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In this lecture, I'm going to discuss how

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we change a protocol to image bypass grafts.

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The major difference between imaging bypass grafts

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and imaging normal native coronary arteries is that

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because bypass grafts can arise much higher up,

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such as from the aortic arch or left subclavian artery,

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so the field of view has to be increased.

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And because the field of view increases, the scan

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time increases, so I'm showing you a scout, and

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when you're doing a bypass study, which sometimes

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isn't always evident from the clinical history,

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so I always ask the patient whether they have

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a stent or a bypass graft, and you can see from

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the field of view that we start above the aortic

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arch, and, um, because we start a little bit

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higher, we have to increase the contrast volume.

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There are essentially two types of bypass grafts.

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There's the arterial and the venous.

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Arterials tend to last longer.

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Arterial grafts are smaller.

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Uh, the most popular arterial graft is the left

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internal mammary artery, shortened to LIMA.

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And there's also the right internal mammary artery,

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radial artery,

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and gastroepiploic artery.

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So to understand the anatomy of the bypass,

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it's good to think about a volume-

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rendered image of the heart.

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And with volume-rendered imaging, we can see

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the coronary arteries outlined very nicely.

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And, uh, here is the left main coming off,

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and that's giving rise to the left anterior

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descending, which then meets the posterior

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descending branch of the right coronary artery.

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And on the mirror image of the right coronary artery

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here, in the posterior AV groove, is the circumflex.

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So keep that in mind, and that will

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help you understand the bypass.

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Bypass is literally what it is said to be,

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bypass, so you're bypassing normal arteries.

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So you can see here, you have the left

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internal mammary artery arising from the

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left subclavian artery, which is mobilized.

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Its origin is left intact, and it's brought,

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usually, to the left anterior descending artery,

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distal to the stenosis.

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So that's what's being bypassed.

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You can see that by virtue of coming off the left

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subclavian artery, if you have any narrowing or

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occlusion of the left subclavian artery, you're going

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to have problems with the graft, which is why when

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you're thinking about bypass imaging, you have to also

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image the arterial supply proximal to the bypass graft.

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Such as the left subclavian, the

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brachiocephalic, or the aortic arch.

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So it's kind of chest imaging,

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it's not just coronary imaging.

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And here you'll notice another vein graft coming

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across from the aortic arch, and another vein graft.

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Vein grafts tend to have origins from the aortic arch.

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The origins are very generally obvious.

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The anastomosis can be difficult to detect.

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But I think as long as you can see the

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origin and see most of the artery and

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exclude an occlusion, that's a good job done.

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Whether or not you can occlude high

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grade stenosis or the anastomosis, it

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tends to be a more difficult matter.

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So a couple of examples on the right here is

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what's known as a curved planar reformation.

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And this is of the right internal mammary artery,

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which is put along its course all the

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way to the point at which it anastomoses

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with the native coronary artery.

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On my left is an example of multiple bypass grafts.

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And this is a volume dimension.

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You can see the arrows are

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pointing to the graft origin.

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They tend to assume the course of the

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artery that they attach to once they're

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attached to that particular artery.

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On this image here, the arrowhead points

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to a vein graft, which, as you can see, is

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larger than what the arrow is pointing to,

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which is an arterial graft, which is smaller.

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So vein grafts tend to be larger.

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Grafts can have stents within them,

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and we'll talk about stent imaging.

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And because grafts can have stents within

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them, you'll have to apply the same

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principles of stent imaging protocol to grafts.

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And here you can see a case of a graft

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having a critical stenosis distal to a patent stent.

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Vein grafts are notorious for getting stenotic.

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So let's think about a few of the nuances.

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The first is patients who have bypass grafts may

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well have had ischemic heart disease for a long time,

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in which case the left ventricular

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function might be depressed.

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And if the function is depressed, then the

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contrast kinetics can change, and it may be,

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on some occasions, better to use the test

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bolus as opposed to the bolus tracking method.

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So, where possible, where I know the

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patient has had a bypass graft, I check

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the LVEF. If it's less than, let's say, 25%,

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I tend to use, uh, test bolus

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instead of bolus tracking.

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Obviously, you don't want to measure calcium score.

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Uh, we do a calcium scan as a

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scout, as the unenhanced scout.

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We don't measure a calcium score because a calcium

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score predicts the long-term, uh, cardiovascular

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events in patients who are asymptomatic.

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And by definition, anybody who has the bypass

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graft may very well have been symptomatic

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at one point, but they've been treated,

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so they're not in the same risk profile.

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And if you miss a portion of the graft, like most

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commonly the origin of the left internal mammary,

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which is very high up in the subclavian, this

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typically happens when you don't know the patient

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has a bypass graft and you end up doing a normal

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coronary, which only gets the distal half of the graft.

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It's okay.

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You obviously don't see the left subclavian origin,

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but as a general rule, if the distal graft is opacified,

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it's unlikely that the proximal graft is occluded

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because these tend to be end arteries that don't have,

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you know, collaterals, um, uh, generally sprouting to

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the proximal to the point of attachment distal to it.

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That's a different matter,

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but I'm talking about proximal.

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Thank you very much.