Interactive Transcript
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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.
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