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In the last lecture, I touched on

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the principles of contrast injection.

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There were some very busy slides,

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so I want to introduce those.

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give you the fuller view of what's going on.

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So first, what happens is that a field of view

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is prescribed, a pre-monitoring slice is chosen, a

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region of interest is placed on it, and then the

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monitoring slice is based on that region of interest.

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Then what happens is that the tech goes

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inside the room, checks and makes sure that

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everything is hooked up properly, and comes

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out of the room and then presses two buttons.

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One alerts that the CT scan is about to start,

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and the other actually starts the contrast.

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So let's say T equals zero, contrast starts.

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So now your contrast is in the veins and it's going

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through the right heart and coming into the left heart.

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At around 10 seconds, you start the monitoring slice.

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In 10 seconds, maybe a little early.

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There may sometimes be situations

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where the circulation is hyperdynamic.

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So it starts looking at the descending aorta and

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taking these low-dose scans with a region of interest.

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And the monitoring of the bolus starts.

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Around 22 seconds, the attenuation in the

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descending aorta reaches our preset, which is 130.

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It can be lower, it can be higher.

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And then, after a trigger delay, let's say 6 seconds,

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maybe 10 seconds or 12 seconds, the acquisition starts.

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And then the breath hold instruction is built

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in so that it's factored in within that delay.

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So let's say at about 28 seconds the

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acquisition starts and at about 34 seconds.

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So you can see that in a scan that is only

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6 seconds long, the whole process, the

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whole orchestra has to be about 34 seconds.

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And most of that time is not scanning; most of

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the time is just the injection getting there.

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That's important to understand, um, because our

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tendency is to think of this as being real time.

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It's not; it's a snapshot of what's going

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on once the injection has been made.

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So what can be the issues with this?

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Well, the first issue is if you

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have a lot of atherosclerosis

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in the descending aorta, or even the ascending

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aorta, which would mean that there's, the

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lumen is not really a lumen; it's black.

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Similarly, if you have a dissection, and accidentally place

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the ROI into the false lumen, that can be a problem.

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You can have streak artifact from metal that can

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falsely elevate the attenuation value, which can

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falsely start the study, or you could have very

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low cardiac output that it takes such a long time

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to reach the descending aorta that the scanner has

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this automatic system where it just scans and then

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you get a suboptimal study as a result of that.

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So how.

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Would you vary?

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I mean, there are a few things in

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your control with this bolus tracking.

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If you want to scan later, so you have

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these presets that are there in the scanner

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and they're based on crude experience,

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but sometimes there's a variation theme.

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You might want to scan earlier,

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you might want to scan later.

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So if you want to scan later,

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there are two things you can do.

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You can either increase the threshold hands

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free unit, let's say from 130 to 180, and I

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honestly, I don't think that does that much.

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If you look at the threshold, it kind

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of goes 90, 130, and it really jumps up.

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The better thing to do is to increase the

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trigger delay; whatever that trigger delay was.

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If you're worried about starting too

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soon, you increase that trigger delay.

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And remember, trigger delay is

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not the same as monitoring delay.

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And also remember, trigger delay

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is not always called trigger delay.

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So just understand what those terms are.

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If you want the acquisition to start

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sooner, you have a couple of options.

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One is to reduce the hands-on unit.

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And again, I think that that is going to be problematic

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because if you reduce it too much, you can be in the

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noise territory where it'll just spuriously start.

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Or you can reduce the trigger delay.

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And that, again, is something that you can

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only do to an extent because there's a certain

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inbuilt minimum that you need to rev the scan up.

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And again, monitoring delay

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is not going to do very much.

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I mean, of course, you can not monitor for

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another 10 seconds, but you might miss the peak.

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It doesn't determine when to start.

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That determines when you're looking at it.