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This is a patient who had right-sided hearing loss and

0:05

had an MRI scan to search for a vestibular schwannoma.

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However, on the non-contrast T1-weighted scan,

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what was discovered was this bright signal

0:17

intensity at the petrous apex,

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which was, in point of fact, encroaching

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on the internal carotid artery,

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which was being displaced laterally and anteriorly.

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As we look at this T1-weighted scan,

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we might say, "Well, it doesn't look the same."

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On the contralateral side,

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endpoint effect is not uncommon to have fat

0:41

in your petrous apex if it is not aerated.

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About 30% of people have aeration of their petrous apex.

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The remainder, 70%,

0:52

have fat bone marrow at the petrous apex.

0:55

So on the left side, this is fat.

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On the right side,

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this is an abnormally expanded petrous apex with bright signal

1:04

intensity. Now, how can I prove that this is indeed fat?

1:08

Well,

1:08

one way of doing is to look at a fat-suppressed sequence.

1:11

So this is a FLAIR scan in which the subcutaneous

1:15

fat has been suppressed.

1:17

And when you look at the petrous apex on the left side,

1:21

where I contended there was fat,

1:24

you notice that there is suppression of signal

1:27

of that petrous apex fat.

1:30

However, the lesion,

1:33

which is the cholesterol granuloma,

1:36

does not get suppressed on the fat-sat FLAIR scan.

1:40

On the T2-weighted scan,

1:43

you can see that this lesion has

1:45

heterogeneous signal intensity.

1:47

You can imagine that the bone has been remodeled here

1:52

and expanded. And you have to ask yourself, "Well,

1:55

where is the internal carotid artery?"

1:57

So if we follow the internal carotid artery,

2:01

we see it's being displaced posteriorly and then laterally

2:05

here and then connects up with the cavernous carotid

2:08

artery. Why is it important to identify this?

2:13

Well,

2:13

you can have a pseudoaneurysm of the petrous internal

2:17

carotid artery, which could look like blood products

2:20

similar to a cholesterol granuloma.

2:22

So it's not unusual when looking at a cholesterol

2:25

granuloma that we perform an MRA at the same

2:29

time to identify where the vessel is.

2:31

And the same thing is true with the CT scan.

2:33

We may also do a CTA just to make sure that this

2:36

represents indeed a cholesterol granuloma,

2:39

hemorrhage, and high protein content

2:41

as opposed to an aneurysm.

2:44

Now,

2:44

the differential diagnosis here includes

2:46

a Petrous apex mucocoele.

2:49

What's the difference between the petrous apex

2:51

mucocoele and a cholesterol granuloma?

2:53

So, a mucocoele in general has homogeneous

2:58

signal intensity because

3:00

the content of the mucocoele is uniform

3:04

with respect to the protein content.

3:07

So the mucocoele is bright on T1-weighted scan because of

3:10

high protein, whereas the cholesterol

3:13

granuloma is bright on T1-weighted scan.

3:15

It's scan because of hemorrhagic products

3:17

and this foreign body reaction.

3:19

When you have hemorrhagic products, it's much more

3:21

likely to look heterogeneous like this.

3:25

Whereas the mucocoeles on the T2-weighted scan

3:28

are homogeneous in their signal intensity.

3:31

And that's how we would make the differential diagnosis.

3:35

Both of them are lytic on CT,

3:38

both of them expand the petrous apex.

3:41

The difference is that on T2-weight scanning, the

3:44

cholesterol granuloma is heterogeneous because

3:47

of the variety of blood products.

3:49

The other thing that we see is that if you were to do a

3:53

post-gad scan here and not have the pre-gad T1-weighted scan,

3:58

there would be virtually no way to know whether

4:01

this is non-enhancing material.

4:03

So thank goodness we have the pre-gad T1 showing it's

4:06

bright and the post-gad. This is not enhancement.

4:09

This is the intrinsic high signal intensity on

4:12

T1-weighted scan of the cholesterol granuloma.

4:15

Now, with respect to the mucocoele,

4:17

by and large, most mucocoeles have enhancement of the

4:21

mucosa which is inflamed around the expansion.

4:25

Whereas, as you see with the cholesterol granuloma, you

4:28

don't see that fine line of enhancement

4:31

that you do in the mucosa of mucocoeles.