0:01

In most institutions, there is ready access to CT

0:06

scanning, and therefore, CT, CT angiography, and

0:11

potentially CT perfusion are the means by which

0:14

most patients get evaluated in the setting of a new

0:18

neurological deficit and the BAT (Brain Attack Team) for brain attack.

0:23

However, if there is an MRI scan available in

0:28

the emergency room or readily available as part

0:30

of the analysis, then you may be requested to

0:34

perform MRI, MRA, and MR perfusion as part of the

0:40

analysis of the patient with a new neurological deficit.

0:44

Johns Hopkins, where I work,

0:46

has an emergency room MRI scanner.

0:50

And therefore, we have a rapid protocol for

0:53

evaluating patients who have a potential stroke.

0:56

Here was a patient who had a left hemiparesis.

1:01

Obviously, the first sequence that we generally

1:03

will look at on the MRI scan is going to be the

1:07

diffusion-weighted scan. Diffusion-weighted imaging,

1:10

as I've said before, is the gold standard

1:13

for imaging for analysis of a stroke.

1:16

Here is a patient who has a diffusion-weighted image.

1:20

This is the B1000 composite image.

1:23

And what we are seeing on this study are high signal

1:28

intensity areas representing acute infarction that are

1:33

within the right middle cerebral artery distribution.

1:36

Now, you see that these are scattered areas

1:39

that don't include the entire

1:42

middle cerebral artery distribution

1:45

but there are areas nonetheless of infarction.

1:48

When one sees high signal intensity on the

1:51

diffusion-weighted imaging, you have to look at

1:53

the ADC map to ensure that these represent areas

1:58

of infarction and not areas of T2 shine-through.

2:03

Because diffusion-weighted imaging is a T2-weighted

2:06

pulse sequence, there are times when vasogenic

2:09

edema, which is bright on T2-weighted imaging, will

2:13

look like bright on the diffusion-weighted imaging,

2:17

which is designed to look for cytotoxic edema.

2:20

So how do we make that distinction?

2:23

Well, what we have to do is

2:24

have a side-by-side analysis

2:27

of the diffusion-weighted image as well as the ADC map.

2:33

The ADC map stands for Apparent Diffusion Coefficient,

2:37

and when we look at the ADC map, it allows us to

2:40

eliminate the possibility of T2 shine-through.

2:44

On an ADC map, low signal corresponds to

2:48

greater restriction of diffusion, which

2:51

is what is present in cytotoxic edema.

2:54

In vasogenic edema, there is increased diffusion,

3:00

and therefore, bright signal on the ADC map.

3:03

So on this ADC map, we see this dark area, darker

3:07

than the neighboring tissue, and we see that

3:10

it corresponds with the bright area on the diffusion-

3:13

weighted imaging, identifying this as representing

3:16

cytotoxic edema, and therefore, acute stroke,

3:20

as opposed to the potential for vasogenic edema.

3:24

There is another area I see here, in the

3:28

sub-insular region, which is dark, showing low ADC.

3:33

This corresponds with the bright area here on the DWI.

3:37

The other area here in the posterior

3:39

temporal lobe, probably in Wernicke's

3:42

area, corresponds to the darker area here.

3:45

So indeed, this patient has an acute stroke.

3:51

So, we've now identified the stroke. As I said in

3:54

our previous snippet, that we want to make sure that

3:57

there is no hemorrhage, so we would go ahead to

3:59

the scans that were done for looking at hemorrhagic

4:03

products—the susceptibility-weighted scans—and

4:07

identify whether or not there was any hemorrhage.

4:09

In this case, there was none.

4:11

This is the gradient echo

4:13

scan, also looking for hemorrhage, and no evidence

4:18

of hemorrhage in the areas of infarction.

4:21

The MRA would be the next pulse sequence to look at.

4:27

Again, I would have called the clinician immediately

4:31

after the DWI sequence that I looked at to let

4:35

them know about infarction because, again, time is brain.

4:39

On this collapsed image from the MRA,

4:43

you can see an occluded M1 segment.

4:46

Let me show you that on the

4:51

MIP images.

4:52

On these MIP images, we have our right internal carotid

4:55

artery, we have our left internal carotid artery,

4:57

we have the right vertebral artery, we have the left

5:00

vertebral artery, and we have the basilar artery.

5:03

We immediately see that there is an occlusion of

5:06

the M1 segment of the right middle cerebral artery.

5:11

And if we scroll along here, we see some other

5:14

elements of atherosclerotic change at the middle

5:17

cerebral artery bifurcation on the left side.

5:20

We see some areas where there is diminished flow

5:23

signal in the cavernous carotid artery that also

5:27

is likely secondary to an area of narrowing.

5:31

And just some irregular blood vessels going out there.

5:35

So, this patient has atherosclerotic

5:38

change in the anterior cerebral artery,

5:41

middle cerebral artery branches,

5:43

as well as this occluded M1 segment.

5:46

So, the question becomes, alright, is it worth going out

5:50

and trying to remove that clot in the right M1 segment?

5:54

For that, we want to see the perfusion imaging.

5:57

The perfusion imaging will tell us whether there's

5:59

tissue at risk or whether whatever has been infarcted

6:03

is already done. So, this is one of the parameters

6:08

for perfusion imaging—time to peak.

6:13

This is mean transit time, this is cerebral

6:18

blood flow, relative cerebral blood flow.

6:22

This is analogous to the cerebral

6:24

blood flow image on the CTA.

6:27

This is cerebral blood volume.

6:31

This is Tmax, and this is K2.

6:35

So, for perfusion imaging on MRA, the best

6:42

thing that we have for detecting a stroke

6:45

is not the perfusion, and we've got the DWI.

6:48

So, the DWI has told us how

6:50

much of the brain is infarcted.

6:51

Let's pull that down.

6:53

So, we're going back to the DWI sequence, and

6:57

these are the areas of infarction.

7:00

So, now let's look at the Tmax,

7:03

and what we see is there is a large area

7:07

of tissue at risk for infarction within this

7:13

right middle cerebral artery distribution.

7:16

And this is also the other parameter

7:19

that we look at that's analogous to Tmax

7:21

on MRI, MRA, and MRP, is time to peak.

7:26

And this shows that there is a large amount

7:28

of tissue at risk in this individual.

7:31

They've only lost this amount.

7:34

We need to go in and take that thrombus out

7:37

in order to save this large amount of

7:41

right frontal, temporal,

7:42

and even parietal lobe that is at risk.

7:46

So, this is the value of MRI, MRA,

7:53

as well as MR perfusion in a patient with a

7:56

right middle cerebral artery occlusion.