through the aortic arch should be included. Nonionic iodinated contrast is helpful for evaluation for lesion conspicuity, assessment of lymph nodes, evaluation of the extent of inflammation in infectious etiology, and for understanding the vascularity of the lesions. Care should be made to avoid the orbits if possible and to use iterative reconstructive dose reduction techniques to minimize radiation exposure, especially in the pediatric population. Rarely should a noncontrast and contrast CT be performed.
A clear advantage of CT scan is exact localization of the facial nerve in the mastoid bone and position of the stylomastoid foramen. This information is useful in cases of tumors invading the mastoid, and tumors that abut the skull base adjacent to the stylomastoid foramen in assessing their relationship to the facial nerve.
MRI of the Parotid Gland
The normal adult parotid gland has a high T1-weighted signal and low to intermediate T2-weighted signal on conventional MRI. As in CT, the fat content of the parotid gland makes it ideal for MRI. Aside from lipomas, most lesions within the parotid gland stand out on precontrast T1-weighted imaging. Additionally, the ability to follow abnormal enhancement along the nerves and assessment of the regional vasculature makes parotid space imaging ideal with MRI ( Fig. 3 ). Contrast can be helpful in lesions that are inconspicuous on T1-weighted imaging or for characterization of nodal spread.
Although imaging protocols vary from center to center, most protocols will include T1-weighted imaging in 3 planes, T2-wegithed imaging with fat suppression to highlight nodal disease and lesion conspicuity, and fat-suppressed postcontrast T1-weighted imaging. The area of coverage should include the base of the mastoid through the mandible for adequate inclusion of parotid glands of varying size. High field strength MRI such as 3T MRI, when available, offers greater signal to noise ratio with a shorter acquisition time. In the pediatric population and the claustrophobic population, this can often shorten the duration of sedation.
Biologic imaging of parotid space tumors
Radionuclide Imaging
Radionuclide salivary studies are also available as a mode of evaluating both the parenchymal and excretory functions of the gland. The parotid glands normally concentrate 99m Tc-pertechnetate, so most lesions within the parotid gland are difficult to identify with scintigraphic studies compared with cross-sectional studies. Single photon emission CT–CT is more promising for its spatial resolution. Radioactive tracers are hypersecreted in cases of acute sialadenitis, lymphoma, and sialosis in addition to tumors such as Warthin’s tumors and oncocytomas. Diminished radionuclide excretory activity is commonly seen in Sjögren syndrome or chronic parotitis. One study comparing 99m Tc and 201 Tl single photon emission CT with CT/MRI imaging demonstrated the ability to differentiate Warthin’s tumors from other major salivary gland tumors with greater sensitivity than conventional CT/MRI imaging. Nonetheless, aside from nonoperative patients, surgical excision is still recommended for these tumors, regardless of histology. The subsequent treatment options and outcomes are only then affected by the histology.
PET Imaging
PET imaging has had little role in initial workup of parotid tumors. The cost of the test, the time involved, and its low specificity preclude routine use of this test. Furthermore, normal activity in the parotid gland may mask an underlying lesion. Both malignant and benign parotid space tumors have increased glucose metabolism and even some inflammatory processes can demonstrate increased uptake. Specificity can be increased by using multiple tracers however these are tracers that are not routinely readily available. There is a role for PET surveillance for recurrent disease or for assessing distant metastases ( Fig. 4 ).
Proton MR Spectroscopy
MR spectroscopy has a unique ability to examine a tumor at the metabolic level. Its limitations relate to the minimum voxel size necessary for spectral analysis in tumors that are often small and irregular and the low signal-to-noise ratios obtained from head and neck imaging. Despite these restrictions, by assessing ratios of metabolites, a predictor of malignancy and cellular turnover can be made. In a study by King and colleagues, the choline to creatine ratio was examined between benign and malignant tumors and between Warthin’s tumor and pleomorphic adenoma. Ratios of greater than 2.4 were predictive of a benign tumor and those greater than 4.5 had a 71% positive predictive value of being a Warthin’s tumor. Spectral analysis, however, has yet to be proven effective in predicting outcome or evaluating treatment efficacy. King and colleagues showed that the choline values on the pretreatment scans and posttreatment studies are not predictive of treatment response.
Computed Tomography Perfusion
CT perfusion (CTP) is a reproducible examination that examines the microvascularity of tumors. With dynamic acquisitions over the tumor region, certain characteristics such as blood volume (BV), blood flow, capillary permeability (CP), and mean transit time can be calculated mathematically. Malignant tumors demonstrate increased BV, blood flow, and CP with a diminished mean transit time. For treatment response and outcomes prediction, studies have shown a role for CTP. Most tumors with early therapeutic response exhibit diminished BV and CP on posttherapy studies and nonresponders showed increased BV and CP. Additionally, marked elevated BV on baseline studies were more predictive of short-term response.
