0:01

How does Coronary CT compare to other modalities for

0:05

radiation and what can we do to reduce radiation dose?

0:09

I want to start talking about radiation, the units.

0:13

The units used are known as sieverts,

0:17

and they're actually a measure; it's a weighted

0:19

measure of the effective radiation dose.

0:22

Weighted because different parts of the body have

0:26

different radiosensitivities and you don't want

0:29

to just give the dose in terms of what's being

0:31

given, but in a sense of what's being received.

0:35

The received dose depends on the radio

0:38

sensitivity, so it's weighted to make it

0:41

comparable across modalities because there are

0:45

some modalities which radiate the bone,

0:49

some which radiate the abdomen, some which radiate

0:51

the neck, so you want a uniform comparison.

0:56

So how does it look?

0:57

The background radiation is 3 millisieverts,

1:00

a chest X-ray, which is very commonly done, is 0.04.

1:02

21 00:01:03,705 --> 00:01:07,765 And coronary CT has been quoted as being 8 to 13.

1:08

And I think these are slightly old figures.

1:10

They're still the ones that I use more often,

1:13

but I'm going to show you how that number has been

1:17

drastically reduced through multiple mechanisms.

1:22

And it's more important for us to understand

1:23

those mechanisms and understand how we

1:25

can reduce the radiation dose further.

1:28

So two basic concepts are

1:31

that of the tube current and the peak voltage; they're

1:34

the ones that produce photons, and then those photons

1:41

generated go through the body and hit the detector.

1:43

So you have three components: you have the generation

1:48

of the photons from the X-ray tube, you have

1:52

the attenuation, the body stopping the photons,

1:55

and then you have the detectors

1:56

being scintillated by the photons.

1:58

A simplistic way of looking at it is that the

2:02

tube current determines the number of photons,

2:05

the KVP determines the strength of the photons.

2:08

So if you have an increase in your tube current,

2:11

you'll have more photons and more radiation dose.

2:13

If you have an increase in your KVP, you'll

2:16

have more photons that go through; they're

2:18

stronger and also increase the radiation dose.

2:23

So how do we reduce the dose?

2:25

I'd say that there are multiple methods

2:28

and the most important, of course, has

2:30

nothing to do with the acquisition.

2:31

It's to make sure that the study is appropriate

2:35

because the best way of reducing radiation is to

2:37

make sure that the study done is clinically indicated

2:41

and the information would change the management.

2:44

That said, once the study has been

2:47

done, think about the field of view.

2:49

If you're only interested in the coronary arteries, you

2:51

might want to just start at the main pulmonary artery.

2:53

If you're interested in the whole chest, then obviously

2:55

go further up, but don't just routinely go further up.

2:59

Make some effort in understanding what the need is.

3:03

An underutilized method is the KVP, which

3:05

I'll get to on a couple of occasions.

3:08

Lower the KVP, lower the radiation dose, and

3:12

it's a quadratic relationship, not just a linear.

3:15

But lower the KVP, higher the iodine

3:19

attenuation, and higher the vascular contrast.

3:24

There are two types of scanning:

3:25

retrospective and prospective.

3:28

With regularity of heart rates, improved

3:31

detector width, and newer scanners,

3:35

more often we can do prospective.

3:37

We don't need to go to retrospective.

3:39

But should you do retrospective, then there is the

3:43

option of tube current modulation, where the tube

3:48

current, which is on throughout the cardiac cycle,

3:53

is dramatically reduced during the points where

3:57

imaging is likely to be noisy because it's

4:02

not diastole; so in systole, the tube current goes

4:06

down dramatically, sometimes 20%, sometimes 2%.

4:11

High-pitch imaging is another

4:12

method, which I will talk about.

4:14

It's basically using the latest technology to provide

4:19

ECG-synchronized acquisitions in one heartbeat,

4:23

taking advantage of two tubes at right angles.

4:27

And then finally, there are reconstruction

4:29

techniques, such as iterative reconstruction,

4:31

which also slightly reduce the radiation dose.

4:34

So, many of these can reduce radiation

4:37

dose by a fair amount, like 40%.

4:42

For example, lowering the KVP

4:45

or tube current modulation.

4:47

So the end result can be quite

4:49

a bit of a dramatic difference.

4:50

And right now, with the latest scanners, with

4:53

attention to the patient details, with, um,

4:57

tailoring the KVP to the patient's weight, you can

5:00

get the dose down to less than 2 millisieverts.

5:06

Reducing the KVP from 100 to 80 reduces

5:08

the dose by 40%, because KVP is quadratic.

5:12

The noise is proportional to multiple

5:16

things, including the milliamperes.

5:20

But here we don't see a constant relationship, because

5:23

if you double the MAS, the KVP, you only increase

5:26

the signal-to-noise ratio by 40%, so you don't have

5:28

the same luxury as you do with reducing the KVP.

5:33

Sometimes you do need to increase the MAS,

5:36

but just bear in mind that the gains aren't

5:38

as much as the gains from reducing KVP.

5:44

Thank you.