MRI AND MENINGIOMAS OF THE POSTERIOR CERE BRAL FOSSA - 31 CASES

To assess the value of MRI for meningioma of the posterior cerebral fo ssa, in correlation with surgical and pathological findings, we retros pectively reviewed 31 cases. The patients (24 females and 6 males rang ing in age from 25 to 79 years) were preoperatively studied on a 1,5 T MR imager (GEMS Signal) between july 1989 and november 1993. The prot ocol included: 1. MR scan with axial sections in T2-weighted spin-echo sequence (3 mm thickness), T1-weighted spin-echo sequence before and after gadolinium injection (3-5 mm thickness), coronal and sagittal T1 -weighted sections performed after injection. 2. Surgery reports. 3. H istopathological reports; the predominant histological subtype of each tumor was graded according to the classification scheme of Russel and Rubinstein.We focused on five items: 1. The site of the dural attachm ent of the meningioma. 2. Tumoral extensions (to the tentorium, to the jugular foramen, to the internal auditory canal). 3. The meningioma s ignal in T1- and T2-weighted sequence using the same visual scoring sy stem for grading signal intensities as Elster and al. 4. Secondary fea tures (necrosis, cysts, calcifications) within the tumor. 5. Interface between meningioma and encephalic structures. Meningiomas arose from the posterior surface of the petrous bone in 74 % of the cases and fro m the clivus in 9,6 %. Meningiomas were bulky at the time of diagnosis as since tumoral arrow overtook 2 cm in 64,5 % of the cases. Surgical approach was guided by an anatomo-radiologic classification based on the exact site of tumoral dural attachement. This determination relied on: 1. Osseous reaction noted in 58 % of the cases (enostosic spur in 19 %, localized osseous thickening in 16 %). 2. The trigeminal nerve displacement by the tumor; in case of clival meningioma extended to th e petrous apex, this nerve is displaced outside; otherwise, meningioma of the petrous bone extended to the clivus displaced the trigeminal n erve inside. 3. Radiate structure within tumor converging to vascular basal pole of the meningioma noted in 42 % of the cases. Tentorial inv olvement remained a difficult diagnosis on MR images. It was affirmed when the tumor extended on the opposing surface of the tentorium and w hen focal hypersignal existed through the usual tentorial hyposignal o n T2-weighted images and T1-weighted images after gadolinium. On the o ther hand, tentorial linear dural enhancement adjacent to the tumor wa s not a reliable sign (error in 15,8 % of the predicted cases). The me ningothelial (syncitial) type was noted in 67,7 % of the cases. On T2- weighted images, the hyperintensity of tumoral signal relative to the cortical gray matter correlated with the meningothelial subtype (p < 0 ,001). Study of the interface between tumor edge and encephalic struct ures revealed 13 distinct features: 1. pools of trapped cerebrospinal fluid surrounding meningioma in 58 % of the cases. 2. curved strip iso intense with cerebrospinal fluid in T1- and T2-weighted images but enh anced after gadolinium suggestive of an <<arachnoid felting>> surround ing the tumor (in 13 % of the cases). 3. pial thickening facing the tu mor as a thin enhanced strip along the surface of encephalic structure s. The diagnosis of posterior cerebral fossa meningioma rests on morph ological features (wide base of dural attachment, osseous reaction on the site of dural attachment). This morphological status and the secon dary features (necrosis, cyst, radiate intratumoral structure) were be tter defined on the thin T2-weighted sections. The exact location of m eningioma - in particular on the posterior face of the petrous bone or on the clivus - is the most important response to give to the neurosu rgeon who chooses specific surgical approach to reach the tumor throug h the petrous bone.