0:00

This is an MRI scan

0:02

in a seven-year-old child with headache.

0:05

And at the level of the body

0:07

of the lateral ventricles,

0:09

there's normal symmetry between both cerebral

0:11

hemispheres, and everything looks okay.

0:14

If we go inferiorly,

0:15

we see this prominent area of bright

0:18

signal on T2-weighted imaging.

0:19

If I go anteriorly on the coronal STIR,

0:22

we also see the anterior aspect of the left

0:25

middle cranial fossa is filled with fluid

0:28

and is also actually,

0:29

if we look at the transverse dimension of the

0:32

anterior aspect of the left

0:33

middle cranial fossa,

0:35

it's slightly larger than the anterior aspect

0:37

of the right middle cranial fossa.

0:39

Enlargement of an area

0:40

is typically a sign of a chronic process.

0:44

If we look at FLAIR imaging,

0:46

we can see that the area that was bright

0:49

on T2-weighted imaging is dark.

0:52

This confirms that we have pure fluid signal.

0:55

On T1-weighted imaging,

0:57

we can see the posterior margin

0:59

of this fluid collection.

1:01

There's brain parenchyma with gray matter.

1:04

This is important because if there was a prior

1:07

stroke involving the anterior aspect

1:08

of the left temporal lobe,

1:10

then we would not have a margin of

1:13

gray matter at the interface.

1:15

The gray matter would have been injured,

1:17

and you would go straight to white matter.

1:18

So this is a sign that this is not

1:21

an acquired injury to the brain.

1:23

Diffusion-weighted imaging shows that this area

1:27

has no restricted diffusion.

1:29

At this point,

1:30

we can be fairly confident that this finding in

1:33

the anterior aspect of the left middle

1:34

cranial fossa is an arachnoid cyst.

1:37

It is bright on T2-weighted imaging,

1:39

it shows suppression on FLAIR imaging,

1:41

it shows absence of diffusion restriction,

1:44

and it does not...

1:46

we do not see any signs of injury

1:49

to the subjacent brain.

1:51

In contrast,

1:52

if we saw bright signal

1:54

on diffusion-weighted imaging,

1:55

that would be suggestive of

1:57

this being an epidermoid.

1:59

Diffusion is always important to look

2:01

at on a suspected arachnoid cyst

2:04

because an epidermoid cyst can look like

2:07

an arachnoid cyst on regular imaging,

2:09

it can be bright on T2-weighted imaging,

2:11

and it can actually suppress on FLAIR imaging.

2:13

While there is no signs of injury

2:15

to the left temporal lobe,

2:17

the space-occupying aspect of this arachnoid

2:20

cyst does result in slight hypoplasia

2:24

of the left temporal pole.

2:25

It also results in slight bony expansion of

2:29

the anterior aspect of the cranial fossa.

2:32

We can see on the right,

2:34

a normal greater sphenoid wing.

2:36

We can see the contour along the anterior aspect

2:39

of the middle cranial fossa.

2:41

On the left side,

2:42

this is bowed anteriorly,

2:44

in contrast to the margins of the right greater

2:47

sphenoid wing.

2:48

The left looks smaller.

2:51

That's because there's been bony remodeling from

2:53

chronic pressure from this arachnoid cyst.

2:55

This is chronic low pressure, not high pressure.

2:59

This is important to notice because these findings,

3:02

if they can confirm an arachnoid cyst,

3:05

indicate this is an incidental finding.

3:07

Arachnoid cysts very rarely cause symptoms

3:11

and very rarely require intervention.