Dura and Brain
Coronal MRI (T1-weighted, gadolinium-enhanced, fat-suppressed) shows enlargement and intense enhancement of the mandibular division of the left trigeminal nerve as it passes through the foramen ovale (large arrow). Normal cerebrospinal fluid in Meckel's cave is seen on the right (arrowhead), whereas there is tumor in Meckel's cave and the cavernous sinus on the left (open arrow). Normal flow voids of the cavernous carotid arteries (a) are seen.Enhancement of dura is often evidence of intracranial extension of tumors arising in the paranasal sinuses, nasal cavity, skull base, nasopharynx, and temporal bone. Subtle dural enhancement is not detected on CT
Computed tomography (soft-tissue algorithm) shows an aggressive mass that has destroyed much of the sphenoid bone (open arrow). Tumor and brain are of different densities, but it is impossible to determine whether dura is invaded (small arrow). The cavernous sinuses (arrowheads) are opacified on this enhanced study, which is normal. However, it is impossible to determine whether tumor invades the cavernous sinus.
Magnetic resonance imaging (T1-weighted, fat-suppressed) more clearly shows the bulky enhancing tumor (large arrows), the interface between tumor and brain, and the abnormally enhancing, thickened aura (small arrows). This could reflect tumor invasion of dura or inflammation in response to adjacent tumor. The signal void of rapidly flowing blood in the left cavernous carotid artery (open arrow) is surrounded by opacified, more slowly flowing blood in the cavernous sinus (curved arrow), making it difficult to determine whether tumor invades the left cavernous sinus. On the right, bulky tumor (small arrows) invades the cavernous sinus.(Figure 10a) but is more easily identified on MRI (Figure 10b). Dural enhancement may indicate tumor invasion or dural inflammation secondary to adjacent tumor or prior surgery.[12] Prior imaging studies are extremely helpful in making this distinction. In the absence of prior studies, it may be necessary to obtain frozen sections in the operating room to determine the appropriate surgical margins.
Imaging perpendicular to the plane of the affected dura facilitates visualization of the enhancement.[4] Coronal images often provide important information about dura along the floor of the anterior and middle cranial fossae and the roof of the temporal bone.
Coronal MR images are far more easily obtained than are coronal CT scans, as the latter require the patient to hyperextend the neck (lying either prone or supine) and the scanning gantry to be tilted. Limitations of patient flexibility and scanner geometry sometimes result in “coronal” CT images that are just slightly oblique axial images. With MRI, however, any scan plane is easily obtainable with software manipulations only. The patient lies in the same supine position throughout the study, whether coronal, axial, sagittal, or oblique images are being acquired.
Vasogenic edema spreading through white matter of the brain may be the initial evidence of tumor invasion. Both CT and MRI (especially T2-weighted pulse sequences) show white matter edema; MRI is superior for the detection of subtle edema. Frank enhancement of brain parenchyma may indicate direct extension of tumor or metastatic disease.[1,3]
Blood Vessels
On nonenhanced CT studies, blood vessels are isodense to other soft tissues, including lymph nodes, muscle, and tumor; consequently, blood vessels may be confused with all of these tissues. After IV iodinated contrast administration, however, blood vessels become hyperdense (white) and can be easily differentiated from other soft tissues. The exception to this is intensely enhancing tumor. For example, it may be impossible to distinguish the carotid artery as a structure separate from an intensely enhancing glomus tumor.[13]
Tumor Invading a Blood VesselTumor invading a blood vessel is usually a different density from the opacified blood in the remaining vessel lumen (Figure 11). On spin echo pulse MRI sequences (T1- and T2-weighted images, for example), rapidly flowing blood generates a signal void (black). This appearance does not change significantly after IV gadolinium administration; rapidly flowing blood is a signal void whether or not it contains gadolinium (Figure 10b).
More slowly flowing blood vessels, such as smaller veins and arteries and the cavernous sinus (Figure 10b), are an important exception; these vessels may enhance after contrast administration.[13] The carotid arteries (Figure 10b), jugular veins, and large dural venous sinuses are all black. More slowly flowing blood yields a signal that may create great confusion. The jugular fossa is one such location; blood flow in the jugular fossa is often slow, creating a bright (hyperintense) signal that can mimic tumor. In this setting, flow-sensitive images tailored to the expected flow velocity may be helpful.
Magnetic resonance angiogramMagnetic resonance angiography (MRA; Figure 12) uses the signal characteristics of flowing blood to generate images of arteries and large veins, such as the internal jugular vein and dural venous sinuses.[14] This imaging study may be obtained without IV contrast, making it a noninvasive procedure that is very different from catheter angiography. Other MRA pulse sequences provide finer detail of blood vessels after administration of IV gadolinium.
