MR IMAGING OF THE LOWER FACE

AND SALIVARY GLANDS

Wade H.Wong, D.O.

INTRODUCTION

Imaging of the head and neck has developed significantly with the advent of CT and MR. These modalities greatly compliment the physical and endoscopic examinations by revealing possible blind areas, such as subtle extension of neoplasms from the lower face and/or salivary glands to deep spaces, nonpalpable adenopathy, bone marrow invasion, and distant metastasis.

MRI has several major advantages over CT. Superior soft tissue contrast is possible with MRI leading to better delineation between tumor and adjacent musculature, especially in the tongue and floor of mouth where relatively little fat is present between tissue planes. Multiplanar imaging can be extremely helpful in appreciating and confirming the extent of disease. Beam hardening artifacts from dental fillings often plague CT examinations, but dental fillings cause only minimal susceptibility artifacts with MR. Non-removable bridgework remain a problem for both techniques. In patients who can not tolerate iodine contrast, MR with gadolinium can still be considered.

However, MR has some drawbacks compared to CT. Patients who are severely claustrophobic in a magnet usually tolerate the more open-spaced feeling a CT scanner offers. General contraindications to MR such as cardiac pacemakers, ocular foreign bodies, aneurysm clips, and surgical ferromagnetic hardware would lead one to depend upon CT rather than MR. Evaluation of calcification for stones in the salivary duct are more easily detected by CT.

TECHNICAL CONSIDERATIONS

Significant problems with motion are frequently encountered when trying to image patients with oropharyngeal, orocavity, and hypopharyngeal cancers because of secretions and gagging. Often, simple encouragement and coaching is all that is needed for such a patient to remain still. Steady, shallow abdominal breathing is helpful. In some cases, deep sedation may be necessary.

Speed of study is also very important, as this limits the amount of motion degradation. Fast spin-echo (FSE) sequences are very helpful in this regard. Since fat remains bright on FSE T2 sequences, fat saturation can be helpful in eliminating competing fat signal from the image, thereby making the pathology more conspicuous. Spin-echo (SE) T1 weighted gadolinium images and T2 weighted FSE images, both utilizing fat saturation, are the key means of evaluating tumors, as this optimizes tumor conspicuity. For fine detail, thin slices from three to five millimeters may be necessary, but when using thin slices, one may encounter signal drop-off. One trick to maintain signal is to use a larger field of view.

While the axial plane is familiar to most who do CT, the coronal plane is extremely beneficial with MR, as it provides a vertical perspective from as far as the skull base to as low as the thoracic inlet. It is often this plane that is most helpful in evaluating tumor spread along important fascial spaces that are vertically oriented, such as the carotid space, the parapharyngeal space, the retropharyngeal space, and the prevertebral space. The coronal plane also provides side to side comparison, and it can assist in determining midline crossing. Also, the lymph node chains are nicely displayed on coronal images, and the round or oval lymph nodes are easily distinguished from the elongated muscles of the neck. For midline lesions such as those on the epiglottis or those invading the spine, the sagittal plane may be helpful.

Artifact suppression techniques such as flow compensation, cardiac gating, presaturation pulses, gradient moment nulling, no phase wrap can improve image quality. Phase direction should be chosen so that any phase artifact is projected away from the area of interest. Most phase artifact is generated from carotid and jugular veins and if allowed to lay along the transverse plane, there is a good chance that the prevertebral space may be obscured. Since this is a very critical area for staging, we prefer to direct the phase direction along the AP plane.

ANATOMY OF THE LOWER FACE

For purposes of discussion, the lower face will include the oropharynx and the orocavity, the hypopharynx, the adjacent deep structures, and the adjacent cutaneous and subcutaneous structures, particularly the lip.

Oropharynx

The oropharynx includes the posterior third of the tongue (or tongue base), the adjacent pharyngeal walls extending from the soft palate to the epiglottis, the palatine tonsils, the tonsillar fossa, and the vallecula. Because squamous epithelium lines the mucosal surfaces of the oropharynx, squamous cell carcinoma is very popular here. However, in addition to squamous epithelium, there is also lymphoid tissue and minor salivary glands, so that lymphoma and salivary gland tumors can also arise in this area1.

Orocavity

The orocavity is situated anterior to the oropharynx. The orocavity includes the anterior two thirds of the tongue (which is separated from the tongue base by the circumvallate papillae), the hard palate, the mylohyoid (which forms the floor of the mouth) muscle, and the buccal mucosa. Two additional spaces related to the orocavity include the submandibular and sublingual spaces, which are separated by the mylohyoid muscle2.

