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Images acquired in cardiac CT

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are synchronized with the ECG.

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In many ways, it's obvious why we do this, but

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I think it's probably worth asking why still.

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The obvious answer is because the heart

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signals its movement, that it has an

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electrical signal, so there may be a compulsion

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to synchronize requisition with that.

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But the real reason is that it is done to

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compensate for an inadequate temporal resolution.

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Imagine if the temporal resolution was

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five milliseconds and you could acquire

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the whole heart in 10 milliseconds.

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I mean, we're not there anywhere.

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But let's say you could, then there would be no part

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point of ECG synchronizing 'cause you'd be able to

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get pretty much the entire heart with very little

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difference between the top of the heart and the

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bottom of the heart in terms of where they were in

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the cardiac cycle in a, uh, flash of the second.

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So we really have to do this because the temporal

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resolution is historically and still is inadequate.

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So what are the modes of ECG synchronization?

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I use that as a generic term.

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And to understand that we have to

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understand the modes of scanning today.

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Modes of scanning today really are two types.

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One is where the table moves and the

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second is where the table is stationary.

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The table is stationary when the gantry also is

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stationary and it just does one rotation.

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And in that one rotation, it covers

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the organ of interest, the heart.

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And that happens with the area detectors that

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have multiple detectors, such as 320 detectors.

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And they're able to scan the whole

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heart in one gantry rotation.

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Mostly this table moves.

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And the table can move in two manners.

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One is sequential and the other one is helical.

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And basically, although this is just a

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very basic distinction, sequential uses

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prospective and helical uses retrospective.

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So what happens with prospective?

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It's also called prospective triggered.

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We say the R wave is very important.

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If you recall your ECG, there's the PQRS

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complex T wave and the R wave is the

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biggest part of the ECG, the tallest part.

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Sometimes that's not the case.

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It may be the T wave.

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So we take a predetermined point from

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the R wave and we say that at that point

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the images are going to be acquired.

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So the gantry moves around the patient.

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That's the shoot.

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Next heartbeat.

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The table advances so that we have a next volume

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that hasn't been scanned, that gets scanned.

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So that's step.

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And then you shoot again.

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This is how we still do the calcium scans.

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We used to do the calcium scans,

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of course, this way we still do it.

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But with increasing number of detectors, we

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can actually employ this method in coronary

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arteriography as well, the angiographic part.

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So this is step and shoot, and there

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are various adaptations of this.

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Which take some stepping, some moving non-

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sequentially, and some moving sequentially.

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So there's a combination of things you can

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do, but the purest form, this is what it is.

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Another is the helical, which

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is the retrospective gating.

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What's retrospective is that we acquire the

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images throughout the cardiac cycle.

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And then we decide later on to put the

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images retrospectively in various bins.

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That can be at 5 percent intervals of the cardiac

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cycle, 10 percent intervals of the cardiac cycle,

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or millisecond intervals, like 50

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milliseconds, 100 milliseconds.

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So the radiation is on continuously, but

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it can be reduced during portions of the

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cardiac cycle that we know the imaging will

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not be that great, such as during systole.

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So that's called tube current modulation

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that saves 40 percent or so radiation dose.

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It used to be the method of choice.

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You know, because the scans were taking so long

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that there was a chance of heart rate variability

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and different arteries had their quiet period

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at different times of the cardiac cycle.

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So we would have to do reconstructions for the RCA

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in a certain phase and the LAD in a certain phase.

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These days we hardly employ retrospective

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unless we want functional imaging either

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of the aortic valve or the heart functional

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imaging to see how the heart is contracting.

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Or if we

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encounter a patient with arrhythmia

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so that we know that there may be

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different phases of the cardiac cycle that will

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work for different portions of the heart.

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So this is a not-so-new now but certainly new

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when this paper was published in 2009 method of

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acquisition which is known as high-pitch imaging.

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Pitch tells you it's like a kind of helical

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spring, and a pitch of one means that the gantry

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moves the same distance as the collimator width.

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A pitch less than one is overlap, which is

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what you need for retrospective gating.

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And a pitch greater than one means that

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there could be informational loss.

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However, that makes the scan faster.

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With two tubes at right angles to each

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other, you can go up to a pitch of 3.2.

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120 00:05:49,829 --> 00:05:53,650 And there are other fancy algorithms used to correct

5:53

for the possibility of informational loss.

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So you can go up to a pitch of

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3.2 without any informational loss.

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So you can start at the top of the heart in

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the beginning of the cardiac cycle as you

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do over here, and by the time you're

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through one or two rotations, pretty much done.

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You've covered the entire volume within the heartbeat.

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So I say one or two, I really mean one heartbeat.

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So this is known as the high

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pitch spiral scan, FLASH.

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Scan time is the time that it takes for the

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table to move from the top of the heart to the bottom

6:39

of the heart to acquire that particular volume.

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Given a pitch of 3.2

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and very fast table speeds, this can

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be done within a single cardiac cycle.

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And this is very different from the prospective

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where you're kind of stepping, shooting,

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moving, shooting, moving, so sequential.

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So this is still helical; the trajectory is helical.

7:04

Very high physiological, except

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it's done within one shot, one sweep.

7:14

Thank you.