Meningiomas account for 15% of all intracranial tumors and are the most common extra-axial tumor. They originate from the dura or arachnoid and occur in middle-aged adults. Women are affected twice as often as men. Meningiomas are well-differentiated, benign, and encapsulated lesions that indent the brain as they enlarge. They grow slowly and may be present for many years before producing symptoms. The histologic picture shows cells of uniform size that tend to form whorls or psammoma bodies.

            The parasagittal region is the most frequent site for meningiomas, followed by the sphenoid wings, parasellar region, olfactory groove, cerebello-pontine angle, and rarely the intraventricular region. Meningiomas often induce an osteoblastic reaction in the adjacent bone, resulting in a characteristic focal hyperostosis. They are also hypervascular, receiving their blood supply predominantly from dural vessels.

            Most meningiomas are isointense with cortex on T1- and T2-weighted images. A heterogeneous internal texture is found in all but the smallest meningiomas. The mottled pattern is likely due to a combination of flow void from vascularity, focal calcification, small cystic foci, and entrapped CSF spaces. Hemorrhage is not a common feature. An interface between the brain and lesion is often present, representing a CSF cleft, a vascular rim, or a dural margin. MR has special advantages over CT in assessing venous sinus involvement and arterial encasement. Occasionally, a densely calcified meningioma is encountered that is distinctly hypointense on all pulse sequences.

            Meningiomas show intense enhancement with gadolinium and are sharply circumscribed. Endnote They have a characteristic broad base of attachment against a dural surface. Associated hyperostosis may result in thickening of low signal bone as well as diminished signal from the diploic spaces. Although meningiomas are not invasive, vasogenic edema is present in the adjacent brain in 30% of cases. Contrast scans are especially helpful for imaging the en plaque meningiomas that occur at the skull base. MR spectroscopy shows elevated alanine and glutamates, no NAA, and markedly decreased creatine. 

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