Hypopharynx

The hypopharynx includes the piriform sinuses, the pharyngeal walls from the pharyngoesophageal junction to the vallecula, and the aryepiglottic folds1-3.

Adjacent Deep Spaces

Because malignant processes tend to invade neighboring spaces, a thorough understanding of the adjacent deep spaces is vitally important in evaluating head and neck pathology4,5,6.

The pharyngeal mucosal space represents the most superficial layer and includes the mucosa of the pharynx, Waldeyer's ring, the cartilaginous eustachian tube, the pharyngobasilar fascia, the levator and constrictor muscles. Common tumors that arise in this space would include squamous cell cancer, lymphoma, adenoid cystic carcinoma, and adenocarcinoma.

The parapharyngeal space is a vertical highway extending from the skull base to the hyoid bone. It contains fat, branches of the trigeminal nerve, and pterygoid veins. Lesions encountered in this space include metastatic carcinoma, especially squamous cell cancer that has penetrated deeply from the pharyngeal mucosal space and squamous cell cancer from the tongue base or tonsillar fossa. Pleomorphic adenomas, branchial cleft cysts, lipomas, and infections can also occur in this space.

Also called the infratemporal fossa, the masticator space contains the muscles of mastication, including the masseter, temporalis, medial and lateral pterygoid muscles. This is another important space by which metastatic lesions, especially from squamous cell cancer, can spread vertically to reach the skull base. One should pay particular attention to tumor invasion intracranially from this space along certain routes that do not require skull base erosion or destruction. These routes include perineural spread along the third division of cranial nerve 57 (the nerve that activates the muscles of mastication) and along the pterygopalatine fossa which leads to the inferior orbital fissure and orbital apex. Common tumors found in this space include deeply invasive metastatic squamous cell carcinoma, adenoid cystic carcinomas, mucoepidermoid carcinomas, lymphomas and rhabdomyosarcomas8.

The carotid space is another vertical highway for neoplasms to extend up to the skull base or down to the aortic arch. The contents of the space include not only the carotid artery, but also the internal jugular vein, the internal jugular chain of nodes, and cranial nerves 9, 10, and 11. Because of nodal drainage, metastasis are frequently found in this space. Metastasis to this space can mean inoperability, particularly if there is carotid encasement that would require sacrifice of the carotid artery. Therefore, the carotid space is an extremely important space in regard to staging.

The retropharyngeal space is a potential posterior midline space which can also provide a vertical highway, extending from the skull base down to approximately the T3 level. Contents of the retropharyngeal space are comprised primarily of lymph nodes. Metastatic lesions, lymphomas, and infections9-11 occur in this region.

Lastly, along with the carotid space, the prevertebral space is another extremely important space in regard to determining operability. The prevertebral space is enveloped by a deep layer of cervical fascia which attaches as far laterally as the transverse process and includes not only the longus colli muscles, but also the paraspinous muscles, the vertebral artery and vein, the spinal cord, and the paraspinous muscles. Metastatic lesions to the prevertebral space can potentially determine inoperability, as the surgeon may be unable to find a tumor free margin once tumor invades into this space. Common tumors found in this space include metastatic lesions, particularly from squamous cell carcinoma. Chordomas can arise in this space. Infections from vertebral osteomyelitis and/or prevertebral abscesses may also be encountered in this region.

Lymph Node Evaluation

Lymph nodes larger than 1.5 centimeters in diameter in the submandibular and/or internal jugular chain (jugulodigastric) should be considered abnormal. Nodes in all other areas of the neck exceeding 1 centimeter in size should also be considered abnormal3,12. Also, benign reactive nodes tend to maintain their normal oval shape, whereas malignant nodes usually have a more rounded configuration. Nodes with necrosis should be considered potentially malignant provided there is no underlying history of tuberculosis and/or previous radiation therapy13. If metastatic disease is confined within a lymph node, a sharp border can be seen between the node and adjacent soft tissues. If the disease breaks out of the nodal capsule, the margins of the node become ill-defined, and there may be infiltration in the surrounding fat planes. This is called extracapsular extension. Some investigators feel that evaluation of lymph nodes for extracapsular and central nodal necrosis is easier with CT than with MR14.

