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 cyst. 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 eroded. 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.