Interactive Transcript
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This is an MRI scan
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in a seven-year-old child with headache.
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And at the level of the body
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of the lateral ventricles,
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there's normal symmetry between both cerebral
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hemispheres, and everything looks okay.
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If we go inferiorly,
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we see this prominent area of bright
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signal on T2-weighted imaging.
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If I go anteriorly on the coronal STIR,
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we also see the anterior aspect of the left
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middle cranial fossa is filled with fluid
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and is also actually,
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if we look at the transverse dimension of the
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anterior aspect of the left
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middle cranial fossa,
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it's slightly larger than the anterior aspect
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of the right middle cranial fossa.
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Enlargement of an area
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is typically a sign of a chronic process.
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If we look at FLAIR imaging,
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we can see that the area that was bright
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on T2-weighted imaging is dark.
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This confirms that we have pure fluid signal.
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On T1-weighted imaging,
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we can see the posterior margin
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of this fluid collection.
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There's brain parenchyma with gray matter.
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This is important because if there was a prior
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stroke involving the anterior aspect
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of the left temporal lobe,
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then we would not have a margin of
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gray matter at the interface.
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The gray matter would have been injured,
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and you would go straight to white matter.
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So this is a sign that this is not
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an acquired injury to the brain.
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Diffusion-weighted imaging shows that this area
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has no restricted diffusion.
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At this point,
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we can be fairly confident that this finding in
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the anterior aspect of the left middle
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cranial fossa is an arachnoid cyst.
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It is bright on T2-weighted imaging,
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it shows suppression on FLAIR imaging,
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it shows absence of diffusion restriction,
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and it does not...
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we do not see any signs of injury
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to the subjacent brain.
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In contrast,
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if we saw bright signal
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on diffusion-weighted imaging,
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that would be suggestive of
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this being an epidermoid.
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Diffusion is always important to look
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at on a suspected arachnoid cyst
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because an epidermoid cyst can look like
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an arachnoid cyst on regular imaging,
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it can be bright on T2-weighted imaging,
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and it can actually suppress on FLAIR imaging.
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While there is no signs of injury
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to the left temporal lobe,
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the space-occupying aspect of this arachnoid
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cyst does result in slight hypoplasia
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of the left temporal pole.
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It also results in slight bony expansion of
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the anterior aspect of the cranial fossa.
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We can see on the right,
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a normal greater sphenoid wing.
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We can see the contour along the anterior aspect
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of the middle cranial fossa.
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On the left side,
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this is bowed anteriorly,
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in contrast to the margins of the right greater
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sphenoid wing.
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The left looks smaller.
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That's because there's been bony remodeling from
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chronic pressure from this arachnoid cyst.
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This is chronic low pressure, not high pressure.
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This is important to notice because these findings,
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if they can confirm an arachnoid cyst,
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indicate this is an incidental finding.
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Arachnoid cysts very rarely cause symptoms
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and very rarely require intervention.
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