0:00

Now, let me go to the second primary factor, which is

0:04

called the tube voltage.

0:07

The tube voltage is also called as kilo volt, kV.

0:12

Because the potential difference between the anode and the

0:16

cathode, you need some energy to attract the electron to hit at

0:20

certain speed on the anode to create the X-rays.

0:24

I'm going to cover this aspect of fundamental X-ray production in a different

0:27

lecture on fundamental on radiation, radiography, and fluoroscopy.

0:32

We'll examine that over there. So right now, when we say

0:36

tube voltage in CT is all kilo voltage, kV.

0:40

What is tube voltage? This is the potential difference between the

0:44

anode and cathode of an X-ray tube.

0:47

So there is an energy to attract the electron to hit at a certain speed.

0:52

Faster the electron hit the anode,

0:55

the X-rays produced will be at a higher energy.

0:59

Therefore, the tube voltage can be looked as

1:03

to defining the quality of X-rays.

1:08

So the tube voltage will define the quality of X-ray.

1:11

The tube current will define the quantity of X-ray.

1:16

The most commonly used tube voltage in CT

1:20

is 120 kV.

1:22

Why so? Because the energy of the X-ray produced

1:26

by 120 kV is quite sufficient to pass

1:30

through the average size patient or even the thick portion of the

1:34

body and so forth. So historically,

1:38

if we look back even 15 years ago,

1:40

every CTs were done at 120 kV.

1:44

But now,

1:45

because of our understanding and way to optimize the

1:49

doses and image quality, we are now doing in different kV.

1:53

We do at a higher kV for a obese patient, lower kV for

1:58

a thin patient, and we are going down in the kV to

2:01

further optimize the radiation dose.

2:04

Especially these days,

2:07

thin and pediatric patients are done at 100

2:10

kV or 80 kV or lesser. That is the

2:14

landmark. So these kVs are calibrated on

2:18

the CT scanner by the vendor. So there is a way they do the

2:21

calibration, so that the scanner, when it is operating, provides the right

2:25

X-ray energies and so forth.

2:28

Why don't we go beyond 140 kV?

2:31

What happen is like, if the X-rays are done at a higher kV,

2:36

the X-ray energy is so great, they pass through the

2:40

body equally without providing any signal

2:43

differences. Therefore, the image quality will becomes

2:47

almost blurred. There's no difference in the contrast.

2:51

On the other hand, why don't we go very low, below 70, 60 kV?

2:56

If you go too low in a kV, the energy of the

2:59

X-rays become very soft. They just don't get penetrate

3:03

through the body. They just get absorbed on the surface of the body.

3:07

Therefore,

3:08

in diagnostic radiology, we are limited to

3:12

the range of kV we can use.

3:15

Anywhere from 50 or 60 kV to 140 to 150

3:19

kV. That is called the diagnostic energy range,

3:23

and that goes back to the principle of radiography and fluoroscopy

3:26

also. So in CT, the most common kV is

3:30

120 kV, and these are the other kVs calibrate on the

3:34

vendors.

3:35

This is a cathode, which has a filament, heats up.

3:38

That heating up is done by mA, the tube current, and the electron

3:42

produced are then attracted towards the anode by the potential

3:46

difference between these two, and that create the X-rays coming out.

3:50

As you all know, the current X-ray tubes which we use

3:55

is one of the very inefficient X-ray tube because

3:59

99% of heat is generated to produce 1% of

4:03

X-rays. And there are a lot of new technology being developed to improve

4:07

this efficiency, but we are still, most of the X-ray tubes are the same

4:11

principle.

4:12

What does the tube voltage has to do with the patient dose and image quality?

4:16

The tube voltage controls the image contrast

4:20

and also the amount of penetration.

4:23

Means the energy of the X-ray beam will

4:27

determine how much is absorbed, how much is transmitted,

4:30

allowing to develop an image contrast.

4:34

The patient dose increases with the tube voltage.

4:38

However, it is not straightforward like a tube current,

4:42

not linear. It is more like kV square.

4:46

I'm going to go into detail later. So

4:49

decreasing the tube voltage will reduce dose when all the other factors

4:53

constant. So that is where the advantage why we are going down in the tube

4:57

voltage.