Magnetic resonance angiography provides little or no additional information beyond that provided by the axial “base images.” The main difference is that, on MRA, the background structures are subtracted by the computer software, and the same software “stacks” the images into a “three-dimensional”–type image that can be rotated and viewed from any angle. However, the tumor itself is not seen on MRA (Figure 12). Unlike conventional catheter angiography (Figure 1), small tumor vessels are usually below the level of resolution of the MRA scan, and the only evidence of the tumor may be either displacement of the affected vessel by the (unseen) tumor or narrowing, indicating encasement by tumor.
In addition, unlike catheter angiography, MRA does not offer the option of therapeutic embolization or intra-arterial instillation of chemotherapeutic agents. Finally, MRA is subject to artifacts that can mimic occlusion or narrowing of a vessel.
New computer software has led recently to the development of CTangiography (CTA). This technique requires a scanner able to do helical scanning, which acquires, in rapid sequence, many thin axial slices (1 to 10 mm thick) with little or no interslice gap. Rapid bolus administration of contrast must be coordinated with scanning so that the area of interest is scanned during maximum opacification of the vessels of interest. Computed tomographic arteriograms and venograms can be obtained by adjusting the timing of scanning with respect to contrast administration; in both cases, contrast is given intravenously.
As with MRA, the CTA software program subtracts background structures and “stacks” the axial data to generate images that closely resemble conventional angiograms. Also, like MRA, the reformatted CTA images contain no information that is not present on the many axial images. Compared with MRA, CTA seems to be subject to fewer scanning artifacts, although atherosclerotic calcified plaque that severely narrows a vessel lumen can mimic a patent vessel if the “scout” images (ie, those obtained before contrast administration) are not reviewed.
Tumor extending through the skull base into the cavernous sinus may be difficult to identify, even on enhanced CT studies, as the enhancing tumor and the enhanced blood in the cavernous sinus are isodense.[13] Abnormal lateral convexity of the cavernous sinus may suggest the presence of tumor, but an ectatic cavernous carotid artery can similarly distort the cavernous sinus contour. In this setting, MRI is extremely helpful, as the rapidly flowing blood in the cavernous carotid artery is black (on spin echo sequences), whereas enhancing tumor is white (on T2-weighted sequences and on gadolinium-enhanced T1-weighted sequences).
Both CT and MRI can show tumor effacement of the normal fat plane around blood vessels. Only at surgery is it possible to determine whether there is frank invasion of the adventitia.
If preoperative studies diagnose or raise the concern of tumor invasion of a carotid artery, preoperative evaluation may include a balloon test-occlusion, to determine whether the artery may be sacrificed.[1,15] The angiographer advances a balloon-tipped catheter into the involved carotid artery. Inflation of the balloon reversibly mimics sacrifice of the artery.
With the balloon inflated, clinical neurologic testing in the angiography suite is the initial evaluation. If no significant neurologic deficits develop, the balloon is deflated but the catheter remains in place. The patient is transferred to the CT scanner, where he or she inhales stable (nonradioactive) xenon gas. The gas diffuses across the alveoli walls into the bloodstream. With xenon in the blood, normally-perfused brain parenchyma is faintly more radiopaque than brain parenchyma that is not as fully perfused. The density of brain parenchyma, therefore, is compared while the balloon is inflated and deflated. If the change in density is less than a predetermined amount, this suggests that the artery can be resected with few or no clinical sequelae.[1,15]
Air
Both CT and MR can differentiate tumor from air in a normally clear paranasal sinus. Air is black on CT (Figure 2a) and on all MR pulse sequences (Figure 2b). Dense tumor calcifications are black (signal voids) on MRI, but calcified foci are usually scattered within the soft tissue mass of a tumor, and not liable to be confused with a clear, normal sinus. As shown in Figure 3, this would not be a problem with CT, as calcifications on CT are dense (white).
Obstructed Secretions
When tumor obstructs a paranasal sinus, obstructed secretions fill the sinus and can mimic tumor in the sinus lumen. On contrast-enhanced CT, tumor is more dense than low-density (watery) secretions (Figure 2a). Chronically obstructed secretions become less watery, more proteinaceous, and more dense on CT. These may be isodense to tumor.
On MRI, the signal intensity of retained secretions depends on the concentration of protein and whether there was any prior bleeding into the sinus.[1,3] As shown in Figure 2b, watery secretions are dark (hypointense) on T1-weighted sequences and bright (white) on T2-weighted images [16]; the appearance is similar to that of other watery secretions, such as cerebrospinal fluid and vitreous humor.