Abnormal supraclavicular lymph nodes may represent metastasis from anywhere, but particularly from lung, breast and esophagus. Abnormal adenopathy along the inferior jugular chain may be due to metastatic disease from the subglottic larynx, esophagus, or thyroid. Tongue, pharynx, and supraglottic laryngeal cancers typically metastasize to the mid-jugular lymph node chain. Jugulodigastric nodes may be related to metastasis from the pharynx, tonsil, tongue, parotid gland, or supraglottic larynx. Submandibular adenopathy suggests metastasis from the adjacent skin (lip), submaxillary gland, or tongue. Posterior triangle nodes may be seen with metastasis from the pharynx, tongue base, tonsil, or thyroid14-20.

PATHOLOGY OF THE LOWER FACE

Malignant Tumors

Squamous cell carcinoma

Squamous epithelium lines the mucosal spaces of the oropharynx, the hypopharynx, and the orocavity. Therefore, squamous cell cancer of the tongue, tonsillar fossa, hypopharynx, and orocavity, including the lip, are very prevalent. In the oropharynx, the most common site of origin of squamous cell carcinoma is the anterior tonsil2. Squamous cell cancer from this site can spread deeply beneath intact mucosa very subtly and silently. It can spread to deep spaces through the superior constrictor muscles into the parapharyngeal and carotid spaces. The internal jugular chain of nodes are often affected, particularly in the jugulodigastric region. Because mucosal lesions are very hard to evaluate on imaging, correlation and knowledge of the physical exam is essential in staging these lesions. Deep lesions, of course, are more accurately evaluated on imaging as opposed to physical examination2,21-23.

The staging of oropharyngeal squamous cell cancer is based upon size of the tumor and invasion of adjacent structures1-3. Therefore, one needs to pay particular attention to tumor extension to deep spaces, especially to the parapharyngeal, carotid, and prevertebral spaces. Evaluation of adenopathy should be made accordingly, particularly in the carotid space.

Tongue base squamous cell cancer can also invade the adjacent deep spaces. Perineural spread is also a possibility. In addition, one should assess for midline crossing, as this can determine whether there can be a hemi rather than total glossectomy. Squamous cell cancer of the tongue base has the propensity to invade inferiorly into submandibular space and extend down to the vallecula and pre-epiglottic space directly. In addition, one should evaluate any oropharyngeal or orocavity cancer for mandibular invasion. Squamous cell cancer of the lip, particularly the lower lip, is the second most common site of squamous cell cancer of the head and neck, the skin being the most common. Squamous cell cancer of the lip can invade the buccal mucosa and the mandible as well as extending into deep spaces2,24.

Squamous cell cancer of the hypopharynx most commonly involves the pyriform sinuses. Cancers that arise in this location tend to be silent and very aggressive. They invade deep spaces early with numerous abnormal nodes. They can invade cartilage and even the larynx. Staging of squamous cell cancer of the hypopharynx is based on location (number of subsites), fixation of the larynx, and invasion of local structures1-3.

Concerning the deep structures, special attention should be made to invasion of the prevertebral space, as this could be an indication of inoperability. Violation of the deep cervical fascia, the longus colli muscles, or the spine itself with bone marrow replacement should be reported. Also, invasion of the carotid artery with encasement would be a prime concern in regard to resectability. Some feel that if tumor involvement of the carotid artery is less than 50%, the tumor likely can be dissected off the carotid. The likelihood of nonresectability increases markedly when the tumor encases more than 75% of the carotid2. Involvement of the masticator space can also occur with oropharyngeal and/or orocavity cancers. Involvement of the masticator space should alert one to the possibility of skull base invasion, particularly through foramen ovale and/or along the pterygopalatine fossa. Other routes of skull base entry are along the carotid canal, jugular foramen, and eustachian tube25-27.

Lymphoma/Leukemia

This malignancy can arise from nodal tissue of the oropharynx, especially in the region of Waldeyer's ring. These lesions are often bulky and also accompanied by large bulky lymph nodes. The most common form of lymphoma in the head and neck is non-Hodgkin's lymphoma. In fact, the head and neck region is the second most common site of non-Hodgkin's lymphoma (the first is the gastrointestinal tract)2. If mediastinal nodes are also present, then one may be dealing with a Hodgkin's lymphoma rather than a non-Hodgkin's lymphoma. Leukemia can present similarly with bulky lymph nodes and enlargement of Waldeyer's ring.

