0:01

Okay, next topic we are gonna discuss is late gadolinium enhancement cardiac

0:06

MRI. Like I mentioned before, late gadolinium enhancement cardiac MRI is

0:10

often the most important sequence that we acquire in our cardiac MRI examination,

0:14

particularly in patients with non ischemic cardiomyopathy. How does it work?

0:19

Well, it turns out that scar and inflammation will enhance on cardiac MRI,

0:24

and this is because in areas with scar and inflammation, there is abnormal

0:29

extracellular volume. So the extracellular volume is increased, and when

0:33

you give gadolinium or any contrast agent really, it works for iodine as

0:37

well, the contrast agent infiltrates the extracellular space. And if you

0:44

wait long enough, the contrast agent will wash in and wash out of

0:48

normal cells, but will hang around in the extracellular space because there

0:52

is no transportation mechanism to remove it from that space. And so what

0:57

happens is we inject the gadolinium, the gadolinium washes into normal tissues

1:01

and washes out of normal tissues, but also washes into areas of scar

1:06

or inflammation, and because of that increased extracellular volume in those

1:11

abnormal areas of myocardium, there really is a very, very slow washout,

1:16

and that leads to enhancement on these delayed enhancement images, and that

1:20

leads to the differentiation between the normal areas and the abnormal areas.

1:25

So what kind of sequence is it? It's ECG gated like every other

1:29

cardiac MRI sequence. It's gradient echo, inversion recovery, so these are

1:34

quick acquisitions. They are acquired roughly 10 20 minutes after the initial

1:40

gadolinium injection. You can even push it, you can go even as early

1:44

as 7 minutes in many cases, particularly if you are giving a lower

1:47

dose, just a single dose of contrast, or 0.1 mmol/kilogram. If you are giving

1:52

a larger dose, generally we tend to wait a little longer.

1:54

When you give a larger dose, you often get better enhancement of those

1:57

subtle abnormalities in the myocardium, particularly in patients with non

2:00

ischemic disease. The problem you run into though is that you also have

2:04

a lot of contrast that's hanging around in the blood pool,

2:07

and so sometimes it can be difficult to differentiate blood pool from a

2:11

small subendocardial infarction, and in those cases, waiting a little bit

2:15

longer, more like 15 or 20 minutes can allow more time for the

2:18

blood pool signal to wash out, and for you to see a little

2:22

bit better definition of the myocardial late enhancement.

2:26

We use inversion, just like with the black blood imaging, we are using

2:29

inversion pulses. In the black blood imaging, we use a double inversion

2:33

pulse or a triple inversion pulse that was used to nullify either blood

2:38

or fat. In this case, we are trying to nullify normal myocardium, and

2:43

so we are gonna select an inversion time that's optimal for nulling normal

2:46

myocardium, and what that's gonna do is it's gonna make normal myocardium

2:50

look really dark, and all the abnormal areas of enhancing myocardium are

2:55

gonna stand out as much as possible. So we are gonna maximize our

2:58

contrast to really get a good visualization of all the areas of scarring

3:03

or inflammation. Typically, the inversion time is 175 250 milliseconds,

3:09

but we do select a specific inversion time per patient. So we modify

3:15

the time, and I will show you the sequence, the inversion recovery sequence

3:18

that we use, to select that inversion time, and the reason we do

3:21

that is because the actual time can vary from patient to patient,

3:24

depending on several factors. One, how long has it been since you injected

3:29

the gadolinium? Two, how much gadolinium did you inject? Three, what's the

3:33

patient's overall blood volume? How big is the patient? Four, what's the

3:39

patient's renal clearance? So, all those different factors contribute to

3:45

the washout of contrast from the myocardium, and therefore

3:49

contribute to the determination of what is the best inversion time

3:54

to null the normal myocardium, and that's why we do it

3:57

on a per patient basis. Okay, so like I mentioned, gadolinium washes out

4:02

slowly from areas of abnormal myocardium, but it's nonspecific. And so what

4:07

I wanna emphasize is that seeing late enhancement does not mean you have

4:11

an infarct, which I think is oftentimes what a lot of people think.

