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I'm here with my colleague, Dr. Ben Laser,

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and he has emphasized repeatedly the importance of

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locating a mass, not just where it is in the brain,

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upper, lower, supratentorial, infratentorial,

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but most importantly,

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what compartment is the lesion in?

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Is it extra-axial or intra-axial?

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So, let's have a look. We've got a mass.

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This mass is extra-axial.

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I've got some little dots here,

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which represent the arachnoid capsules,

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which is where meningiomas arise.

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And we've got the extra-axial space with some CSF.

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Now, this gray area is CSF,

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but also the black area is CSF,

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meant to delimit the CSF

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with the folds of the dura in it.

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So the dura is displaced,

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and within those dural folds

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are displaced CSF vessels.

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So just for purposes of education,

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if I was to say, "Well, water is blue,"

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well, this is water too.

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The black area as well.

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I've just drawn it a little bit differently because

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you've got some folds of dura in there also.

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So, let's look at some of the signs that we would use

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to isolate a mass as being extra-axial.

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First, an extra-axial lesion

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will push this gray cortex inward.

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Never will the cortex go all the way around

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the lesion circumferentially.

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So this is known as the cortical

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and white matter buckle sign.

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Then we've also got expansion or widening

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of the subarachnoid space.

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This widening may make a thin slit between the lesion

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and the brain parenchyma,

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consisting of cerebrospinal fluid.

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I'm going to put a little blue on there

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just to emphasize that it's water.

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And this is known as the hyperintense on

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T2 or water-weighted imaging cleft sign.

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With expansion of the subarachnoid space,

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you'll also get a little meniscus of

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water signal around the lesion,

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and this is known as the meniscus sign.

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Now, unlike a parenchymal lesion,

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which may efface brain parenchyma,

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an extra-axial lesion is not going to

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interfere with brain parenchyma,

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and you're going to have preservation

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of brain boundaries.

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And then finally,

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the vessels in an extra-axial lesion

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are going to be pushed in.

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So, we've got several signs that all of you residents

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and fellows, and young attendings

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can use to decide that a lesion is extra-axial.

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You've got the meniscus sign,

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you've got the cleft sign,

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you've got the displaced vessel sign,

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and you've got the gray and white matter inward buckle

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sign to tell you you've got an extra-axial lesion,

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which denotes frequently that you have a meningioma.