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Back to physics of cardiac imaging.

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This is an important topic, pitch intuitive once you

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understand it, and it is often made very confusing

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by the definition because lots of terms are used,

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such as beam width, detector width, collimator

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

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

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So the pitch is how far the detector moves in

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one rotation, or rather the CT moves in one

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rotation, divided by the collimator width.

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The key thing to understand here

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is what's the collimator width.

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A collimator width is the number of detectors

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multiplied by the individual detector width.

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So if you had a 64 detector CT,

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that was 64 detectors, each of which were

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0.625 millimeters.

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So 64 multiplied by 0.625,

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which is 4 centimeters, is the collimator width.

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But to understand this a little bit better,

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

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And to understand the CT the term helical,

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which is used synonymously with the term spiral.

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When CT was first invented and through its

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initial iterations, what happened was that

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the CT rotated and then wires were jammed up,

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and then the wires had to be unfolded.

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And then the CT rotated again.

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So the whole thing took a long time until they came

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up with this interconnect called the slip ring.

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And you needn't have put the wires back again

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

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So the rotation of movement created

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a path known as the helical path.

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And when multi-detector CT came out, the eyes

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of the CT scanner flared up, became larger.

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So the multi-detector CT essentially meant

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that there was more than one eye looking at

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

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So a pitch tells you how much of the body is

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being seen by the detector in one gantry rotation.

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

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distance of the collimator width, so if that is four

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centimeters, it's going to move four centimeters,

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which means that the detectors will see part of

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

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

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

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Firstly, the scanner will go slower.

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So it'll take longer to get through

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a particular anatomical area.

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The second thing is that each part of the

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body will be seen by more than one detector.

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And if the pitch is greater than

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two, the scanner moves quicker.

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And now you have a different problem.

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Now the problem you have is that some parts

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

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So there'll be gaps, informational gaps.

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

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kind of beyond the scope of this talk.

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I just want you to understand what pitch is.

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And the importance of that for coronary imaging.

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So what of collimator width?

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So things have improved since

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we first started doing MDCT.

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So in the beginning, for example, you had

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four detectors, each of one millimeter.

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So the collimator width was four millimeters.

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A big leap came with 64 detectors,

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because as detectors kind of doubled, there

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came a point where it made a big difference.

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The 64 detectors, two things happened.

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First, the collimator width actually increased

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that you could go further with the same pitch.

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Second thing is that you could afford to have

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thinner collimators, thinner detectors, without

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compromising the overall collimator width.

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So with the four, you had to have one

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millimeter, otherwise you really weren't

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seeing much of anatomy whatsoever.

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With 64, you could go down to 0.625,

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

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And now, scanners are offering 320 detectors,

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which, if you take the thinnest detector with 0.625,

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you see 16 centimeters in one gantry rotation.

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

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You can see the entire anatomy of

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interest in one gantry rotation.

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So the concept of pitch really only applies

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when you're moving the pitch.

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

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So let's look through this again.

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If the pitch is one, in one gantry

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rotation, the table moves 40 millimeters.

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If the pitch is 0.25,

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it moves 10 millimeters.

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If the pitch is 1.5,

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it moves 60 millimeters.

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So now I spoke about the importance, or rather

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the consequence, of having a pitch less than one,

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which is that you have redundant information,

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but information isn't always redundant.

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Sometimes you want more of it for a particular reason.

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And in coronary imaging, you want that if

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

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Multi-phase data is when you see all parts of

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the heart in all parts of the cardiac cycle.

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For that to happen, you need retrospective

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gating, which means that the image

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must be acquired continuously.

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The radiation is on continuously.

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It may not be of the same intensity

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continuously, but it is acquired continuously.

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And the purpose of that is perhaps you might want

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functional information to see how the heart moves, how

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the valves move, or you might simply be worried that

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

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You may want a different phase

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to capture the coronary artery.

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So low pitch is essential for a

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particular form of coronary imaging.

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

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low pitch or as low pitch as we used

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to need, but nevertheless, it's an

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important concept in coronary imaging.

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And then another important thing to understand is that

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the optimal pitch is very much heart rate dependent

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because what the heart rate does, the slower the heart

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

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And the longer the interval between the R waves,

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the longer you're in that particular cardiac cycle

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and the longer you are in that particular cardiac cycle,

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the more there is happening that has to be captured.

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So lower the heart rate, lower the pitch,

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slower the scan; slightly higher heart rate,

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you can go with higher pitch and faster scan.

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

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advancement of CT is this increase in the, whatever

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

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When CT first came out, we had four detectors.

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And that took about 35 seconds to

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travel through an area like the heart.

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

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With 320 detectors, now, the whole

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scan can be done in less than a second.

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Because you could just need one

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rotation to capture the entire heart.

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And the resolution is 0.5 millimeters,

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

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which was a leap from 16.

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The resolution went down to 0.625

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with a scan time of 5 to 8 seconds.

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So what we've been seeing progressively

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in the advancement of CT, two things.

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First is that we've been able to improve

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the Z resolution because we've been

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able to make the detectors smaller.

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Because there are so many of them, you

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don't get penalized for making them smaller.

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The second thing is that the scan is faster,

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and with the scan being faster, it means

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that the contrast volume can be lower.

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And there's also a reduction in the radiation

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dose because 320 detector CT doesn't necessarily

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need low pitch imaging because it can just

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

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So when you look at the gains of

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CT development over the years,

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small things have led to dramatic

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reduction in scan time and radiation dose.

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Thank you.