Metastasis

A likely cause of metastatic disease is squamous cell cancer arising at other sites in the head and neck and spreading along the deep spaces such as the carotid or parapharyngeal space, the vertical highways. Popular hematogenous sources, however, would include lung, breast, colon, kidney, and thyroid. Naturally, the job of the diagnostic radiologist is not to try to guess the histology, but rather to determine the extent of involvement.

Adjacent masticator space lesions

Squamous cell cancer can invade deeply from the oropharynx or orocavity and penetrate deeply into the muscles of mastication. This is not at all uncommon. Masses can arise within the masticator space and further invade the region of the oropharynx or orocavity. In children, the rhabdomyosarcoma is the most common soft tissue sarcoma2,3. The head and neck represents the second most common site of rhabdomyosarcomas, as approximately 40% occur in the head and neck. There is propensity for skull base invasion and coronal MR is extremely helpful in this regard. The embryonal type is more commonly found in the head and neck, whereas the alveolar type is more commonly found in skeletal muscle. Other sarcomas that can arise in the masticator space would include liposarcomas which arise from lipoblasts and not pre-existing lipomas. They contain some elements of fat, but also sarcomatous elements. If they have more sarcomatous elements than fatty elements, their signal may not be predominantly that of fat on T1 weighted MR images. Other sarcomas arising in this region may include osteosarcomas and chondrosarcomas. Metastatic lesions can also spread to the masticator space as with other parts of the lower face by hematogenous means25,26.

Mandibular Lesions

Squamous cell cancer of the tongue or tonsillar fossa can invade the mandible. On MR one should look for signal drop-out in the bone marrow on T1 weighted images. High signal may be seen on T1 weighted images with gadolinium and fat saturation.

Many of the primary mandibular lesions are cystic. Many will therefore appear bright on T2 weighted images and dark on T1 weighted images. Some may be hemorrhagic and hyperintense on both T1 and T2 weighted sequences. The most common odontogenic cystic mandibular lesion is the periapical cyst2,28,29. This lesion is associated with an infected tooth and can be located along the mandible or maxilla. It is usually detected on dental films and treated successfully, so it rarely comes to imaging by MR. Another common odontogenic lesion is the dentigerous cyst which is a lytic unilocular lesion of the mandible. It is located adjacent to an unerupted tooth. It has sclerotic borders and may be better evaluated by xray techniques rather than by MR. The ameloblastoma is another common mandibular lesion. This lesion tends to be multiloculated, lytic, and expansile, and the bony cortex may be eroded30,31. It can be associated with an underlying dentigerous cyst. Again, CT or panorex films may be preferable to MR for evaluation of cortical changes.

The malignant fibrous histiocytoma is a lesion that can appear to be similar to the ameloblastoma on panorex or CT in that it is lytic, expansile, and causes cortical erosions. However, there is no underlying cysic characteristic of this lesion, rather it is fibrous. Therefore it can be characterized by MRI which reveals it to be somewhat isodence with muscle on both T1 and T2 weighted imaging.

Of the malignant tumors, metastasis from squamous cell cancer ranks high, particularly if there is an adjacent head and neck cancer. Other sources would include metastasis from other sites such as breast or lung. Osteosarcomas of the mandible can occur, especially following radiation therapy for previous head and neck neoplasms. An aggressive periosteal reaction might be the prime differential finding and this may be better appreciated with CT or panorex rather than MR. However, if it is necessary to define the extent of a soft tissue component related to a destructive mandibular mass, MR can be very helpful. A host of other less common odontogenic and non-odontogenic tumors are often diagnosed satisfactorily by dental panorex and are never subjected to MR for further evaluation.

Benign Cystic Lesions

Ranula

The ranula is a mucous retention cyst secondary to obstruction of a minor salivary gland or the sublingual gland. They represent a unilocular cyst in the sublingual space. One often sees the tail. They are considered simple if they are confined to the sublingual space. If the capsule breaks and there is extension from the sublingual space into the adjacent submandibular or parapharyngeal space, they are considered a diving or plunging ranula2,3,32-34.

Thyroglossal duct cyst

The thyroglossal duct cyst arises from a remnant of the thyroglossal duct which follows a course from the thyroid isthmus to the tongue base. Most of the thyroglossal duct cysts will be in the midline, but approximately 25% may be paramedian. About 20% are suprahyoid in location1-3.

