0:00

So what is contrast resolution?

0:03

Contrast resolution is the ability to differentiate objects

0:07

of different intensity. See, the CT has such a high

0:11

contrast resolution because it can distinguish between the different

0:15

objects because you are scanning through a very thin part of the object.

0:19

For that, we use the same phantom, which has an embedded

0:23

material of certain dimension and certain resolution

0:27

capability. So in this object, a 25 millimeter

0:31

object is shown here, low contrast object compared to the background.

0:35

And then there is four, five millimeter and six millimeter

0:39

objects,

0:40

four, three and two millimeter sized objects are embedded.

0:44

And if you can see the smallest one easily distinguishable compared to the

0:48

background, that is the limiting capability of the contrast resolution.

0:53

And again, contrast resolution is affected by the tube current,

0:57

tube voltage, pixel size, slice thickness, and

1:01

reconstruction algorithm. Again, I want to emphasize this.

1:05

These factors are influenced by the secondary factor because the raw

1:09

data is already applied and now you can either affect the contrast

1:12

resolution by either making it into thin slices or thick slices and so

1:16

forth.

1:18

So here is this object. So this is a low contrast object,

1:22

which has these size objects six, five, four, three and two

1:26

millimeter size object of varying contrast resolution.

1:30

In this clinical image, scanned image, you can see resolved only this much.

1:34

The object in the four millimeter you are not able to resolve, so the limiting

1:38

factors goes up to five millimeter object you can resolve, and

1:42

that's how we quantify the capability of the contrast

1:45

resolution.

1:47

There is also another way we calculate is by putting the region of

1:50

interest on the object and the adjacent and see the signal

1:54

difference. The signal difference will give you the actual contrast

1:58

resolution.

2:00

The contrast resolution is also impacted by the amount of radiation

2:03

dose, which means amount of X-ray photon contribute.

2:08

So here in these two panels, what radiation dose is

2:12

necessary to visualize an object with certainty?

2:16

For example, if this image was obtained at 0.1 milligray

2:20

of dose, you can see here object of this size can resolve.

2:25

It can begin to lose object of this because this

2:28

object will become merged with the background noise, so you are

2:32

not able to visualize.

2:35

On the other hand, if you increase the dose, more X-ray

2:39

photons will contribute to the image.

2:41

You can begin to see these objects showing up,

2:45

and that's almost four times more dose, but here is the

2:48

visibility. So there is a trade-off between what you want to

2:52

see and how much patient dose can be done.

2:55

So that is the relationship between the radiation dose and contrast

2:58

resolution.

3:00

Shown here is two extreme end. One image obtained at

3:04

800 mAs, the other image obtained at 100 mAs.

3:09

Hundred mAs means there's more image noise, therefore, there is lot

3:13

of background rise that will automatically merge

3:16

these low contrast object of large size.

3:20

Whereas if the dose is higher, then you can begin to

3:24

see this object very clearly. That's where the trade-off

3:28

between contrast resolution and the mAs and patient

3:32

dose vary. So the decreasing tube current

3:36

will increase image noise, therefore, decrease contrast resolution.

3:41

The second thing is if you reconstruct the slice into more thicker part,

3:45

that can improve contrast resolution, but that will degrade the spatial

3:49

resolution.

3:52

So as the patient size increases, contrast

3:56

resolution will decrease for the same technique.

3:59

So here is a five millimeter slice, and this is

4:02

0.625 millimeter slice. In this particular slice, there is

4:06

less photons that are contributing to the image, and that's why it's

4:10

noisier and you can't see the image with

4:13

every other factor keeping same.

4:16

Whereas now you can reconstruct into thick slice and immediately you can see this

4:20

object. So what we do normally is to trade off between

4:25

contrast resolution and spatial resolution.

4:27

We acquire the clinical images in a very

4:31

thinnest slice possible. That is determined by the

4:35

thinnest size of the detector in the Z direction.

4:39

So once you have acquired the data in a thin slice, you can

4:43

always reconstruct into thick slice to improve the contrast

4:47

resolution. However, the caveat is you can't go

4:51

back. You can't acquire a thick slice and reconstruct into thin

4:55

slice.