CT and MR Features of the Intracranial Schwannomas

PURPOSE: To evaluate CT and MR findings of the intracranial schwannomas arising from variable cranial nerves. MATERIALS AND METHODS: The authors retrospectively analyzed CT (n=21) and MR(n=15) findings of 24 cases in 23 patients(M : 7, F : 16) who had suffered from surgically-proven intracranial schwannomas over the previous fiveyears. RESULTS: Schwannomas arose from the acoustic nerve(n=18), the trigeminal nerve(n=2), the glossopha-ryngeal-vagal-accessory nerve complex (n=2), and the olfactory nerve(n=1). Intracranial schwannomas were welldefined, lobulated and inhomogeneously or homogeneously enhancing masses on CT and MR, and were located along the course of the specific cranial nerve. Acoustic schwannomas involved both the internal auditory canal(IAC) and the cerebellopontine angle(CPA) in 14 cases, the IAC in three, and the CPA in two. Two trigeminal schwannomas involved both middle and posterior cranial fossa and were in the shape of a dumbbell. One of the two schwannomas that invelved lower cranial nerve complex(9-11th) was located in the medullary cistern and jugular foramen ; the other was located in the central posterior cranial fossa. A case of olfactory schwannoma was located in the right cribriform plate. The precontrast CT scan showed low density in 13 cases(62%), isodensity in seven(33%) and highdensity in one(5%). On postcontrast CT scan, enhancement was seen in 20 cases(95%). Of the 15 cases with MR, 12had low signal intensity on T1 weighted image and 14 had high signal intensity on T2 weighted image. MR imaging after Gd-DTPA infusion showed enhancement in 14 cases. Enhancement was inhomogeneous in 14 cases on CT and in 13 on MR. Of 24 cases, intratumoral necrosis was seen in 19, ring enhancement in five and severe cystic change inone. Other findings were intratumoral calcification(21%), hemorrhage(8%), pressure bony erosion(70.8%), midline shift(58%), peritumoral edema(29%) and hydrocephalus(33%). On MR, there was in all 15 cases a peritumoral lowsignal intensity rim on T1- and T2-weighted images and on a T1 weighted image following gadolium infusion. A caseof olfactory groove schwannoma was associated with neurofibromatosis type I and a case of bilateral acoustic schwannoma with neurofibromatosis II. CONCLUSION: Schwannomas can be easily diagnosed when a well defined, lobulated and inhomogeneously enhancing mass with intratumoral necrosis, cystic change, calcification orhemorrhage is seen along the course of a cranial nerve. Peritumoral low signal intensity rim on MR may be helpful in differentiating intracranial schwannomas from other tumors.