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Contrast Injection for Coronary CTA

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I'm going to talk about principles of contrast

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injection and these principles apply to all

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vascular imaging and not just coronary CTA but

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they're very important to understand because

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that's how you get a great study like this one

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on the left and you also need to understand why

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they're sometimes bad, such as the one on the right.

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Now ordinarily radiologists aren't

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involved in this because this is entirely

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taken care of by the technologists.

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Protocols have been set up and you're kind

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of only informed when things go wrong.

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And, um, it's sort of like, you know,

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the MD informed, you're informed, but what

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are you going to do about it differently?

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And to understand what to do differently,

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you have to understand the principles as well

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as the technologists and better than those.

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So what is the fundamental

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principle of contrast injection?

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The fundamental principle is that.

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You want to time the acquisition of CT, the picture

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taking, to when the arteries are maximally opacified.

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Now that sounds fairly simple,

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except there are two things.

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Firstly, when they're maximally opacified

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arteries, it's only a short time, because

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in arteries, contrast goes in and goes out.

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Veins, they tend to hang out for a bit longer,

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so veins are more forgiving than arteries.

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The second thing is that different arterial

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trees are opacified at different times.

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Slightly different times, but getting optimality

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means you need to think carefully about the

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arterial tree that you're trying to image.

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And the arterial tree, as in the case of the

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coronary artery, is so small that for you to

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actually time it, you'd have to see the coronary

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arteries light up, and that can be quite difficult.

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So what you do is you end up choosing the

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artery closest to the arterial tree

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of interest, which most resembles that.

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And in the case of the coronary

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artery, that happens to be the aorta.

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And you can choose the descending

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or the ascending aorta.

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I don't think it makes that much of a difference.

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And so now what you're doing is you're

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trying to get the timing of the CT to

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correspond with the peak maximal enhancement.

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And that in itself is a bit of a science.

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There are multiple factors to it.

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There's timing, which I think is

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what we have most control over.

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There's breath hold.

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Where, of course, you have control over, but

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not that much, because ultimately it's patient

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dependent, but you can educate the patient.

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Bolus geometry, which is, again,

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what we have control over.

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And then there are patient factors, such as their

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ejection fraction, which it's worth bearing in

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mind, because you can do a variation on the theme.

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And finally, there's the kVp, which I'll talk

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about in more detail in another lecture, but,

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yes, that's something to also bear in mind.

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Peak maximum enhancement (PME) and time to peak (TTP)

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are two important concepts which

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happen behind the scenes, but they're happening

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all the time and they depend on a bunch of factors.

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Firstly, the rate of injection volume of contrast.

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The amount of iodine in the contrast, so rate

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volume iodine, so that's kind of like the iodine

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flux, the amount of iodine that's going through.

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And lastly, the saline push.

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So if you look at the graph here, you'll notice

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that as you increase the rate of contrast, if you

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start at one milliliter per second, then you move on to

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three milliliters and then five milliliters, the peak gets larger.

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As you inject quicker, the peak

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increases and the time to peak reduces.

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So you get higher, quicker with

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greater contrast injection.

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That kind of makes sense.

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This is an interesting one, the volume of contrast.

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You'll notice that as the volume

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increases, three things happen.

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First is that your peak increases.

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The second thing is your time to peak also increases.

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It's kind of the opposite with the injection rate.

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Injection rate peak increases.

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Time to peak decreases here, the peak

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increases, time to peak increases.

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The third thing that happens, which may

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not be best illustrated in this graph,

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is that the time of that peak increases.

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So instead of that pointy peak, it becomes

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more like a nice little mountaintop.

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So, basic contrast kinetics, also, if you're

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increasing the amount of iodine, which you

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don't really have an option of doing, because

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you kind of get one iodinated agent, so you

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can't say, let me put it in more contrast.

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But something to bear in mind, if you

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increase the amount of iodine, then it's

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the same as increasing the injection rate.

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So this graph here shows more iodine,

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and then the blue shows less iodine.

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More iodine, higher peak.

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Quicker time to peak.

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And then there are body factors as well,

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but I think the most important one to understand

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is this one here, which is what happens when

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you have a reduction in the cardiac output.

