Upcoming Events
Log In
Pricing
Free Trial

Cardiac CT Physics: Pitch

HIDE
PrevNext

0:01

Back to physics of cardiac imaging.

0:04

This is an important topic, pitch intuitive once you

0:09

understand it, and it is often made very confusing

0:14

by the definition because lots of terms are used,

0:17

such as beam width, detector width, collimator

0:22

width. So if you think of pitch as a spring, then the

0:28

definition won't matter too much, but let's define it.

0:32

So the pitch is how far the detector moves in

0:39

one rotation, or rather the CT moves in one

0:42

rotation, divided by the collimator width.

0:44

The key thing to understand here

0:46

is what's the collimator width.

0:48

A collimator width is the number of detectors

0:51

multiplied by the individual detector width.

0:54

So if you had a 64 detector CT,

0:57

that was 64 detectors, each of which were

1:00

0.625 millimeters.

1:02

So 64 multiplied by 0.625,

1:05

which is 4 centimeters, is the collimator width.

1:09

But to understand this a little bit better,

1:11

you have to understand what trajectory of a CT scan is.

1:16

And to understand the CT the term helical,

1:21

which is used synonymously with the term spiral.

1:24

When CT was first invented and through its

1:26

initial iterations, what happened was that

1:31

the CT rotated and then wires were jammed up,

1:36

and then the wires had to be unfolded.

1:40

And then the CT rotated again.

1:44

So the whole thing took a long time until they came

1:48

up with this interconnect called the slip ring.

1:51

And you needn't have put the wires back again

1:55

because the scan could rotate and move at the same time.

2:01

So the rotation of movement created

2:04

a path known as the helical path.

2:07

And when multi-detector CT came out, the eyes

2:11

of the CT scanner flared up, became larger.

2:15

So the multi-detector CT essentially meant

2:18

that there was more than one eye looking at

2:19

the scanner in what was known as the z-axis.

2:24

So a pitch tells you how much of the body is

2:29

being seen by the detector in one gantry rotation.

2:34

So, if the pitch is one, the table moves the

2:40

distance of the collimator width, so if that is four

2:43

centimeters, it's going to move four centimeters,

2:47

which means that the detectors will see part of

2:53

the CT body, but there will not be any overlap of

2:55

data, so there's going to be no redundant data.

3:00

If the pitch is less than one, two things will happen.

3:03

Firstly, the scanner will go slower.

3:10

So it'll take longer to get through

3:12

a particular anatomical area.

3:14

The second thing is that each part of the

3:17

body will be seen by more than one detector.

3:21

And if the pitch is greater than

3:22

two, the scanner moves quicker.

3:26

And now you have a different problem.

3:28

Now the problem you have is that some parts

3:31

of the body won't be seen by any detectors.

3:35

So there'll be gaps, informational gaps.

3:36

Now there are ways to fix it, and that's

3:39

kind of beyond the scope of this talk.

3:41

I just want you to understand what pitch is.

3:43

And the importance of that for coronary imaging.

3:47

So what of collimator width?

3:49

So things have improved since

3:51

we first started doing MDCT.

3:54

So in the beginning, for example, you had

3:57

four detectors, each of one millimeter.

4:01

So the collimator width was four millimeters.

4:03

A big leap came with 64 detectors,

4:09

because as detectors kind of doubled, there

4:13

came a point where it made a big difference.

4:15

The 64 detectors, two things happened.

4:18

First, the collimator width actually increased

4:22

that you could go further with the same pitch.

4:27

Second thing is that you could afford to have

4:30

thinner collimators, thinner detectors, without

4:34

compromising the overall collimator width.

4:37

So with the four, you had to have one

4:39

millimeter, otherwise you really weren't

4:41

seeing much of anatomy whatsoever.

4:43

With 64, you could go down to 0.625,

4:44

84 00:04:45,725 --> 00:04:50,415 yet see 10 times more than you could with 4.

4:51

And now, scanners are offering 320 detectors,

4:55

which, if you take the thinnest detector with 0.625,

5:02

you see 16 centimeters in one gantry rotation.

5:07

So technically, you no longer have to move the patient.

