0:01

We have different CT dose descriptors.

0:04

The most common dose descriptor is called computed tomography

0:08

dose index weighted.

0:11

That is given by this formulation.

0:13

You are taking the measurement done on the surface, two-third of the

0:17

surface measurement, add with the one-third of the center

0:22

will give us a weighted average of the computed

0:25

tomography dose index.

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To add to the complexity of this one, if the

0:31

acquisition is done in a helical way, we have to take into account

0:35

the pitch factor.

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And the pitch, as defined earlier, is the table feed per

0:40

rotation divided by the nominal scan width.

0:45

If you take that into account,

0:48

the CTDI weighted was measured from one axial scan.

0:52

You divide that by the pitch used in the clinical image, you get what

0:55

is called as CTDI volume.

0:59

It is an estimate of an average patient dose in one

1:03

single slice. And here is how it varies.

1:07

The other factor in CT is the concept of pitch.

1:11

As I mentioned earlier,

1:13

if the pitch is greater than one,

1:16

there is some anatomical gap, therefore, the radiation dose is

1:20

less because that's our relationship.

1:23

The radiation dose to the patient is inversely proportional to the

1:27

pitch value.

1:28

So you can see here,

1:30

if the pitch is less than one, there is a lot of overlapping, that

1:34

can result in a higher patient dose, which is what we

1:37

saw in a retrospective ECG gating in cardiac

1:41

CT.

1:42

If the pitch is greater than one, it is like this extended

1:46

imaging. You see some gap, and that means it reduces the

1:50

patient dose, but there are some trade-off with respect to axial

1:53

resolution.

1:55

A lot of the body protocol you can do with a pitch greater than

1:59

one because there is not many organs which are moving as fast as the

2:03

heart.

2:04

For only in cardiac CT, the pitch is typically less than

2:08

one if you are acquiring with the retrospective ECG gating.

2:13

Generally, we don't do one slice. We do a length of a scan on a

2:16

patient. To account for that, we use a term

2:20

called dose length product, DLP.

2:24

This represent the total dose in terms of the

2:28

total scan length.

2:30

And this is important for estimating the risk to the patient from

2:33

radiation dose, which we do.

2:36

So related to energy important but more intuitive than use of

2:40

energy, we use DLP is equal to

2:43

CTDI volume expressed in milligray

2:47

multiplied by the scan length expressed in

2:49

centimeter. That concept is

2:52

described distinction here.

2:55

Here are two objects, two patient,

2:58

whereas everything, the scan protocol is exactly same.

3:02

Therefore, the CTDI volume in

3:06

both the patient will be same.

3:09

On the other hand, in this patient had only 10

3:13

slices, the DLP is 20 milligray centimeter.

3:17

Ten times two is 20 milligray centimeter.

3:20

In the second scenario, the length is twice that length,

3:25

therefore, the DLP is 40 centimeter by milligray

3:28

centimeter.

3:30

How does it make a difference? Because here, larger anatomical area

3:34

was exposed, therefore, the DLP in a way

3:38

represent the greater biological risk, indicator of biological

3:42

risk.

3:43

The CT dosimetry can be thought of as this follows.

3:47

One is we started with the phantom and measured CTDI

3:51

100.

3:52

Then you take into account the surface and the edges and

3:56

introduce a term called CTDI weighted.

4:00

You take into account the pitch value, that's when you come what is called a

4:03

CTDI volume.

4:05

Multiply that by the scan length, you get DLP.

4:10

From here, we have lot of approximation models to

4:14

calculate the effective dose required for estimating the

4:17

long-term risk.

4:20

Therefore, as of now, the main CT

4:24

dose descriptors are only two.

4:26

One is computed tomography dose index expressed in milligray,

4:32

and the second one is dose length product expressed as

4:35

DLP expressed in milligray centimeter.