Ct And

Both CT and MRI provide information superior to that provided by other imaging techniques or by physical examination.12 To obtain the maximum amount of information possible, CT scanning should be performed with intravenous injection of contrast material. The normal parotid gland has a high fat content and is easily visualized on both CT and MRI; therefore, both techniques can demonstrate whether a mass is intra- or extraglandular. Generally, CT/MRI do not provide information regarding the specific histologic diagnosis, except rarely. An example of such a rare scenario is with lipoma of the parotid gland. However, CT/MRI can give useful information that may differentiate benign from malignant tumors. In contrast to benign tumors, which invariably have well-defined margins, malignant tumors usually exhibit irregular margins. Extension of the tumor beyond the confines of the gland can be adequately seen on both CT and MRI. Bony destruction of the mandible or skull base is best visualized on CT, whereas bone marrow involvement is better demon strated on MRI. Both studies can adequately evaluate the neck for metastatic adenopathy. CT has the advantage of being less expensive and more available than MRI. However, CT images are more susceptible to degradation by dental arti-fact.12 MRI is superior to CT in demonstrating the internal architecture of salivary gland tumors in a multiplanar fashion, and in delineating the interface between tumor and normal salivary gland.13

Perineural spread of parotid malignancy along the facial nerve have a profound negative impact on survival and can drastically change the therapeutic plan, including the surgical approach and adjuvant therapy.14 Although perineural spread may present as abnormal nerve function, it often is asymptomatic. High-resolution imaging may be helpful in such cases in detecting perineural involvement. The criteria of nerve involvement on CT rely on bony changes along the course of the facial nerve. These changes include bone erosion, sclerotic margins, and widening of the normal diameter of the fallopian canal or stylomastoid foramen. However, these findings are late indicators of perineural spread. Perineural spread can be detected earlier on MRI, because of the better soft tissue delineation. The capability of MRI to detect the different signal intensity of tumor, fat, and nerve allow for better assessment of perineural spread. The criteria of nerve involvement on MRI include replacement of normal perineural fat with tumor, enhancement with gadolinium (regardless of size), and increased size of the nerve in question (regardless of enhancement). Using these criteria, MRI is more sensitive and specific in evaluating perineural spread than CT.14

Although parapharyngeal space masses are well visualized by both techniques, they are better delineated with MRI than CT. This is because of the different signal intensity of tumor, fat, and muscle on MRI. Most salivary tumors have low to intermediate T1 signal intensities and intermediate to high T2 signal intensities. The differential diagnoses of parapharyngeal masses include deep lobe parotid tumors, minor salivary gland tumors, and neurogenic and vascular tumors. Deep lobe parotid tumors and minor salivary gland tumors of the parapharyngeal space lie in the prestyloid compartment, anterior to the carotid artery, and displace the parapharyngeal fat medially. Deep lobe tumors are connected to the parotid gland at least in one imaging section. Minor salivary gland tumors are completely surrounded by fat.12 By contrast, neurogenic tumors and glomus tumors lie in the poststyloid compartment, posterior to the carotid artery, which is displaced anteriorly. Neurogenic tumors usually enhance intensely with gadolinium, whereas glomus tumors have a characteristic serpiginous flow voids (salt-and-pepper appearance) on MRI.

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