0:00

Let's look a couple of trade-off very important in understanding some of these

0:04

factor.

0:05

So the effect of slice collimation on

0:09

spatial resolution and image quality.

0:12

I just want to let you know, this is assuming all the factors are the same.

0:16

Only one factor is changed, that's the slice thickness in the Z direction.

0:20

The same abdominal image here, acquired and reconstructed

0:24

0.625 and double the size, double the slice

0:28

thickness, and double the slice thickness.

0:30

So if you look in here, gradually, the primary

0:34

dose to the patient is the same. Now you are reconstructing to all

0:38

four different panel. Adjusting CT acquisition

0:42

parameter to reading image collimation is one way to help

0:46

target protocol and reduce dose. So you acquired a thinner slice,

0:50

and whatever you can, then you can reconstruct into thick slice to image

0:54

improve the contrast resolution.

0:56

So you have a very high spatial resolution, but the image quality can

1:00

be improved by simply adding the slice thicknesses.

1:05

This is another trade-off. This is the effect of tube

1:08

voltage on dose, image quality, and

1:12

contrast. So here is the study which we did in the

1:15

past, acquired at 100 kV tube voltage,

1:19

and here is the axial image and the coronal image.

1:23

And you can see here, this is done at 120

1:27

kV and coronal with contrast. See the difference between 120

1:31

and 100.

1:32

Between these two, the radiation dose is less with 100 kV,

1:36

but the coronal contrast resolution can be jeopardized.

1:40

Therefore,

1:41

to irregularly enhancing mass in the rectum is more

1:45

conspicuous due to lower tube voltage here in this area.

1:49

So at that time, you may have to increase the tube voltage to trade off.

1:53

This is with respect to iterative reconstruction.

1:56

Iterative reconstruction was done back in early days into

2:00

2006 onwards to keep the radiation dose

2:04

as low as possible. However, to mathematically do some

2:08

iterative iteration on the image reconstruction to improve the

2:12

image noise and reduce the image noise.

2:15

If you do too much mathematical averaging, the image can look

2:18

really, really like a phantom image and a plastic image.

2:22

So there is a trade-off between. Here is an image with a very

2:25

noisy image acquired at a certain

2:28

technique that on applied

2:31

iterative reconstruction was slightly about 30.

2:34

There are different vendors use different nomenclature.

2:38

This is one vendor utilizing what is called as adaptive statistical

2:42

iterative reconstruction. Using that model of the

2:46

strength of 30%, it slightly improves the image noise.

2:50

But if you go to higher and higher, you can beginning to see more like

2:54

a plastic image. So what is the effect of iterative

2:57

reconstruction? Image noise decreases with higher

3:01

percentage of iterative reconstruction used, but there's also the

3:04

effect of smoothing artifact.

3:07

So now I want to show you a series of trade-off.

3:10

One is with respect to tube current.

3:13

If the tube current is high,

3:16

you have less image noise,

3:18

better contrast resolution, and higher patient dose.

3:22

On the other hand, if the tube current is low, you have higher image

3:26

noise, lower patient dose, and decreased contrast

3:30

resolution.

3:31

As a clinician, one needs to pick where you want to go at a high tube

3:35

current or a low tube current, understanding this

3:39

in fact, which is a trade-off.

3:41

The trade-off with respect to tube voltage is as follows.

3:44

If the tube voltage is high, you have greater penetration.

3:48

It can also result in higher dose, and this is

3:52

usually desirable in obese patient to improve the signal-to-noise

3:56

ratio. Because if you don't increase the tube work voltage, the

4:00

scanner can only go up in tube current up to one point beyond

4:04

which it cannot exceed, therefore, image noise will

4:08

become predominantly very significant.

4:11

At that instance, for obese patient, it advised to increase the tube

4:14

voltage to 140 instead of 120.

4:18

If you go down on the low tube voltage, the contrast in the

4:22

image increases. There is a less patient dose,

4:26

and especially this is a desired point to select

4:30

for tube voltage for thin and pediatric subjects.

4:34

The trade-off with respect to pitch.

4:37

Pitch was defined earlier as the ratio of the table

4:41

travel to the total beam width. If the pitch is

4:45

higher, greater than one, imagine a Slinky, pull the

4:49

Slinky out, that's a pitch of greater than one, which means there are

4:53

some gap between the object, and that will result in a lower

4:57

patient dose. This will also allow faster scan time.

5:01

However, if you stretch the Slinky too much, the error in the

5:05

reconstruction can appear that can impact the image resolution and image

5:09

quality. So there is a range of pitch you can increase,

5:13

but anything greater than one pitch results in a lower

5:16

patient dose and faster scan time.

5:20

A pitch less than one is higher patient dose and

5:24

also less spiral artifact, and that's typically done

5:28

with cardiac CT we're going to study later, where if

5:32

you acquired a very low dose, very low pitch means you are

5:36

overlapping the same anatomy that can increase the patient dose.

5:41

The next trade-off is the slice thickness.

5:44

In the slice thickness, there are two ways.

5:47

If the slice thickness is large, means like five millimeter, 10 millimeter slice,

5:52

it is less noisier because there are more photon are

5:54

contributing. However, it leads to a poor spatial

5:58

resolution. It can improve this contrast resolution, higher

6:02

contrast resolution, but the spatial resolution is jeopardized.

6:06

That's where the trade-off. If on the other end, the slice thickness is very

6:10

thin, small, it has higher spatial resolution in

6:14

the Z direction and provide lower contrast

6:17

resolution, but also results in a greater noise.

6:21

So there is a trade-off between these two.