Cardiac magnetic resonance imaging in myocardial infarction
In this video, cardiologist Andrew Houghton will explain all you need to know about CMR’s strengths when it comes to myocardial infarction. Watch out for even more to come soon!
One of the most useful aspects of cardiac magnetic resonance (CMR) is its ability to identify and assess areas of myocardial infarction. In this video from our Cardiac MRI Essentials course, you'll go through a step-by-step process for assessing myocardial infarction using CMR, and learn how to determine when the presence of late gadolinium enhancement indicates an MI (and not something else).
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[00:00:00] One of CMRs most useful aspects is its ability to identify areas of myocardial infarction. This is helpful diagnostically and also plays a key role in the assessment of myocardial viability, which in turn helps with decision making about when revascularization is likely to help and when it isn't. If we understand what CMR can offer in the assessment of viability, we can make better decisions about revascularization procedures. The key to imaging areas of myocardial infarction
[00:00:30] is the use of late gadolinium enhancement. And that's what we're going to learn about now. In this lesson, we're going to look at the assessment of myocardial infarction using CMR. CMR offers good sensitivity and specificity in the detection of myocardial infarction. With CMR, we can identify the affected coronary artery territories, we can assess the extent of infarction, and also quantify the degree
[00:01:00] of any left ventricular systolic dysfunction. We can also assess the viability of the remaining myocardium either using late gadolinium enhancement CMR or low dose dobutamine stress CMR. And we can also identify complications of myocardial infarction such as left ventricular aneurysm and thrombus. When we report areas of myocardial infarction using CMR then we should refer
[00:01:30] to the standard myocardial segments to do so. So, most centers use a 16- or 17-segment model. And in this the basal slice of the left ventricle, this is a short access view of the left ventricle with the right ventricle seen just, here and the orifice of the mitral valve, here. In the basal slice of the left ventricle, there are six segments: the anterior and anteroseptal, inferoseptal, inferior, inferolateral, and anterolateral
[00:02:00] segments. There are also six equivalent segments at the mid-left ventricular level as identified by the presence of papillary muscles. And there are four segments of the apical level: anterior, septal, inferior, and lateral. And each of these segments is usually supplied by a specific coronary artery. So, the left anterior descending coronary artery normally supplies the basal anterior and anteroseptal segments, the mid-anterior and
[00:02:30] anteroseptal segments, and the apical anterior and septal segment. The right coronary artery, it normally supplies the basal inferior and inferoseptal segments, the mid-inferior and inferoseptal segments, and the apical inferior segment. And the circumflex coronary artery normally supplies the anterolateral and inferolateral segments basally, and at the mid-level, and the apical lateral segment. Some centers
[00:03:00] also include the apical cap, the very tip of the left ventricle is the 17th segment and that is supplied by the LAD. So, when we identify areas of myocardial infarction, we can relate these to the specific myocardial segments and then we can make deductions about which coronary artery is likely to have been the culprit for the infraction. And this can have significant implications for decision making about coronary
[00:03:30] revascularization. So how do we identify areas of myocardial infarction? Well, we do so using late gadolinium enhancement imaging. So, we inject an intravenous bolus of gadolinium contrast and then we wait for ten minutes. And after ten minutes has elapsed, we then undertake imaging of the myocardium. Gadolinium contrast normally enters the myocardium and is washed out to the myocardium very quickly. So, by the time ten minutes has elapsed, there's normally no contrast left in
[00:04:00] the myocardium and the myocardium looks dark as it does on the image, here, on the left. This is a short-axis view of the left ventricle taken at the papillary muscle level and avoids the mid-left ventricular cavity level. We can see the two papillary muscles just, here and here. The image on the right, however, shows the presence of gadolinium contrast. We can see an area of contrast, this bright material here in the
[00:04:30] anteroseptum, which is still present ten minutes after the bolus was given. That indicates that there's an area of myocardial infarction here. The contrast has entered but is very slow to wash out. So we can identify an area of myocardial infarction in the basal anteroseptal segment. Now, we can see late gadolinium enhancement in a number of conditions not just
