0:00

The fourth primary factor is pitch.

0:04

The concept of pitch came only after the introduction of

0:08

helical CT scanner or the spiral CT scanner,

0:13

and that is defined as the following.

0:16

This is the table. The patient is moved in the CT table,

0:20

I is called the table feed per rotation, how much

0:24

table is transported through the gantry.

0:28

While this is the X-ray tube, the side view of the X-ray tube and the

0:31

multi-detector CT,

0:33

the pitch is defined as the following.

0:36

Pitch is the ratio of the table feed per

0:39

rotation divided by the X-ray beam width.

0:45

When I say X-ray beam width, if it is a single detector, that is the

0:49

dimension of the detector.

0:50

With the multiple row detector, the width is basically a

0:54

product of the number of detectors multiplied by the

0:58

active channel. That's called the size of each detector,

1:02

multiplying the number of channels we are using, and that gives the beam

1:06

width. Make sense? Now, the radiation dose

1:10

to the patient goes inversely as the pitch value.

1:15

Radiation dose is directly proportional to mAs,

1:18

but is inversely proportional to pitch.

1:22

How do we picture this in mind? Here's an example.

1:26

When pitch is equal to one means the table is

1:30

translated

1:32

in such a speed such that the beam width is

1:36

almost same as the table speed,

1:39

which means it's kind of blocking a Lego box one after the other

1:43

and there's no gap in this one. Imagine a Slinky,

1:47

and if the Slinky is collapsed, brought back to come tightly, there is

1:51

no air gap in between. That is a pitch of one.

1:54

Means the table movement is same as the beam width,

1:59

so every anatomy is covered. That's a pitch of one.

2:03

A pitch greater than one means

2:06

you are moving faster to the table, faster

2:09

than the beam width,

2:12

which means here we have certain gaps developed here,

2:17

and the pitch greater than one means you are extending the

2:20

imaging with reduced patient dose because there are

2:24

certain part of the anatomy is not exposed to radiation dose,

2:28

therefore, you are able to reduce the radiation dose, and

2:32

that's why this pitch value greater than one means the patient

2:36

dose will be less.

2:39

The other aspect in when the Slinky is collapsed

2:43

means if the table is traveling very slowly

2:47

while the beam is larger, then you have lot of overlap in the

2:51

anatomy. That implies a pitch less than one.

2:55

If the pitch is less than one means the dose will be higher,

2:59

and there are scenarios which I'm going to discuss later in cardiac CT,

3:04

where we use a very low pitch value, therefore, the

3:07

patient doses can be higher. That happen because of the

3:11

overlapping and higher patient dose.

3:15

Bringing back to the same simulation here,

3:18

we are now keeping all the techniques same except the

3:22

pitch factor.

3:23

And the pitch factor is changed, its pitch is

3:27

0.64, pitch is 0.83, pitch is

3:30

0.148.

3:32

If we look this one, compared to the CTDI value of 37

3:36

milligray,

3:37

a lower pitch means usually it's a higher value of DC

3:41

and radiation dose.

3:43

That's about 30% higher.

3:46

Whereas a pitch greater than 1.1, which here is

3:49

1.48, the CTDI is now 45%

3:53

lower.

3:55

Now, the question comes how high pitch you can go,

4:00

and there are certain limitation.

4:02

If you open up the Slinky too much, the interpolating

4:06

the data from adjacent area becomes incorrect.

4:10

So it depends on the vendor. Usually, you can go up to

4:14

1.5 pitch value. Beyond that, the error

4:18

creeps in, the axial resolution goes down, therefore, it is not

4:21

advised use.

4:23

So in which clinical scenario you use a pitch greater than one

4:28

or a pitch less than one?

4:31

For typically for most abdominal CT scans,

4:35

where you have the lesser movement of the abdomen and

4:39

everything, we can get away with a pitch greater than one,

4:42

therefore, we can reduce the radiation dose to the patient.

4:46

On the other hand, for cardiac CT, where the

4:50

heart is beating very fast, you tend to do a pitch

4:54

less than one,

4:55

therefore, the retrospective ECG gating techniques tend to

4:59

result in a higher radiation dose to the patient.

5:03

Typically, for a head CT, even though sometime we use a pitch greater

5:07

than one, generally use the pitch of one because of the dense

5:11

skull involved, and most vendor prefer to keep it

5:15

a pitch of equal to one.

5:17

This affects number of things. Pitch will affect the total

5:21

scan time. If it is greater than one, you're going to be covering a larger area.

5:25

It has an impact on the radiation dose.

5:29

It also has an impact on noise resolution.

5:32

Here is a phantom image of showing low contrast resolution.

5:37

If this is a pitch of 0.56, you can see this six millimeters

5:41

object, five millimeter object, and even in fact four millimeter object,

5:45

and if you squint the eyes, you can even see the three millimeter object.

5:49

Now, with the pitch greater than one, which means less

5:53

X-ray photons are contributing the image, you can immediately see the

5:57

object becomes diminishes. The noise dominates,

6:01

therefore, you can hardly see the six millimeter or the five millimeter,

6:05

definitely not the three or smaller than that.

6:08

This is a concept of pitch.

6:10

There is a caveat to the pitch concept.

6:13

Two vendors, Siemens and Philips, uses a concept called

6:17

effective mAs,

6:19

which means they account the pitch inside,

6:23

therefore, if we change the pitch value, the scanner

6:27

will automatically adjust the mA or the scan time

6:31

to get the same effective mAs.

6:34

So

6:36

in this particular vendor's concepts here, the change in the pitch

6:40

does not have any impact on the radiation dose, either increase or

6:44

decrease, because that automatically corrected inside by

6:47

either increasing the mA or decreasing the mA to maintain what is called as

6:52

the effective mAs. This concept is based on

6:55

maintaining image quality independent of pitch setting.