Branchial cleft cyst

Approximately 95% of branchial cleft cysts arise from a remnant of a second branchial cleft which has failed to obliterate. They are usually found at the angle of the mandible along the anterior border of the sternocleidomastoid, but their location can be variable as the course of the second branchial cleft extends from the tonsillar fossa down to the supraclavicular region38. They are usually found in young adults. They often present when they become infected, and at this point they can have the appearance of an abscess. Like other cystic lesions, they tend to be bright on T2 and dark on T1 weighted images. If they are infected, they can have an enhancing rim when contrast is given with T1 weighted imaging.

Cystic hygroma, lymphangioma

Cystic hygromas and lymphangiomas arise from lymphoid tissue. They tend to have cystic signal characteristics and may present as tortuous cystic masses. There can be intratumoral hemorrhage. The posterior triangle tends to be a very common site1-3.

Benign Tumors

Dermoid, epidermoid, teratoma

Differential considerations of cystic lesions in the location of the tongue base and sublingual space should include epidermoid and dermoid tumors. These tumors are slow-growing cystic masses, usually located under the oral tongue. They tend to be unilocular. The dermoids tend to have mixed contents of fat and fluid as well as other substances and tend to be inhomogeneous in signal intensity on all sequences34,35. The epidermoids tend to be cystic and can be very difficult to differentiate from a ranula. However, neither epidermoids or dermoids tend to communicate with the parapharyngeal or sublingual space36.

Teratomas are congenital neoplasms that contain elements of all three germ layers. Therefore, a mixture of hair, bone, cartilage, water, and muscle may be encountered, which can lead to variable appearances, sometimes quite complex and even bizarre.

Lipomas

Lipomas often present as palpable masses. Both MR and CT can be very specific in evaluating the location and extent of these lesions as well as tissue characterize them as fatty tumors very accurately. They are most common in obese females. They have a tendency to occur in the posterior triangle, but can really be present anywhere.

Vascular Lesions

The hemangioma is a very common benign pediatric neoplasm. These often extend from the superficial soft tissues to the deeper underlying tissues. They tend to be bright on T2 and even brighter on longer TE images. When gadolinium is given, they usually enhance very brightly1-3.

Arterial venous malformations are rather uncommon abnormalities in the head and neck, but they can be seen as serpentine tangled flow voids or areas of flow related enhancement due to slow-flowing draining veins. Phase artifact may be a tip off, which can be accentuated with administration of gadolinium.

Infectious/inflammatory Disease

Laryngitis and tonsillitis are the most common inflammatory lesions in the oral pharynx however usually no imaging is necessary as this area is very amenable to direct visualization. However, if tonsillitis goes untreated and becomes complicated by peritonsillar abscess which can spread to the parapharyngeal and lateral retropharyngeal spaces easily, MRI may be necessary to evaluate spread to the deep spaces. In children, a retropharyngeal abscess can develop in response to an underlying inflammation usually from tonsillitis however a traumatic perforation of the posterior pharyngeal wall can also be a cause. Masticator space infections are often due to dental infections that become complicated, but sometimes they can be due to otitis externa. Ludwig's angina which is usually due to complicated streptococcus or staphyloccoccus infections are often of dental origin. This represents an extensive infection of the floor of the mouth and can extend inferiorly along deep spaces to the mediastinum.1-3

On imaging abscesses typically contain fluid centrally with a wall that enhances brightly when contrast is administered. A phlegmon, cellulitis, or fasciitus involves an area of soft tissue enhancement without fluid or gas formation.

In this age of intravenous drug abuse and aids, one may find unusual abscesses often with multiple large nodes, that can easily be mistaken for malignancy with metastatic lymphadenopathy. Clinical history of intravenous drug abuse and in particular attempted needle access to the jugular veins would be helpful in considering a different diagnosis.9-11,13

SALIVARY GLANDS: ANATOMY

The major salivary glands include the parotid, the submandibular, and the sublingual glands. The minor salivary glands are widely distributed over the mucosal surfaces of the orocavity, the oropharynx, and the hypopharynx, and to a lesser extent, the nasopharynx, sinuses, and respiratory tract1-3.

The parotid space contains the parotid gland, the facial nerve, the external carotid artery, and the retromandibular vein. The parotid gland is divided into deep and superficial lobes with the stylomandibular tunnel (which encloses the facial nerve) being the dividing line. Therefore, a portion of the parotid lies superficial to the mandibular ramus, and another portion lies deep. The parotid gland is drained by Stensen's duct which exits adjacent to the second maxillary molar.