[00:05:00] myocardial infarction but also different forms of myocardial fibrosis. So how do we know that late gadolinium enhancement when it's present, is actually the result of the myocardial infarction? Well, the answer lies in its distribution. So let's imagine that this is a short-axis slice of the left ventricle. If we have an area of myocardial infarction, then we get late gadolinium enhancement and it occurs subendocardially. In other words, we see the patch
[00:05:30] of bright gadolinium present just beneath the endocardium. Now, it may extend further out. So, if the patients had a big infarct, it may extend from much of the wall of the left ventricle. And they may have actually had a full thickness infarction, which involves the whole of the wall of the left ventricle. But nonetheless, it always occurs subendocardially. So, we always see it going from the subendocardium outwards towards
[00:06:00] the outer epicardial layer of the left ventricle. If we see late gadolinium enhancement, which occurs in the mid-wall or which occurs epicardially, then we know that that is not due to myocardial infarction because it is not occurring subendocardially. And if we see it, here, in the subendocardium or extending from subendocardium, all the way out to the epicardium, then we know that this is likely
[00:06:30] to be due to myocardial infarction. So, here's an example. This is a patient who has had a focal inferobasal infarct as a consequence of an inferior myocardial infarction. So, this is a two-chamber view of the heart: the left ventricle and the left atrium, here. This is the anterior wall apex and inferior wall of the left ventricle. And we can see this patch of bright material, here, this is late gadolinium enhancement in the
[00:07:00] inferobasal segment of the left ventricle. And we can see quite clearly how it extends from the subendocardium outwards. So, this is classical of a myocardial infarction. Here's a much more extensive inferior infarct. This patient has had an inferior ST segment elevation myocardial infarction. And again, we can see this bright gadolinium contrast, which is present in the subendocardium. In fact, inferobasally,
[00:07:30] it extends from much of the wall thickness of the myocardium. This patient is developing an inferobasal aneurysm. We can see how the left ventricle is starting to pouch outwards, here. And again, this is entirely consistent with an inferior myocardial infarction. We don't just have to use late gadolinium enhancement to indicate areas of myocardial infarction as with echocardiography. We can also look for regional wall motion abnormalities.
[00:08:00] So here is a patient who has had a circumflex territory myocardial infarction. This is a short-axis view of the left ventricle. The right ventricle is here. And we can see that the lateral wall of the left ventricle is both slightly thinned and is virtually akinetic as a result of a myocardial infarction. So we would expect to see significant subendocardial late gadolinium enhancement when we undertake
[00:08:30] delayed imaging in this patient. And we can assemble all of this information, both the regional wall motion information and also the late gadolinium enhancement information to fully describe the consequences of the myocardial infarction. Here is a different example. This is a patient who has had an inferior myocardial infarction due to occlusion of the right coronary artery and again we can see a short-axis view of the left ventricle. The right ventricle
[00:09:00] is here and the inferior wall is slightly thinned and is virtually akinetic as a result of this inferior infarct. So, on delayed imaging, we'd expect to see late gadolinium enhancement in the RCA territory. When we undertake late gadolinium enhancement imaging we may also see the phenomenon of microvascular obstruction. Here, we have a patient who has had an infarct in the LAD
[00:09:30] territory. This is a short-axis view of the ventricle and we can see that the anterior wall and anteroseptum have got this bright late gadolinium enhancement subendocardially, indicative of a LAD territory infarct. But if you look carefully, you can also see a dark area within the infarction itself. This is an area of the infarction that the gadolinium contrast has not been able to reach and that's why it appears dark, rough, and bright.
[00:10:00] And the reason that it hasn't reached that area is because of microvascular obstruction. This is either because of small pieces of clot and debris blocking the vascular bed supplying this area or edema of the myocardium resisting the flow of blood into that area. So, these dark patches are indicative of microvascular obstruction.