Specific Imaging Findings

Cranial nerve (CN) schwannoma (neuroma, neurinoma, Schwann cell tumor, neurolemmoma, neurilemmoma) is typically welldelineated, hypodense on CT and T1 hypointense to isointense to the pons on pre-contrast images and with avid post-contrast enhancement. Schwannomas can arise along any cranial nerve,most commonly the vestibular nerve in the internal auditory canal (IAC), followed by the trigeminal nerve (including Meckel's cave), facial nerve, glossopharyngeal, vagus, and spinal accessory nerve. Schwannomas involving oculomotor, trochlear, abducens, and hypoglossal nerves are rare. Enhancement may be homogenous or heterogenous, based on the absence or presence of necrosis and/or intramural cysts. Calcifications are not present. Smaller tumors are usually round, ovoid-shaped or less frequently poly-lobular, T2 iso- to hypointense. Larger tumors are typically of high T2 signal and heterogenous with internal cystic areas. Schwannomas exhibit increased diffusion and are hyperintense on ADC maps. Eighty-five percent of vestibular schwannomas are centered at the IAC meatus. Larger tumors show the typical “ice creamon cone” appearance and usually lead to smooth widening of the internal auditory canal. Arachnoid cysts/trapped CSF are found around the tumor in a minority of cases, while increased signal within the ipsilateral inner ear structures is frequently present on FLAIR images.