4:59

So here is a scenario of the same two simulated images abdomen.

5:03

Now, I'm keeping the tube current same,

5:07

but the tube voltage is changed. Everything else is

5:11

the same. The images are displayed.

5:14

As you can see here, this is a grainier image.

5:17

This is a much more nicer image.

5:20

This was acquired at 135 kV,

5:23

which resulted in a dose of 37 milligray.

5:27

And if you put a region of interest on this area, you get what is called

5:31

as the standard deviation of the CT number, which is an

5:35

indicator of the image noise. That's eight.

5:38

Whereas here, this is done at 120 kV.

5:42

The CT volume is 29 milligray,

5:45

but there is a slight increase in the image noise. The standard deviation is 10.

5:50

One other principle in CT is the way image noise

5:54

is measured.

5:56

There are very sophisticated way to measure the image noise and

6:00

which we use it routinely for all of our research purpose.

6:03

But for simplicity case, if you take a region of

6:07

uniform region, like in the liver, and measure the CT

6:11

number of this ROI,

6:13

it will measure what is called the mean CT number in this particular region of

6:17

interest you drawAnd the standard deviation will

6:21

tell you the quantity of the image noise.

6:24

So the image noise is kind of indicated by the standard

6:29

deviation of a particular region of interest.

6:32

The standard deviation will tell whether it's high or low, whether it's

6:36

noisy or not. The derivation of these two images is like

6:41

a 12% change in tube voltage. There is only 12% change in

6:45

tube voltage,

6:46

led to almost 27% increase in

6:50

dose.

6:52

At the same time, led to 25% decrease in noise

6:56

from 120 to 135.

6:58

So then now question is, do we need

7:02

this amount of decrease in noise or you can save off this

7:06

dose? That is where the trade-off comes into picture.

7:10

So I'm showing here is for a head CT versus a tube body

7:14

CT, simulating values based on a phantom measurement.

7:18

This is a technique used, and here is showing is like for 120

7:22

kV.

7:23

If the CTDI was 50, if I go to

7:27

100, the CTDI value will be 32. That's

7:31

almost decrease of 36% dose reduction in head CT.

7:35

Head CTs are smaller size.

7:38

If you go down to 80 or 70, that number goes on even

7:41

further.

7:43

If I go to 140, that increase the dose by 44%.

7:47

So here in the body CT or a body protocol,

7:51

if it is at 120 kV, the CTDI volume

7:54

is 24 for this particular technique.

7:58

Then by going down in kV, you are saving a dose of

8:01

30%. If you go further down to 80 kV, you save a

8:05

percentage of almost 70% dose.

8:08

And that's where the current status is now with lot of the CT

8:12

protocol optimization to take into account

8:15

how we can optimize the radiation dose to the patient by adjusting

8:19

these parameters. The relationship between the tube

8:23

voltage and the patient dose is very little bit more complex.

8:28

It is tube voltage significantly reduces tube dose

8:31

by kV square.

8:33

It is not kV, but kV square.

8:36

So what I meant to say is, like a small change in the

8:40

tube voltage will have a larger impact in reducing

8:44

or increasing or reducing the radiation dose.

8:47

So you can see it, 140, 120, or 100,

8:51

and so forth. See the amount of radiation dose.

8:53

This decreased by 0.6 millisievert or 0.5

8:57

millisievert here with the 80 kV.

9:00

The most common CT application even today uses 120

9:04

kV.

9:05

However, for thin patient and infants,

9:09

we are now recommending or using 100 kV,

9:13

80 kV, and 70 kV can be used.

9:16

The advantage of using a lower tube voltage, especially

9:20

for CTs done with contrast, because it increases the

9:24

image contrast and also decreases the radiation dose.

9:28

So in a way, one of the few times when we

9:32

find dual advantage is going down to lower

9:36

tube voltage in those protocol with

9:39

contrast will have not only decreased the radiation dose to the

9:43

patient, but you are also going to increase the image contrast.

9:46

This is one of the few times there is hardly any trade-off in going

9:50

in the lower tube voltage.

9:52

At the same time, if the patient is obese or thicker anatomy, it

9:56

is advised to go into higher tube voltage to maintain the image

10:00

quality.