The submandibular space contains the submandibular gland, submandibular nodes, a portion of the facial artery and vein, and a small segment of the 12th nerve. The submandibular gland is drained by Wharton's duct which exits at the frenulum at the floor of the mouth39.

The sublingual space is a potential space which does not have a true fascial covering. It is located in the floor of the mouth superior and medial to the mylohyoid muscle. The sublingual gland is the smallest of the major salivary glands and may not be apparent on imaging. Compared with other major salivary glands, the sublingual gland tends to have the fewest lesions associated with it2,40.

SALIVARY GLANDS: PATHOLOGY

Malignant Tumors

Mucoepidermoid carcinoma

Mucoepidermoid carcinomas comprises about 30% of salivary gland malignancies. They are located most commonly in the parotid, and of parotid malignancies, they represent the most common malignancy. They are also the most common pediatric salivary tumor. Their appearance is extremely variable as the neoplasm can vary from high to low grade. The higher grade tumors tend to be extremely aggressive and infiltrating, whereas the low grade tumors may be well encapsulated and appear similar to benign lesions, such as a pleomorphic adenoma or a Warthin's tumor. The prognosis varies with the grade of the tumor41-43.

Adenoid cystic carcinoma

The adenoid cystic carcinomas are the most common salivary tumor of the minor salivary glands and also of the submandibular and sublingual glands. They tend to be slow growing, but extremely persistent. They are known to have a high propensity for perineural spread, and one should look very carefully for signs of perineural spread along the courses of the facial nerve, as well as the third and second divisions of the 5th cranial nerve, particularly on follow-up examinations for known adenoid cystic carcinoma2,41,42.

Squamous cell carcinoma

Squamous cell carcinoma can be found in the salivary glands, often as the result of direct spread from adjacent mucosal neoplasia or from metastasis to lymph nodes in the parotid or submandibular spaces. Therefore, one needs to search for a primary tumor whenever there is a diagnosis of squamous cell cancer in a salivary gland. Infrequently, squamous cell cancer can arise denovo, probably as the result of metaplasia of ductal columnar epithelium within the salivary gland. On imaging, these neoplasms tend to be hypointense on T2, but they do enhance on T1 weighted postgadolinium images with fat suppression1,2,41,44.

Adenocarcinoma, expleomorphic carcinoma

Less commonly, adenocarcinoma involves the glandular tissue of the salivary glands, and it usually carries a bad prognosis. The signal intensity tends to be variable depending on whether the nature of the tumor is solid, mucinous, or cystic. Carcinoma expleomorphic adenoma may be the result of a pleomorphic adenoma that degenerates into a carcinoma. It is estimated that perhaps 20% of pleomorphic adenomas can do this. These malignancies to be very aggressive neoplasms with metastasis to the lungs and adjacent lymph nodes, in addition to very rapid growth of the primary tumor2,3,40,42.

Lymphoma

Non-Hodgkin's lymphoma can affect almost any organ in the head and neck and the salivary glands are not immune. These tumors tend to be quite bulky and are often associated with large bulky homogeneous lymph nodes. They can exhibit very rapid enlargement.

Benign Lesions

Pleomorphic adenoma

The benign mixed cell or pleomorphic adenoma comprises about 80% of parotid neoplasms, representing the most common salivary gland neoplasm. They are usually solid, rounded, well defined masses, most commonly found in the superficial lobe of the parotid. On T1 weighted images, they tend to be low signal relative to the gland, and they are generally high signal on T2 weighted images. There may be some fluid containing spaces within the tumor. Unlike a typical cyst, however, they tend to contrast enhance rather brightly with some heterogeneity. There may be calcification within the tumor. A small number can degenerate into carcinomas.

Pleomorphic adenomas can arise from the deep lobe of the parotid and extend into the parapharyngeal space with an epicenter away from the parotid, but well centered in the parapharyngeal space. At times, these lesions can be difficult to differentiate from other parapharyngeal/carotid space lesions such as schwannomas and glomus tumors. However, lesions that arise from the parotid tend to have a prestyloid location. They are separated from the carotid space by the tensor palattini veli fascia. Therefore, a mass extending into the parapharyngeal space that is of parotid origin will push the carotid artery and jugular vein posteriorly, whereas those that push the carotid anteriorly are more likely to be a schwannoma or glomus tumor2,21,45-48.

