|Case 24 :|
female presents with recurrent headaches. No other positive history is elicited.
Clinical examination is normal, with no positive neck signs or any focal
neurological deficit. An MRI of the brain is requested.
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MRI shows an extraaxial mass in the right middle cranial fossa, as evidenced by the CSF sleeve around the mixed intensity mass in the right middle cranial fossa, with vasogenic odema in the temporal lobe. Contrast shows homogenous enhancement with a dural tail that is suggestive of meningioma.
Several criteria help to establish the extracerebral localization of the meningioma, and these criteria represent the key to the diagnosis. A broad, dural-based margin is strongly suggestive, but not definitive for this localization. Bony hyperostosis and/or invasion is highly specific for an extra-axial origin. However, it is infrequently present. Another highly specific characteristic for extra-axial localization is the identification of various anatomic interfaces interposed between the tumor surface and the brain surface. Three different anatomic interfaces may be identified with MRI. These consist of pial vascular structures, CSF clefts, and dural margins. With high-resolution and multiplanar MRI, one or more of these interfaces can usually be identified in essentially all cases. The interfaces are frequently not found along the full tumor-brain margin but in most instances are sufficient to make a highly reliable determination of extra-axial localization.
For those tumors located at the base of the brain or in the proximal portions of the sylvian and interhemispheric cisterns, displaced larger brain arteries are likely to be identified at the interface. Meningiomas tend to develop large marginal draining veins located at their brain surface interface. These veins are most likely the large marginal vascular structures seen with the more peripherally located meningiomas and at tumor-brain interface locations, where large arteries are not expected to be seen. CSF interfaces are also identifiable in about 80% of meningiomas on MRI. Vascular rims may be seen to lie within regions of the CSF clefts. In approximately two-thirds of the meningiomas, vascular rims and CSF clefts are both present at the brain-tumor interface. The dural margin interface is seen primarily in meningiomas of the cavernous sinus. It appears as a low-intensity rim on all imaging sequences covering the lateral margin of the tumor and separating it from the adjacent temporal lobe.
In comparison with
CT, MRI is superior for extra-axial localization of the tumor. CT without
and with contrast can define these interfaces in less than 50% of the
cases in which they are identified on MRI. Contrast enhancement on MRI
is almost always identifiable, even when meningiomas are densely calcified.
Enhancement may either be central or ring-like. This enhancement relates
to the fact that meningioma capillaries have no blood-brain barrier. Intravenous
contrast is useful in demonstrating and defining the borders of symptomatic
small lesions that compress or infiltrate around cortical neural structures
such as the optic nerves and other cranial nerves, which may be occult
on nonenhanced MRI. The anatomic boundaries of larger lesions that may
be isointense to brain are clearly definable on T1-weighted enhanced scans.
The most striking finding of contrast-enhanced MRI in meningiomas is the
finding of dural enhancement adjacent to the lesion. En plaque meningiomas
as well as globular convexity and basal meningiomas may infiltrate adjacent
dural surfaces for several centimeters. Recognition of this infiltration
can be of significant importance in surgical planning for complete tumor
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Dr. Ashok Raghavan, Manipal Hospital, Bangalore