0:01

So, this case nicely illustrates the

0:03

two principles of head-neck.

0:05

When we're looking, especially

0:07

at the posterior fossa,

0:09

the purpose of the imaging is to define the

0:11

anatomy, and eventually, the end

0:13

goal is to see the anatomy.

0:15

So here's an example of a patient that

0:18

if you understand the anatomy,

0:20

you can actually predict why they presented.

0:23

So here's a heavily weighted T2-weighted image.

0:26

And when we look at this heavily

0:28

weighted T2 image,

0:29

what we see is there's a cystic mass that's

0:32

located right here in the left

0:34

cerebellar pontine angle.

0:36

Here is the porus Acusticus and extending

0:39

through the porus Acusticus is

0:41

the 7th-8th nerve complex.

0:42

Now normally, the 7th-8th nerve complex gets from

0:46

point A to point B in the straightest

0:48

and shortest line.

0:50

But look what's happening to the superior

0:53

vestibular nerve. Instead of the superior

0:55

vestibular nerve going straight,

0:57

notice how it is being displaced

1:00

anteriorly by this cystic mass that's

1:02

located in the posterior fossa.

1:05

So we see a cystic mass located

1:07

in the posterior fossa.

1:09

This patient presents with some type of dizziness,

1:11

which we can explain by the distortion in the

1:15

normal course of the superior vestibular nerve.

1:17

And the next thing that we want to be able to do

1:20

is better characterize this on the

1:22

T1 with and without contrast.

1:25

So below is the non-contrast T1-weighted image.

1:28

And we can see that this is a cystic mass.

1:31

Again, from an anatomical standpoint,

1:33

here is the porus Acusticus located here.

1:36

And now we can see anterior displacement of the

1:39

superior vestibular nerve when we give contrast.

1:42

Notice how there is no enhancement whatsoever.

1:45

So now we have to wonder,

1:47

is this an arachnoid cyst or is it an epidermoid?

1:51

Because as we'll see,

1:52

epidermoids don't necessarily

1:54

have to enhance either.

1:56

But what allows us to differentiate

1:58

between the arachnoid

2:00

and the epidermoid is this sequence

2:02

on your top right,

2:03

which is the diffusion-weighted imaging sequence.

2:06

So this DWI sequence obtained at the exact level

2:10

of this T2-weighted image and the contrast-

2:12

enhanced T1-weighted image showed that there's

2:15

no increased diffusion signal on this sequence.

2:18

And the lack of increased signal on the diffusion-

2:21

weighted sequence confirms that this is

2:24

an arachnoid cyst and not an epidermoid.