0:00

Now to some technical aspects of cardiac

0:04

CT physics, notably kilovoltage, or kVp.

0:10

What is kVp?

0:12

It is a part of the strength of the

0:17

x-ray spectrum, the peak energy of it.

0:22

It determines dose, just as the milliamperes

0:26

does, except it determines dose

0:31

quadratically, meaning small changes

0:35

in kVp can lead to dramatic changes in dose.

0:39

And the default for most imaging is

0:42

often set at 120; for vascular, it's set at 100.

0:46

And so, I'm going to discuss the

0:48

significance of this entity in coronary imaging.

0:53

As I'm going to do that, I also want to show you

0:54

this particular graph, which on the y-axis, you see

0:58

the linear attenuation coefficient, which is this.

1:02

A log scale of the amount of x-ray attenuation and

1:07

by different tissues, and on the x-axis, you can

1:10

see the x-ray energy, and what you'll see in this

1:15

is that for most tissues, soft tissue, fat, there

1:23

isn't much difference between the attenuations.

1:27

In the typical energy range that we use.

1:31

So the typical energy range of CT

1:35

tissues aren't easily distinguishable.

1:38

And you know that from looking at the unenhanced

1:40

scans, how difficult it can be to distinguish tissues.

1:43

And that's why we give contrast, because the moment

1:47

we give contrast, you can see what happens is this

1:50

little peak and things start appearing different.

1:53

So the whole idea of giving contrast

1:56

is to make tissues appear different.

1:59

And it does so because the availability of iodine

2:02

changes the attenuation coefficient dramatically.

2:08

So, there are two elements of a kVp.

2:11

A kVp is kilovoltage; it gives you

2:14

the photons, the energy, increases the

2:19

average energy and the peak energy.

2:22

And a high kVp reduces the artifacts from

2:26

calcium and metal, artifacts that can

2:30

cause there to be blooming artifacts.

2:35

And a high kVp increases the number of photons

2:41

that go through and reduces the artifact.

2:44

That comes at a penalty.

2:45

First penalty is that you have increased dose.

2:51

And the second penalty is that you

2:53

have a slight reduction in contrast.

2:56

This is a little counterintuitive, but a

3:00

low kVp, what it does, it increases the

3:05

attenuation value from iodinated contrast.

3:10

We normally think about higher kVp

3:12

increasing attenuation values because

3:14

it increases the energy of the photons.

3:17

But here's a bit of a flip over here

3:19

where it's the low kVp that increases the

3:22

attenuation value from iodinated contrast.

3:25

And why it does that is because the lower the

3:28

kVp, the more chances you have of incurring

3:34

what's known as the photoelectric effect.

3:37

So you don't have to rely on, um, the two

3:40

types of, uh, methods of getting x-rays.

3:47

One is a photoelectric effect and the other one

3:49

is a Compton scatter. The photoelectric effect is very

3:52

powerful, a lot more energy in it, but you really

3:55

need to approach what's known as the K-edge energy.

3:58

So, the closer the kVp is to the

4:01

K-edge—so 80 is closer than 120—

4:05

um, the more bang for the iodinated buck you'll get.

4:09

Now, all of this is automated, so now scanners

4:11

can actually decide what the optimal kVp is.

4:14

But in the event that you can't,

4:16

then think about the patient.

4:17

If the patient is young and you want

4:20

to minimize radiation dose, then you

4:23

can choose a lower kVp, an 80 or a 70.

4:27

And if the patient has a stent, has metal, has

4:30

calcium, then go towards the higher kVp, 120 kVp.

4:35

So aside from the reduction of the radiation

4:38

dose, which is really quite a quadratic

4:40

reduction, so going from 100 to 80 kVp

4:45

reduces radiation dose by 50 percent.

4:48

Aside from that, the other thing you can do is

4:50

reduce the contrast volume that you can give.

4:53

So let's say that the standard protocol

4:55

is to give 100 mL of iodinated

4:57

contrast of a certain density like 317.

5:03

If you lower the kVp to 80 or 70, you can get away

5:07

with giving 50 mL, and that's obviously valuable

5:09

in patients who have renal impairment because of

5:12

the possibility of contrast-induced nephropathy.

5:16

So does it make a difference?

5:17

No.

5:17

It makes a huge difference.

5:20

I should say it makes a small difference,

5:22

which makes a large difference.

5:23

The top images you can see were both

5:27

taken of the same anatomical part.

5:30

Everything is the same.

5:31

The patient's the same.

5:32

One is 100 kVp, one is 120.

5:36

Um, A is brighter than B, so it

5:39

won't surprise you that A is 100 kVp.

5:42

The bottom image shows the, um, curved planar

5:46

reformation of the RCA taken at 100 kVp, um, 80 kVp.

5:54

80 kVp is on the left, 100 kVp is on the right.

5:58

So you can see the difference it makes.

6:01

Uh, here is a more systematic analysis of the dose

6:04

and contrast volume, and you'll see that as the

6:09

kVp reduces, let's say from 120 to 80, dramatic

6:13

reduction in the dose, quite a huge reduction.

6:17

And the other thing to notice is, as the kVp

6:21

reduces, the volume of contrast also goes down.

6:25

So it's one of the few win-win situations we have in

6:28

imaging, this is one of them, where low kVp,

6:31

as long as it's arterial imaging, does two things.

6:35

It lowers the contrast volume needed,

6:38

and it reduces the dose as well.

6:41

When it comes to penalties, there's more

6:42

noise, and obviously if you have calcium

6:46

or stents, then there's a problem because

6:48

you're going to get beam hardening from it.

6:51

Thank you.