Tumors of the salivary gland origin in the parapharyngeal space arise either from the parotid gland, or salivary rests within the prestyloid compartment fat of the parapharyngeal space, or in the minor salivary glands of the pharyngeal mucosa. The commonest benign neoplasm is pleomorphic adenoma which on CT appears as a ovoid masses of soft tissue attenuation, typically homogenous when small but when larger one show areas of low attenuation representing sites of cystic degeneration or seromucinous collection (Fig 3) [6].

The malignant salivary gland lesions have irregular, ill-defined margins with heterogenous contrast enhancement and bone destruction (Fig 4).

Figure 5

Figure 4. Adenoid cystic carcinoma of the parotid in a 56yr-old-man. Contrast enhanced axial CT scan shows a heterogenous enhancing lesion with irregular margins, muscle infiltration, vascular encasement with loss of fat planes and bone destruction.

On certain occasions rare lesions of salivary gland origin are in the parapharyngeal space. One among them is cystic choristoma of salivary gland which demonstrated cystic density on plain scans (10-20HU) with no enhancement on post contrast studies (Fig 5). Choristoma of salivary gland are very rare lesions with imaging features which parallel epidermoid cysts [7, 8].

Figure 6

Figure 5. Cystic choristoma in a 1 month-old-infant. Contrast enhanced axial CT scan showing a nonenhancing cystic lesion in the prestyloid space (arrowhead).

Among the neurogenic lesions which form the next most common group schwannoma (76-80%) is more common than the neurofibroma [4]. Schwannomas are usually ovoid or fusiform masses with well-delineated margins and are higher in attenuation than adjacent muscle or isodense, less commonly of lower attenuation than the adjacent muscle (Fig 2a &b). These schwannomas show typical anteromedial displacement of the internal carotid artery and lateral displacement with compression of the internal jugular vein. The neural sheath tumors with extension into the jugular foramen show smooth scalloped jugular foraminal widening (Fig 6).

Figure 7

Figure 6. Axial CT scan at the skull base in bone window. Smooth scalloped widening of the right jugular foramen seen (arrows)

Neurogenic tumors show a density less than that of muscle with cystic areas and minimal post contrast enhancement (Fig 7). The hypodensity of these neurogenic tumors has been related to regions of both low cellularity and high lipid content [9]. They have also been attributed to entrapment of perineural adipose tissue by plexiform neurofibroma or cystic degeneration secondary to infarction or necrosis.

Figure 8

Figure 7. Neurofibroma in a 22yr-old-man. Contrast enhanced axial CT scan shows the minimally enhancing tumor in the post-styloid space (star) causing anterior displacement of ICA (arrow).

The glomus tumors are ovoid lesions with welldefined margins showing a density more than or equal to that of muscles and intense homogenous contrast enhancement (Fig 8a) [6]. The high cellularity and vascularity of these tumors may account for their isodensity on CT (Fig 8b). Their marked vascularity accounts for their homogenous hyperdensity after contrast enhancement [9]. The carotid body tumors caused splaying of ICA and ECA (Fig 8) while glomus vagale tumors caused anteromedial displacement of ICA (Fig 9) [10]. These typical features of intense contrast enhancement and vascular displacement patterns are helpful in arriving at a precise preoperative diagnosis.

Figure 9

Figure 8a. Carotid body tumor in a 26 yr-old-man. Post contrast axial CT shows intensely enhancing tumor in the post-styloid space (star) causing splaying of internal and external carotid artery (arrows).

Figure 10

Figure 8b. Photomicrograph (100x) shows solid aggregates of glomus cells around small capillary sized vessels in a myxoid stroma. The glomus cells are round, regularly shaped with sharply punched out rounded nucleus.

Figure 11

Figure 9. Glomus vagale in a 43 yr-old-woman. Post contrast axial CT shows intensely enhancing vagale tumor (star) causing anterior displacement of the internal carotid artery (arrow).

Primary malignant lymphomas of the parapharyngeal space are rare and involvement is secondary to the involvement of the head and neck. On imaging studies, they are homogeneous, enhancement is mild to moderate, may display necrosis and display calcification post-treatment. They are isodense to muscle on CT and circumscribed with distinct margins that occasionally display extra nodal extension with less well-defined margins and areas of necrosis within the tumor matrix (Fig 10). CT is also useful for detection of bone destruction involving the base of the skull, paranasal sinuses, and the mandible or maxilla [11]. As a general rule, lymphomas tend to be less infiltrative and cause relatively little bone erosion compared to carcinomas and most sarcomas [6]. The involvement of Waldeyer’s ring is an important feature as it gives vital clue to the diagnosis as the imaging features of lymphomas are quite nonspecific.

Figure 12

Figure 10. Non-Hodgkin’s lymphoma in a 32 yr-old-man. Post contrast axial CT shows a heterogeneously enhancing mass causing encasement of vessels (arrow).