Desiccated secretions may be hyperintense on T1-weighted images (Figure 2a and figure 2b), [16] but very desiccated secretions may have no mobile hydrogen protons to generate an MR signal and, therefore, may yield a signal void indistinguishable from an aerated sinus.[16] This is an important pitfall to keep in mind when using MRI to evaluate the paranasal sinuses.
Inflamed, nonneoplastic sinus mucosa enhances. This is better seen on MRI than on CT studies. The thin, white line of enhancing mucosa is not likely to be mistaken for tumor. The availability of T1- and T2-weighted pulse sequences and IV gadolinium make MRI superior to CT for differentiating tumor from obstructed secretions.
Lymph Nodes
Enhancement of Lymph NodesImaging studies have contributed to the identification of metastatic cervical lymphadenopathy. A clinically N0 neck may contain metastatic disease that can be identified only by CT or MRI.
Normal lymph nodes are the same CT density as muscle and are isointense to muscle on T1-weighted MR images. Normal nodes are usually slightly hyperintense (white) on T2-weighted images and enhance slightly and homogeneously after contrast administration.
Imaging criteria for normal and pathologic lymph nodes vary. The goal is to maximize sensitivity, so that nodes that contain tumor are not overlooked as normal and histologically normal nodes are not misidentified as pathologic.[17]
Many authors accept as normal the criterion of 1 cm in greatest axial diameter for cervical lymph nodes. Some authors extend this to 1.5 cm for jugulodigastric (zone II) nodes that drain much of the upper aerodigestive tract. Homogeneous density or signal, an oval shape, and intact surrounding fat are also normal.
Lymph Node NecrosisA heterogeneous appearance, enhancement (Figure 13), and/or frank central necrosis (Figure 14) strongly suggest the presence of metastatic tumor in the appropriate clinical context. (Infectious lymphadenitis may have the same appearance, however.) Even small lymph nodes (£ 5 mm) with a necrotic center are abnormal. These are often not clinically palpable, and their identification on imaging studies may alter the clinical stage of the tumor.
Abnormal Number of Lymph NodesIt is important not to mistake the normal hilar fat in a node for central necrosis. This is only an issue on CT, as fat and necrosis have different signals on MRI.
Normal lymph nodes are oval, whereas round nodes often contain tumor. It is important to make sure that the scan is oriented along the long axis of the node before identifying a node as round. An increased number of nodes may also be abnormal (Figure 15).
Imaging studies are especially important for detecting clinically nonpalpable nodes.[17] These are not just small nodes. Retropharyngeal lymph nodes (nodes of Rouviere) are difficult or impossible to appreciate clinically but are easily identified on imaging studies (Figure 14). This important information may alter surgical planning or treatment options.
New Techniques
Three-dimensional reconstructionThree-dimensional–type images generated from the axial CT data facilitate visualization of the anatomy (Figure 16). These images contain no additional information, but it is sometimes easier to conceptualize the relationship of tumor to normal structures after the computer software rearranges the data.
Intraoperative localization using CT and MRI guidance is now possible.[18] New computer software programs link the axial CT or MRI data acquired preoperatively to a probe placed in the surgical field at the time of surgery.
In the operating room, the surgeon co-registers fixed surface landmarks on the patient (such as the medial canthus of the eye and tip of the nose) with a three-dimensional reformatted image made from the same patient’s axial data.[18] When the localizing probe is placed on a structure in the surgical bed, the computer displays the axial image, plus the reformatted coronal and sagittal images, all with a marker that indicates the location of the probe. Intraoperative image-guided navigation has proven to be quite useful in facilitating the surgical approach to skull base tumors.[18]
In addition to preoperative embolization of a highly vascular tumor, the angiographer can provide other forms of therapy. Therapeutic embolization of surgically unresectable bleeding tumors can be a life-saving intervention. Also, intra-arterial administration of chemotherapy allows for the delivery of a higher concentration of the drugs to the tumor, with fewer systemic side effects.
Positron emission tomography (PET) may prove to be of great value in differentiating tumor from scar, edema, and other nonneoplastic soft tissues.[19] A radioactive isotope is attached to a substance that the body metabolizes (usually fluorodeoxyglucose). Tumor accumulates and metabolizes the tracer-labeled glucose more avidly than does scar, normal muscle or lymph node, or edema.[19]
Conclusions
Imaging studies provide important information about the location and extent of head and neck neoplasms. This information facilitates preoperative surgical planning and can be used to evaluate response to nonsurgical therapy.
Imaging studies can also identify residual and recurrent tumor and clinically occult disease. With imaging-guided biopsy and intraoperative localization; therapeutic embolization and intra-arterial chemotherapy under angiographic guidance; and PET studies, the radiologist has assumed a more active role in the evaluation and treatment of head and neck neoplasms.