5:10

You can see the entire anatomy of

5:12

interest in one gantry rotation.

5:15

So the concept of pitch really only applies

5:18

when you're moving the pitch.

5:19

Without moving the pitch, pitch ceases to be a concept.

5:26

So let's look through this again.

5:30

If the pitch is one, in one gantry

5:33

rotation, the table moves 40 millimeters.

5:36

If the pitch is 0.25,

5:38

it moves 10 millimeters.

5:40

If the pitch is 1.5,

5:40

it moves 60 millimeters.

5:43

So now I spoke about the importance, or rather

5:51

the consequence, of having a pitch less than one,

5:53

which is that you have redundant information,

5:57

but information isn't always redundant.

5:59

Sometimes you want more of it for a particular reason.

6:03

And in coronary imaging, you want that if

6:09

you want what's known as multi-phase data.

6:11

Multi-phase data is when you see all parts of

6:15

the heart in all parts of the cardiac cycle.

6:21

For that to happen, you need retrospective

6:23

gating, which means that the image

6:25

must be acquired continuously.

6:28

The radiation is on continuously.

6:30

It may not be of the same intensity

6:31

continuously, but it is acquired continuously.

6:34

And the purpose of that is perhaps you might want

6:41

functional information to see how the heart moves, how

6:46

the valves move, or you might simply be worried that

6:52

the coronary artery isn't captured in the right phase.

6:54

You may want a different phase

6:56

to capture the coronary artery.

6:58

So low pitch is essential for a

7:04

particular form of coronary imaging.

7:05

If we want multi-phase data, these days, we don't need

7:12

low pitch or as low pitch as we used

7:14

to need, but nevertheless, it's an

7:17

important concept in coronary imaging.

7:20

And then another important thing to understand is that

7:23

the optimal pitch is very much heart rate dependent

7:25

because what the heart rate does, the slower the heart

7:28

rate, the longer is the interval between the R waves.

7:35

And the longer the interval between the R waves,

7:37

the longer you're in that particular cardiac cycle

7:40

and the longer you are in that particular cardiac cycle,

7:42

the more there is happening that has to be captured.

7:46

So lower the heart rate, lower the pitch,

7:51

slower the scan; slightly higher heart rate,

7:54

you can go with higher pitch and faster scan.

7:59

So one thing to, um, appreciate about the, um,

8:02

advancement of CT is this increase in the, whatever

8:08

you want to call it, beam width, collimated width.

8:11

When CT first came out, we had four detectors.

8:15

And that took about 35 seconds to

8:19

travel through an area like the heart.

8:22

And the resolution in the z-axis was one millimeter.

8:26

With 320 detectors, now, the whole

8:32

scan can be done in less than a second.

8:34

Because you could just need one

8:35

rotation to capture the entire heart.

8:38

And the resolution is 0.5 millimeters,

8:39

152 00:08:41,325 --> 00:08:43,755 um, with 64 detectors,

8:44

which was a leap from 16.

8:47

The resolution went down to 0.625

8:49

with a scan time of 5 to 8 seconds.

8:53

So what we've been seeing progressively

8:55

in the advancement of CT, two things.

8:59

First is that we've been able to improve

9:04

the Z resolution because we've been

9:06

able to make the detectors smaller.

9:10

Because there are so many of them, you

9:12

don't get penalized for making them smaller.

9:14

The second thing is that the scan is faster,

9:17

and with the scan being faster, it means

9:21

that the contrast volume can be lower.

9:25

And there's also a reduction in the radiation

9:31

dose because 320 detector CT doesn't necessarily

9:35

need low pitch imaging because it can just

9:39

deal with, look at the whole heart in one go.

9:41

So when you look at the gains of

9:43

CT development over the years,

9:46

small things have led to dramatic

9:48

reduction in scan time and radiation dose.

9:53

Thank you.

Report

Faculty

Saurabh Jha, MD

Co-Program Director, Cardiothoracic Imaging Fellowship, Associate Professor of Radiology

University of Pennsylvania

Tags

Vascular

Coronary arteries

Cardiac

CTA

CT

© 2024 Medality. All Rights Reserved.

Contact UsTerms of UsePrivacy Policy