Warthin's tumor

Warthin's tumors are often bilateral and multicentric. Their imaging features resemble a pleomorphic adenoma in that they tend to have sharp borders. They also enhance with contrast, however, more centrally and less peripherally1,2. They tend to be more common in males. They are not infiltrative and are usually limited to the gland.

Cystic lesions

Of the cystic lesions, the lymphoepithelial cysts are being seen more and more commonly with the increase in AIDS patients being imaging49. These cysts tend to be multiple and are often located in the parotid gland. They are often associated with multiple cervical lymph nodes. They tend to have sharp borders and appear similar to adenomas on non-contrast MR, but maintain cystic characteristics as they do not enhance1,2,3,50.

The branchial cleft cyst can arise adjacent to the parotid gland, particularly if it is a second branchial cleft cyst, which has a propensity to arising near the angle of the mandible. However, the first branchial cleft cyst is more likely to be found within the parotid, as the remnant first branchial cleft extends from the submandibular triangle to the external auditory canal. First branchial cleft cysts are often located at the inferior aspect of the parotid, either in the superficial or deep lobe. They can also present as a mass at the external auditory canal. If branchial cleft cysts become infected, they can have an enhancing rim and mimic an abscess. They can also mimic a Warthin's tumor or even a mucoepidermoid carcinoma with central necrosis2,3,21,38.

As mentioned previously, potential cystic masses at the floor of the mouth in the vicinity of the sublingual gland and submandibular glands also include ranulas, plunging ranulas, dermoids, epidermoids, and thyroglossal duct cysts. Of these, the thyroglossal duct cyst can be paramedian, but is rarely has any communication with the sublingual or submandibular spaces. Retention cysts and simple congenital cysts may also be found in the salivary glands, that are not associated with the branchial apparatus or other congenital remnants34.

Inflammatory Disease

Inflammatory lesions can also present as cystic abnormalities, including abscesses, some of which may be secondary to obstruction of the gland. Sialoadenitis may be the underlying cause and can present either with large abscesses or multiple microabscesses. Inflammation of the peripheral salivary ducts may be related to an underlying autoimmune condition. An example is Sjogren's disease, which is related to chronic sialoadenitis and leads to a dry mouth as the result of fibrous changes of the salivary glands. It is often related to an underlying collagen vascular disorder such as rheumatoid arthritis or systemic lupus erythematosus.

A sialocele is a collection of saliva that is often located outside of the gland or duct and is caused by leakage as the result of penetrating or blunt trauma.

Salivary gland calculi can cause obstruction of the glands. The diagnosis of calculi is made more easily and more accurately by x-ray techniques, such as CT or panorex, rather than MR1,2,3,21.

POSTSURGICAL/POSTRADIATION CHANGE

Following treatment by surgery and/or radiation therapy, the adjacent soft tissue changes and enhancement patterns can be very confusing. Following radiation, there tends to be a loss of the normal signal in the adjacent subcutaneous fat. Loss of soft tissue planes also occurs, especially around the carotid sheath. There can be thickening of the mucosa, especially at the epiglottis. Postsurgical edema can last for over eight weeks. Realizing that there can be a recurrence of head and neck cancer in up to 90% in the first year, close follow-up, perhaps at six month intervals, is recommended. Some of the key concerns include an increase in the size or bulk of the original tumor mass, perineural spread, bone marrow changes suggesting a loss of bone marrow signal rather than simply radiation change, and increase in nodal size, but not necessarily necrosis, as radiation change can cause nodes to become necrotic in appearance. Any changes that develop beyond the expected time intervals for radiation or surgical changes should draw suspicion, such that either close follow-up or biopsy can be directed to the area of concern2,51,52.

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14. Yossem, D., Som, P.M., Hackney, D.B., et al: Central nodal necrosis and extracapsular neoplastic spread in cervical lymph nodes: MR imaging versus CT. Radiology 182:753-759, 1992.

15. Holliday, R.A.: Neck nodes and masses. ASHNR 26th Annual Conference and Postgraduate Course. May 13-16, 1993:87-98.

16. Steinkamp, H.J., Hosten, N., Richter, C., et al: Enlarged cervical lymph nodes at helical CT. Radiology 191:795-798, 1994.

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22. Hudgins, P.A.: The suprahyoid neck: Normal anatomy. ASNR Core Curriculum Course. May 1-2, 1994:169-173.

23. Mancuso, A.A., Harnsberger, H.R.: Diseases of the suprahyoid neck spaces. ASNR Core Curriculum Course. May 1-2, 1994:175-180.