Perfusion-Weighted MRI
Like CTP, perfusion-weighted MRI examines the vascular infrastructure of tumors. Unlike CTP, there are variations in how the images are acquired as well as dependent on which vascular model is applied for determination of BV and permeability. Additionally, perfusion MR acquisitions require more time for scanning. These have generally limited its use to a few centers. Recent studies have looked at permeability on the pretreatment studies. As in CTP, an increased permeability in the treatment responders was higher than in nonresponders on the baseline study.
Diffusion-Weighted MRI
Diffusion-weighted imaging looks at water molecule motion through the cell membranes of the tissues in the scan field. When the NA–K pumps fail or when the extracellular space is minute, there is increased in diffusion restriction of the water molecules (shown as increased DW signal and low apparent diffusion coefficient [ADC] values). Diffusion-weighted imaging has been introduced into many head and neck MRIs owing to its short acquisition time and its reproducibility. In parotid imaging, restricted diffusion with low ADC values are predictive of tumor malignancy. Most studies have shown ADC values below 0.9 to 1.3 × 10 −3 mm 2 /s are typical for malignant tumors or metastatic lymph nodes. Values greater than 1.8 × 10 −3 mm 2 /s have a high positive predictive value for pleomorphic adenomas. Diffusion-weighted imaging is another biological imaging tool to predict tumor behavior ( Fig. 5 ).
A combination of dynamic contrast-enhanced MR and ADC is another preoperative tool to assess benignity of parotid tumors. Yabuuchi and colleagues classified parotid tumors based on their time–intensity curves. Tumors with flat contrast enhancement and gradual contrast enhancement were highly predictive of benign tumors. Those tumors with rapid peak enhancement but gradual washout were suggestive of malignancy and those tumors with rapid peak enhancement but steep washout were more suggestive of benign histology. These last 2 categories often had overlap. Adding the cutoff ADC values of 1.4 × 10 −3 mm 2 /s between pleomorphic adenomas and malignant tumors and 1.0 × 10 −3 mm 2 /s between Warthin’s and malignant tumors further helped to accurately identify tumors preoperatively (accuracy 82% vs 94%; positive predictive value 67% vs 92%).
Furthermore, functionality of the gland can be assessed by looking at the ADC values. In normal parotid tissue, during the early gustatory stimulation, there are low ADC values that steadily increase over time. In a gland previously treated with radiotherapy, this gradual increase to greater than the baseline value is not seen; the ADC values remain increased at baseline and after stimulation.
Pictorial essay of parotid space lesions
Lipoma
Lipoma of the parotid space accounts for 10% of all parotid lesions. Clinically, lipomas are found either incidentally or from painless swelling. There is no predilection for malignant transformation and surgical excision is reserved for cosmetic purposes. On US, lipomas are compressible, echogenic lesions without posterior acoustic shadowing. On CT, they have Hounsfield units of -20 to -100 with possible thin rim of enhancement. No nodularity is noted. They are T1 and T2 hyperintense on MRI, similar to subcutaneous fat with complete fat suppression. CT/MRI is helpful to examine the extent of lipomatous infiltration and to assess for any nodularity suggestive of sarcomatous degeneration ( Fig. 6 ).
Intraparotid Lymph Nodes
Intraparotid lymph nodes are normally seen within the parotid glands. Approximately 20 lymph nodes reside within the parotid space owing to late embryologic encapsulation. Intraglandular lymph nodes are not seen in the other salivary glands. They serve as the primary lymphatic drainage of the external auditory canal, pinna, and regional scalp (see Fig. 3 A, D).
Branchial Cleft Cyst (Types I and II)
First branchial cleft cysts are usually periaricular or periparotid/intraparotid. These latter cysts/sinus tracts are classified as type II and are the most common 1st branchial cleft cyst. On US, they are usually solitary anechoic masses with posterior acoustic shadowing. On CT, they are well-circumscribed, rim-enhancing lesions within the substance of the parotid, periparotid space or parapharyngeal space. They are hyperintense on T2-weighted imaging and hypointense on T1-weighted imaging. A thickened rim is suggestive of acute/chronic infection. ( Fig. 7 ) In contrast, a second branchial cleft cyst is often seen deep to platysma, anterior to the sternocleidomastoid muscle, posterior to the submandibular gland, and lateral to the carotid sheath. It often lies inferior to the parotid gland ( Fig. 8 ).
Parotitis
Parotitis is often a clinical diagnosis; the majority of these cases have a viral etiology. They tend to be bilateral with glandular enlargement, whereas bacterial parotitis is uninlateral with associated cellulitis with or without abscess. Autoimmune parotitis is often difficult to distinguish from viral parotitis because it is often bilateral. Periglandular inflammation is less common. Serology is often required for diagnosis ( Fig. 9 ).