Most infections that result in a parapharyngeal space abscess arise either in the palatine tonsil or pharynx or are odontogenic in origin. The CT findings of cellulitis are those of “dirty” fat and some swelling of the fat in the prestyloid compartment. The abscesses in our study had a low-attenuation necrotic, pus-filled center with a thick, irregular, enhancing rim on CT (Fig 11). Other features include evidence of air pocket with myositis, loss of fat plane with the great vessels and trans-spatial configuration. The presence of the abscess rim separates cellulitis from an abscess and this differentiation is important as it has significant implications on management [12].

Figure 13

Figure 11. Parapharyngeal Abscess in 69 yr-old-man. Post contrast Axial CT shows a peripherally enhancing right parapharyngeal abscess with air pockets (arrows).

Tubercular lymphadenitis usually shows a central low attenuation necrotic area with peripheral rim enhancement and minimal perilesional fat stranding (Fig 12). This is in contrast to pyogenic lymphadenitis and abscess which show more surrounding fat plane obliteration with thickening of the surrounding muscles and skin [13].

Figure 14

Figure 12. Tubercular lymphadenitis in a 12 yr-old-boy. Axial post contrast images showing multiple peripherally and densely enhancing lesions (arrows)

The metastatic lymphnode in the parapharyngeal space in our study was from a primary in the oropharynx (squamous cell carcinoma) which forms the drainage area of the level II group of lymphnodes. The lymphnode measured 3 cm in diameter showing heterogenous contrast enhancement. The lymphnodal mass demonstrated central areas of necrosis and evidence of extracapsular spread features which are considered as a specific criterion for diagnosing neck metastases (Fig 13) [14]. The CT features and presence of a primary lesion helps to confirm the diagnosis of metastases as their imaging features of overlap with that of infective lymphadenitis [6].

Figure 15

Figure 13. Metastatic lymphnodes in a 66 yr-old-woman. Axial post contrast images showing bilateral heterogenously enhancing lymphnode with areas of central necrosis (arrow), extracapsular spread and vascular invasion (arrowhead).

The vascular lesions in the parapharyngeal space include aneurysms of the internal carotid system. Extracranial aneurysms of the internal carotid artery are rare and usually secondary to atherosclerosis. On CT scan, extracranial internal carotid aneurysms, particularly the fusiform type, may have peripheral eggshell calcification and exhibit arterial enhancement after intravenous contrast (Fig 14) [15]. Of all the masses that occur in the parapharyngeal space, the aneurysm is most immediately life-threatening [15].

Figure 16

Figure 14. True internal carotid artery aneurysm in a 8 yr-old-girl. Post contrast axial CT shows intensely vascular saccular aneurysm in the post-styloid space.

Pseudoaneurysms of the carotid system are rare and usually are secondary to infection or fine needle procedures in the neck. The pseudoaneurysm shows vascular enhancing and nonenhancing areas and anatomical continuity with the ICA (Fig 15). Features indicative of inflammation may be seen in the surrounding soft tissues [16].

Figure 17

Figure 15. Internal carotid pseudoaneurysm in a 2 yr-old-girl. Post contrast axial CT image shows a large mass with central tubular area of arterial enhancement (star) surrounded by nonenhancing fluid density areas (arrow).

Meningiomas are the most common of the intracranial neoplasms to extend into the parapharyngeal space. They exit the intracranial compartment either by extending through the jugular fossa or other neurovascular canals or by invading the skull base bone [17]. Meningiomas are enhancing, homogeneous tumors that may have calcifications either in a localized portion of the lesion or scattered throughout the tumor (Fig 16a & 16b). These tumors may reveal a dumbbell configuration with a smaller intracranial and larger extracranial component [6]. Hyperostotic thickening of the bone is a common finding (Fig 17) [2].

Figure 18

Figure 16a. Meningioma in a 24 yr-old-woman. Axial CT shows a enhancing heterogenous density mass (arrow).

Figure 19

Figure 16b. Photomicrograph (100x) shows the characteristic meningothelial cells (star) and the psammoma bodies (arrow).

Figure 20

Figure 17. Meningioma in a 39 yr-old-woman. Skull base extension of the meningioma seen as hyperostosis of the adjacent skull base (star) with a small calcified intracranial infratentorial component (arrow).

Solitary fibrous tumors are uncommon spindle cell neoplasms in the parapharyngeal space with most of the cases documented in literature being case reports. The imaging features are fairly nonspecific.

Chordomas arising from the clivus may rarely extend caudally into the parapharyngeal space. On CT, most tumors have a nonhomogeneous enhancing appearance, with multiple scattered areas of “calcification” that presumably represent residual skull base fragments. In general, because of their bony matrix and the presence of scattered calcifications, these tumors are more easily diagnosed on CT than on MR imaging studies [18-20].

Among the lesions of the parapharyngeal space with skull base extension the pattern of involvement of the skull base gives a clue to the probable histopathological diagnosis. Meningiomas cause hyperostoses of the ipsilateral portion of skull base a feature typical of meningioma. The schwannomas which show extension into the skull base cause smooth scalloped widening of the jugular foramen, unlike in cases of paraganglioma where the involvement is said to be permeative and destructive [9]. Chordoma causes destruction of the sphenoclival synchondroses while lymphoma caused nonspecific bony destruction of the skull base.