24. Lufkin, R.B., Wortham, D.G., Dietrich, R.B.: Tongue and ovopharynx: Findings on MR imaging. Radiology 161:69-75, 1986.

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28. Abrahams, J.J.: Mandibular lesions and dental implants. ASHNR 26th Annual Conference and Postgraduate Course. May 13-16, 1993:63-71.

29. Underhill, T.E., Katz, J.O., Pope, T.L., et al: Radiologic findings of diseases involving the maxilla and mandible. AJR 159:345-350, 1992.

30. Minami, M., Kaneda, T., Yamamoto, H.: Ameloblastoma in the maxillo mandibular region: MR imaging. Radiology 184:389-393, 1992.

31. Weissman, J.L., Snyderman, C.H., Yossem, S.A., et al.: Ameloblastoma of the maxilla: CT and MR appearance. AJNR 14:223-226, 1993.

32. Coit, W.E., Harnsberger, H.R., Osborn, A.G., et al.: Ranulas and their mimics: CT evaluation. Radiology 163:211-216, 1987.

33. Batsakis, J.G., McClatcheg, K.D. Cervical ranula. Ann Oto Rhinol Laryngol 97:561-562, 1988.

34. Vogl, T.J., Steger, W., Ihrler, S., et al.: Cystic masses in the floor of the mouth: Value of MR in planning surgery. AJR 161:183-186, 1993.

35. New, G.B., Erich, J.B.: Dermoid cysts of the head and neck. Surg Gynecol Obstet 65:48-55, 1937.

36. Levegue, H., Sarasew, C.A., Tang, C.K.: Dermoid cysts of the floor of the mouth and lateral neck. Laryngoscope 89:296-305, 1979.

37. Miller, M.B., Rao, V.M., Tom, B.M.: Cystic masses of the head and neck: Pitfalls in CT and MR interpretation. AJR 159:601-607, 1992.

38. Salazar, J.E., Duke, R.A., Ellis, J.V.: Second branchial cleft cyst: Unusual location and a new CT diagnostic sign. AJR 145:965-966, 1985.

39. Thibault, F., Halimi, P., Bely, N., et al.: Internal architecture of the parotid gland at MR imaging: Facial nerve or ductal system. Radiology 188:701-704, 1993.

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41. Vogl, T.J., Dressel, S.H., Spath, M., et al.: Parotid gland: Plain and gadolinium enhanced MR imaging. Radiology 177:667-674, 1990.

42. Schlakman, B.N., Yossem, D.M.: MR of intraparotid masses. AJNR 14:1173-1180, 1993.

43. Casselman, J.W., Mancuso, A.A.: Major salivary gland masses: Comparison of MR and CT. Radiology 165:183-189, 1987.

44. Horowitz, S.W., Leonetti, J.P., Azar-kia, B., et al.: CT and MR of temporal bone malignancies primary and secondary to parotid carcinoma. AJNR 15:755-762, 1994.

45. Curtain, H.D.: Separation of the masticator space from the parapharyngeal space. Radiology 163:195-204, 1987.

46. Abramowitz, J., Dion, J.E., Jensen, M.P., et al.: Angiographic diagnosis and management of head and neck schwannomas. AJNR 12:977, 1991.

47. Van Gills, A.P., Vandenberg, R., Falke, T.H.: MR diagnosis of paraganglioma of the head and neck: Value of contrast enhancement. AJR 162:147-153, 1994.

48. Vogl, T., Bruning, R., Schedel, H., et al.: Paragangliomas of jugular bulb and carotid body: MRI imaging with short sequences and Gd DTPA enhancement. AJNR 10:823-827, 1989.

49. Holliday, R.A., Cohen, W.A., Schinella, R.A., et al.: Benign lymphoepithelial parotid cystic and hyperplastic cervical adenopathy in AIDS-risk patients: A new CT appearance. Radiology 168:439-441, 1988.

50. Minarmi, M., Tanioka, H., Oyama, K.: Warthin tumor of the parotid gland: MR-pathologic correlation. AJNR 14:209-214, 1993.

51. Som, P.M., Urken, M.L., Biller, M., et al.: Imaging the post operative neck. Radiology 187:593-601, 1993.

52. Gussack, G.S., Hudgins, P.A.: Imaging in recurrent head and neck tumors. Laryngoscope 101:119-124, 1991.

53. Curtain, H.: MR of the salivary glands. Rad Clinics of N.A. 27(2):381, 1989

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