Nerve sheath tumors

            Tumors of schwann cell origin include schwannoma and neurofibroma. Schwannomas are more common and most arise from the 8th cranial nerve. Neurofibromas are usually associated with neurofibromatosis. Acoustic neuromas originate on the vestibular division of the eighth cranial nerve just within the internal auditory canal. Bilateral lesions are common with NF 2. They usually present in middle-aged adults with a sensorineural hearing loss, but other symptoms include headache, vertigo, tinnitus, unsteady gait, and facial weakness. Large tumors may fill the cerebellopontine angle cistern and compress adjacent brain structures, producing additional symptoms.

            Most schwannomas are isointense to the brain on MR images, but some are distinctly hyperintense with T2-weighted sequences. Occasionally, a schwannoma will be hyperintense on T1-weighted images owing to foci of hemorrhage. They may be heterogeneous on T2-weighted images as well, particularly the larger ones, due to necrosis, hemorrhagic components, and occasional calcification. With small intracanalicular tumors, partial voluming effects may result in uneven signal intensity. Endnote

            Gadolinium causes approximately 50% shortening of the T1 relaxation time of schwannomas, making them appear very bright on T1-weighted images. Those lesions that are heterogeneous on plain scan will likely exhibit heterogeneous enhancement as well. Endnote  

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