Computed tomography (CT) with thin sections (1-3 mm) is an excellent imaging technique by which to identify paragangliomas and document the extent of tumor with precise evaluation of bone invasion. The classic CT findings of paragangliomas include a homogeneous mass with intense enhancement following administration of intravenous contrast (Figure 2). Although this appearance is typical of these tumors, it may also be seen with other vascular tumors such as neurolemmoma and meningioma.
Tumor location, displacement of major vessels, and patterns of involvement or invasion of surrounding structures aid in the diagnosis of paraganglioma. For example, carotid body tumors typically display the common carotid bifurcation with posterolateral displacement of the internal carotid artery (Figure 3A). By contrast, vagal paragangliomas will displace both the internal and external carotid arteries anteriorly (Figure 3B), and are associated with erosion and widening of the jugular foramen.
Jugular paragangliomas may be distinguished from tympanic tumors based on early involvement of the skull base, erosion of the caroticojugular spine,[27,42] and destruction of the ossicular chain, which is unusual with tympanic tumors.
Magnetic Resonance Imaging
Although CT is the study of choice for the evaluation of bone involvement, magnetic resonance imaging (MRI) provides meticulous soft-tissue detail and defines skull-base, intracranial, dural, and neural involvement, especially with the use of gadolinium enhancement (Figure 4). Augmented by its ability to produce images in multiple planes, MRI is superior to CT in defining the relationship of paragangliomas to adjacent vascular and skull-base structures.
MRI studies of paragangliomas demonstrate a background tumor matrix of intermediate signal intensity on T1-and proton density-weighted images and moderately high signal intensity on T2-weighted images, along with scattered areas of focal signal voids, reflecting high-flow blood vessels. Intense homogeneous contrast enhancement is seen. On T2-weighted images, the classic MRI "salt and pepper" appearance, originally described by Olsen et al and present in most lesions greater than 1.5 cm, reflects signal voids intermixed with regions of focally high signal intensity, the latter of which are probably due to sites of slow flow within the image plane (Figure 4).
These findings, however, are not specific for paragangliomas, and may be seen with other hypervascular lesions (eg, metastasic renal cell carcinoma and thyroid carcinoma). That said, the typically smooth contour, sig- nal characteristics, and location of paragangliomas, coupled with a detailed clinical history and physical findings should result in an accurate diagnosis.
It is also worth noting that MRI is more effective than CT in identifying small synchronous paragangliomas, especially those smaller than 5 mm. (CT is most effective in demonstrating lesions greater than 8 mm.)
Magnetic resonance angiography (MRA) provides excellent visualization of the major head and neck vasculature and can demonstrate vessel displacement, gross tumor involvement, and possible compromised blood flow. It may be useful in defining flow-related enhancement of lesions greater than 1.5 cm. While three-dimensional time-of-flight angiography appears superior to other MRA techniques in identifying some tumor feeders, its sensitivity is not high enough to demonstrate detailed tumor vascular supply, which is best defined by digital subtraction superselective angiography.
Paragangliomas, like other neuroendocrine tumors, have been found to have a high density of somatostatin(Drug information on somatostatin) type 2 receptors on their cell surface. Octreotide(Drug information on octreotide) (Sandostatin) is a somatostatin analog, which, when coupled to the radioisotope indium-111, creates a scintigraphic image of tumors expressing somatostatin type 2 receptors. In a study of 21 patients with presumed head and neck paragangliomas who underwent indium-111/ octreotide scintigraphy, Telischi et al demonstrated an accuracy of 90%, a sensitivity of 94%, and a specificity of 75% in detecting tumors with this technique.
Radioisotope scintigraphy provides a noninvasive imaging modality that is particularly useful in screening families with a history of paragangliomas. In addition, it is useful in detecting both synchronous and metachronous metastatic tumors.
Angiography plays an important role in the evaluation of paragangliomas if surgery is being contemplated. It provides exquisite detail of the vascular anatomy, delineating the tumor blood supply (and possible anastomoses),[ 2] displacement of vessels, potential vessel compromise by tumor invasion, and adequacy of intracranial circulation if internal carotid artery sacrifice is necessary; this technique can also reveal previously undiagnosed synchronous paragangliomas.[ 49] Superselective angiography allows safe preoperative embolization of the tumor vasculature, hopefully avoiding proximal vessel occlusion and unexpected migration of embolization material into the cerebral or systemic circulation.
Our method of internal carotid balloon occlusion performed under local anesthesia with monitoring of mental functions for 20 minutes seems to be reliable in predicting a patient's tolerance to the interruption of carotid blood flow. More recently, we have relied on angiographic evidence of adequate contralateral cerebral blood without performing elective internal carotid balloon occlusion (Figure 5).
In our study of 28 patients undergoing cerebral angiography, 26 (93%) demonstrated angiographic or balloon occlusion testing evidence of tolerance to possible carotid sacrifice.[ 28] With this preoperative information, surgical decisions about carotid bypass options can be made. Even with preparation for internal carotid artery grafting, a review of multiple series reports a 9.7% rate of strokes and a 2.4% incidence of mortality.
In our series, 4 of 47 patients with previously undiscovered paragangliomas were diagnosed on angiography (three carotid body tumors, one vagal paraganglioma). For these reasons, bilateral carotid angiography is an important part of the protocol in our evaluation of patients who are to undergo surgery.
If a diagnosis of paraganglioma is suspected, an incisional biopsy is contraindicated due to the risk of hemorrhage and subsequent fibrosis at the operative site. A fineneedle aspiration biopsy may be performed safely with minimal risk of bleeding.
The most commonly used staging systems for carotid body tumors and jugulotympanic paragangliomas are listed in Table 4.