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So low ejection fraction cardiac failure situation

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that you will frequently encounter.

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And sadly, often only when something goes wrong.

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So if you reduce your cardiac output, it's

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the same as increasing the volume of contrast.

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That's very interesting.

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So it takes longer to get to the peak.

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And the peak is higher.

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And I think the peak is higher because your effective

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circulating volume, because there's all these

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kind of jazzy stuff that, uh, renal physiology and

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pathophysiology, that what happens with heart failure

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is even though you have more water on board, what's

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actually in effective circulating volume reduces.

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So you have hemoconcentration.

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So that's the effect, same

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effect as giving more volume.

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So all of these things you want to bear in mind.

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And also I didn't mention the saline push,

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the saline push has the same effect, to an

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extent, where it'll increase your time to peak.

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And slightly increase your peak as well.

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So in terms of timing, in terms of saying,

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okay, when am I going to take the acquisition,

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3T acquisition versus, you know, giving the

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contrast, there are two ways you can do it.

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You can do an empirical delay, you can make a

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guesswork, and this is what we do with veins.

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So, you know, we kind of say with a portal

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vein, 70 seconds, the lower extremity veins,

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120 to 140 seconds, or you can trigger.

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Trigger is when you try and be precise about it.

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There are two ways to trigger, one is called bolus

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tracking and the other one is called test bolus.

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I'll go through test bolus because that's

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used less often than bolus tracking.

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So in test bolus what you're doing is you're

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injecting a small amount of contrast, let's say

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about 20 mL or so, in a predetermined segment,

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such as the aorta, and then you're seeing

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how long does it take for it to reach the peak.

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The peak itself isn't important, but

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it's the time to peak that's important.

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And then however long it takes to get to that peak.

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With the real injection, which is going to be much

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more, I factor in a fudge factor of about five to

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six seconds, because if you recall, when you give

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more contrast, it takes longer to get to the peak.

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So it's a method of, um, calibrating.

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When do I use it?

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I probably should use it more often,

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but I a hundred percent use it if I know

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the patient has low ejection fraction.

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So I do get the texts for

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you know, things like runoffs and,

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aortic dissection studies and even

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coronaries to see if their ejection fraction is low.

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So how low is low?

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I mean, this is an arbitrary number I'm putting

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out, but you know, less than 20%, absolutely.

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Yeah.

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20 to 30%.

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You might get away with the other method,

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but yes, less than 20 percent clearly.

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All right.

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So this is the method we use all the

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time, which is the bolus tracking method.

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In the bolus tracking method, what we do is we

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put a tracker on the abdominal aorta or somewhere

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and watch how it increases in signal intensity.

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We specify a threshold such as 130 Hounsfield

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units and we then specify a trigger delay.

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So, the trigger delay is the time from

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reaching the Hounsfield unit threshold

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to the start of the acquisition.

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You do need to have some trigger delay,

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because if you don't, you're going to end up

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with, I mean, it's not possible that there

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has to be this deep breath in and hold it.

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So I want you to look at this, because it's a very

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busy graph, and I'm going to go through this again.

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But an important point to understand over

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here is that when you give the injection,

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you don't take the images straight away.

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Because when you give the injection, you

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give it through a vein, it goes through your

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circulation, goes through the right heart, comes

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to the pulmonary artery, then pulmonary vein,

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then left atrium, left ventricle, then aorta.

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And then you do what's called a monitoring.

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You start monitoring the arrival of

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contrast in that predetermined segment.

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When it reaches that threshold, then after that

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trigger delay, you start the scan with the hope of

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coinciding the acquisition with the peak maximum

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enhancement. It's important to understand that the

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actual scan happens after 15 to 20 seconds towards

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the tail end of the injection, and that's why your

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injection can't be too short, because you'll miss that.

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There are two types of delay.

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There is the monitoring delay, which

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is the delay between the start of the

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injection and the first monitoring slice.

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You have to use that judiciously, but the

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trigger delay is the one that's different.

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So you have to, when communicating to techs, make

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sure you understand the distinction between trigger

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delay, which has been in the attenuation, reaching

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a preset threshold and the start of the acquisition.

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

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