Interactive Transcript
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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.
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