4:14

It can certainly be seen in acute infarction, and that's because necrotic

4:19

myocardial cells, basically they lyse in the setting of ischemia, and then

4:24

you have all this new extracellular fluid volume that is just floating around

4:28

there and all the gadolinium washes in. But there are other reasons to

4:31

get enhancement. Those include scar. So in scar, you are not dealing with

4:35

myonecrosis, but rather you are dealing with the replacement of normal tissue

4:39

with collagen. The collagen and the scar has a lot higher extracellular

4:43

fluid volume than normal tissue. So, you see enhancement both with an acute

4:49

infarct and with a scar, and so it actually becomes tricky.

4:52

You can't really distinguish between acute infarct or scar just on late

4:57

gadolinium enhancement and cardiac MRI. You have to use other findings.

5:00

And so, on our ischemic cardiomyopathy series, we'll talk about a lot of

5:04

that, but you look for clues such as wall thinning, edema,

5:08

and then obviously the patient's history can be really helpful. Tumors.

5:12

Tumors also have late gadolinium enhancement, and that's because they have

5:16

a lot of disorganized vascularity. The extracellular space is increased

5:20

sometimes, and so you can get enhancement in this patient, for instance,

5:24

who has a large melanoma metastasis in the right ventricle. You can see

5:27

a lot of enhancement there. And then inflammation. Myocarditis, sarcoidosis.

5:33

These are all areas where there is more edema in the heart.

5:36

There is expansion of extracellular space, and that can lead to enhancement

5:40

because of all the processes we talked about already.

5:45

Okay, so I mentioned how we use an individualized inversion time for each

5:50

patient, depending on all those various factors that can influence the amount

5:54

of gadolinium within the myocardium, and here is an example of this sequence.

5:59

This is called the TI Scout. TI, as we know, stands for inversion

6:04

time, and so the TI Scout, what it is it's basically a set

6:07

of images that's acquired at different inversion times throughout the cardiac

6:11

cycle. Here is a graphical representation of the TI Scout. You start at

6:17

zero, and then you basically wait various increasing inversion times

6:23

all the way up to approximately 600 milliseconds. And while that's happening,

6:27

your T1 signal is recovering. So imagine this TI Scout is basically multiple

6:33

acquisitions along this T1 recovery line, and what you are looking for is

6:39

the spot where normal myocardium here crosses the zero point and is most

6:44

nulled. So in this case, somewhere around this point here, I see the

6:49

normal myocardium is really dark. So I would look at the image data

6:55

for this particular slice, find the inversion time, and then use that on

6:59

my subsequent late gadolinium enhancement image acquisitions. Okay, on this

7:04

next slide, I am showing differences between both PSIR and Magnitude

7:09

late gadolinium enhancement images. And the left hand slide is what I want

7:14

to call your attention to first. This is kind of the most traditional

7:18

late gadolinium enhancement image, and this is a nice example of what happens

7:21

when you select the wrong inversion time. So if you think about that

7:25

last graph, if you select an inversion time that's very early on the

7:29

recovery curve, you will actually end up with these artifacts where instead

7:33

of having nulled myocardium, you are gonna have myocardium that's kind of

7:36

bright. As you move closer to the correct null point, you'll see that

7:40

you get slightly less bright myocardium. You often get this etching artifact

7:44

around the borders, and then eventually you get a nice dark myocardium and

7:49

kind of a sweet spot right around here in the 250

7:51

300 millisecond range. So the bad thing about the magnitude images is if

7:57

you accidentally put in the wrong inversion time, you may end up with

8:01

images that look like this, which are really non diagnostic. So more recently,

8:06

the PSIR technique was developed. This is a modification of the Magnitude

8:10

technique, basically using various physics tricks, taking the same raw data.

8:15

The scanner is able to produce images like this, which really do a

8:19

really nice job of compensating for any incorrect inversion time selection.

8:24

So PSIR images are much, much more friendly if you accidentally select an

8:29

inversion time that's a little bit too long or a little bit too

8:31

short. You tend to get still really nice nulling of the myocardium in

8:35

any situation. Having said all that, we still like to always acquire TI

8:41

Scout and make sure we get our inversion time

8:43

as optimal as possible, because despite these really perfect images,

8:51

reality can be slightly different. And sometimes even with PSIR images,

8:55

if you select the wrong inversion time, you may get some funny artifacts.

8:58

So it's always good to try and get the optimal inversion time in

9:03

every case.