0:01

In this lecture, I will talk about how

0:04

coronary stents are imaged, particularly

0:07

the nuances, uh, for imaging stents.

0:12

Now, coronary stents are controversial,

0:15

but for the purposes of images, that's

0:19

not so important to, um, understand.

0:23

What is important to understand is that, uh, though

0:26

they come in all shapes and sizes, uh, two things.

0:30

Firstly, size is important.

0:32

If the stent is less than three millimeters,

0:34

it becomes very difficult imaging it.

0:36

If the stent is greater than three millimeters,

0:38

our chances of imaging it are greater.

0:42

Stents have different types of material,

0:46

some of which can barely be seen on CT.

0:51

So clearly, if that's the case,

0:53

then that makes life easier.

0:55

But I'm talking about stents that have

0:58

a great deal of metallic component.

1:01

And here are some examples.

1:03

I know examples of the quality.

1:05

You can see that three very different stents over here.

1:08

The one on the left, A.

1:10

The quality is good, you can see inside the stent,

1:14

the one in B, you can kind of see inside the stent,

1:16

but the one in C, you know, all bets are off.

1:19

So the marker of stent imaging is being able to

1:23

see through that stent strut, inside the stent.

1:29

And when done properly, you can have an image as

1:32

nice as this, which carries multiple stents and

1:36

you can see the opacification, it doesn't change

1:38

very much prior to the stent and going into the

1:42

stent, and you can see with great confidence.

1:44

The stent is patent.

1:49

In other scenarios, you have a situation where though

1:55

the image quality isn't great, you could still say

1:58

with some degree of confidence, as in this case where

2:01

inside the stent you really see a very low attenuation

2:04

area, which is, uh, suggestive of stent occlusion.

2:08

But stents have problems and the three

2:12

problems with the stents all arise

2:14

with the fact that the stent has metal.

2:18

And because the stent has

2:19

metal, it creates an artifact.

2:22

The metal, what it does, it reduces the

2:26

transmission of X-rays so that only those X

2:30

rays that are really strong can go through

2:33

the metal, and that's called beam hardening.

2:37

And beam hardening means that only the strongest

2:40

of the X-ray beams can get through the metal.

2:43

And once the strongest of the X-ray

2:44

beams goes through the metal, it gives

2:46

a false sense of a lower attenuation.

2:52

Metal can bloom, meaning that it can look larger than

2:56

it actually is, and the net effect of all of this is

3:00

that the stenosis gets inflated, which means that our

3:06

specificity for detecting critical stenosis is reduced.

3:12

So how do we reduce the artifact?

3:14

Any consideration of stent imaging, you have to think

3:17

very much about the nuance of the stent imaging.

3:21

Well, first and foremost, every good

3:24

practice for cardiac imaging must

3:27

be adhered to when there's a stent.

3:31

So we must control the heart rate to preferably

3:35

less than 70, particularly if there's a stent,

3:39

because whatever suboptimality that occurs

3:43

in a normal CT will just get exaggerated

3:45

when there's a stent.

3:46

So first and foremost, you want to have good contrast

3:50

dynamics, make sure that you have heart rate control.

3:55

Other things that one needs to

3:56

think about, one is raising the KVP.

3:59

KVP is often set as a default in many places.

4:04

And that default may be 100 KVP, which

4:08

is the default for vascular imaging.

4:10

So you have to raise that to 120 KVP.

4:15

We obviously need to use thin sections.

4:17

This is for

4:18

metallic stuff anywhere.

4:21

However, for coronary imaging, we are

4:24

going about as thin as we can, and I don't

4:27

think that's going to go much thinner.

4:30

So that leaves two methods.

4:33

One is using a sharper kernel, and the

4:35

other one is iterative reconstruction.

4:37

So that's within your control, the sharper kernel.

4:41

The iterative reconstruction actually

4:44

depends on the, um, CT platform.

4:48

So before the acquisition has been made,

4:51

be careful to make sure the KVP is 120.

4:55

After the images have been acquired, then

4:57

ensure that you're using the appropriate kernel.

5:00

So what is a sharp and what is a medium kernel?

5:04

A kernel is a kind of reconstruction algorithm that

5:07

is a trade-off between two types of resolution.

5:10

One is the soft tissue resolution, the ability

5:12

to tell different tissues apart, different

5:15

tissues of similar attenuation apart.

5:18

And the second thing is spatial

5:20

resolution, particularly edge definition.

5:23

And it would seem that in coronary imaging, soft tissue

5:28

resolution is more important, and it is, but it comes

5:32

at a penalty that you don't get the edge definition.

5:35

And as these things are almost always

5:36

opposing forces, sometimes you get too much

5:39

soft tissue contrast without enough edge.

5:43

So here are three images.

5:45

Image A shows you a reconstruction with

5:49

a soft tissue kernel, image B shows you a

5:52

reconstruction with a sharp kernel, you can

5:56

instantly see the difference between the two.

5:58

It's very important seeing the edge of the stent,

6:01

because if you don't see the edge of the stent,

6:03

how will you know where the lumen begins and ends?

6:06

And if you don't know where the lumen begins and

6:07

ends, how do we know what's stenosis, what's not?

6:10

So a sharp kernel instantly

6:12

gives you that edge definition.

6:14

Image C is actually a sharp kernel

6:17

bolstered by iterative reconstruction.

6:20

That bolstering has made a little bit of

6:23

an effect, but I'd say that the biggest

6:24

effect came from going from A to B.

6:27

Another set of images.

6:28

Again, the purpose here is to show you

6:31

the incremental gains going from soft

6:35

tissue, which is A, to sharp, which is C.

6:42

D is sharp plus iterative

6:44

reconstruction, which is the best.

6:47

And in D you really can see through the stent.

6:50

You can see through the stent in C as well.

6:52

You can do a soft tissue plus iterative

6:54

reconstruction, which is B, but that's not going

6:56

to give you as much as the sharp is.

6:59

So two methods.

7:02

Used concurrently, are more powerful than used singly.

7:07

But of them, the sharp reconstruction

7:09

is the most important one.

7:12

And, um, often you can get a sense

7:16

that there is a high-grade stenosis.

7:19

If you look at A, you'll see that the top half where

7:22

the arrows are pointing is of a lower attenuation.

7:25

The bottom of B, by virtue of simply that you can

7:27

infer that there is a, because there's

7:30

a gradient, there's an attenuation

7:32

gradient that there's a high-grade stenosis.

7:35

But it's made much clearer in C and

7:39

confirmed by a catheter angiogram.

7:41

Thank you.