How to score coronary artery calcium (CAC) using cardiac CT
Learn how to measure coronary artery calcium and catch cases of patients with calcified plaque before they show symptoms.
Coronary CT is the only imaging method that can detect subclinical plaque and early vessel calcification before blood flow reduction occurs. In this video taken from our Cardiac CT Essentials course, you will learn how to measure coronary artery calcium so that you can catch cases of patients with calcified plaque before they become symptomatic.
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[00:00:00] This line shows the classical progression of atherosclerosis. The vessel lumen starts off by being normal. This is then followed by fatty streaks accumulating with the internal wall and thereafter by white blood cells and red blood cells. Later on, foci of calcification accumulate as calcified plaque. At this stage, there is no significant reduction in the luminal caliber and blood flow to the heart muscle is preserved. Stress testing with echocardiography, myocardial
[00:00:30] perfusion scintigraphy, and treadmill testing only have the ability to detect coronary disease that causes a significant reduction in blood flow. Coronary CT, on the other hand, is the only imaging modality that has the ability to detect subclinical plaque and early vessel calcification, in the wall of a vessel, before a reduction in blood flow occurs. This differs from invasive angiography, which looks at a reduction in luminal caliber but cannot detect the pathological processes occurring within the vessel wall, as per coronary CT
[00:01:00] and calcium scoring can. So, what is coronary artery calcium scoring? If we consider a normal coronary artery, the blood flow is preserved to the heart muscle. When one develops significant plaque, the blood flow to the muscle is compromised and this can become manifest with symptoms of angina. Coronary artery calcium scoring is a technique that detects and measures the amount of calcium within the coronary arteries. By doing this, clinicians are able to identify patients who may have calcified plaque
[00:01:30] but who have no symptoms. The benefit of this, is that patients who are at a high risk of future cardiac events, can receive thereafter early risk factor modification such as a commencement of aspirin, statins, and receive appropriate lifestyle changes. So, how is coronary artery calcium performed? A coronary artery calcium scan is a low radiation dose, non-contrast, prospective ECG-gated scan. The scan is usually acquired from the level of the bifurcation of the main pulmonary artery, as a starting point,
[00:02:00] since this point is almost invariably above the origins and proximal segments of the coronary arteries. Images are then acquired every 3 mm, to the bottom of the heart. This slide shows an example of a patient with no coronary artery calcium. When calcium is present, it appears similar to that seen of the spine and the sternum and is depicted by a white color. This is an example of a patient who has calcification present in all three of their coronary arteries.
[00:02:30] There is calcification present in the left main stem, proximal and middle LAD segments, the first diagonal branch, the left circumflex, and the mid-right coronary artery. You can also see calcification in the aortic valve and the descending aorta. Here, we can see the calcification in the left main stem and also the proximal LAD segment. As we scroll down through the imaging set, we can see further coronary artery calcium in the proximal to mid-LAD segment and also within the left circumflex artery.
[00:03:00] If we scroll down further, we can see calcification in the first diagonal branch and also within the mid-segment of the right coronary artery. After a coronary calcium scan is acquired, this then requires quantification using a system that was developed by Arthur Agatston. This takes the mean Hounsfield unit of the calcified plaque being evaluated and depending upon the value, attributes this to a multiplication factor. For example, if a calcified plaque had a Hounsfield unit of 320,
[00:03:30] the multiplication factor would be 3. If 600, the multiplication factor would be 4 and so forth. Here, we can see how this multiplication factor comes into play. If we take a vessel that has two sequential calcified plaques, the first has a Hounsfield unit score of 250, so the multiplication factor will be 2. This is then multiplied by the area of the plaque, giving a total value of 10 Agatston units. But the second plaque, the mean Hounsfield number is 450, so the multiplication factor, in this case,
[00:04:00] is 4. This is multiplied by the area, which is 10, to give 40 Agatston units. The total coronary artery calcium score is thereafter the sum of the Agatston score, for every calcified plaque, within the coronary arteries. And in this case, the Agatston score, would be 50 Agatston units. Now, fortunately, every CT workstation has software that is able to calculate the coronary calcium score, using this technique, automatically. All that is required, from the operator, is to mark the calcified plaque
[00:04:30] within the coronary arteries. In this example, the software color codes anything with a Hounsfield unit score above 130, which represents calcium into yellow. There is only a tiny amount of calcium in the proximal LAD segment and the total coronary calcium score was 0.8 Agatston units. Let us work through a case together. Using the same principle, this is an example with more extensive coronary artery calcification. The total coronary artery calcium, in this case,
[00:05:00] was 1,061 Agatston units. What does the coronary artery calcium score actually mean? Coronary artery calcification only occurs in the presence of atherosclerosis. It, therefore, shows the effect of all of an individual's cardiovascular risk factors, on their coronary arteries, at a single point in time. It is influenced by smoking, diabetes mellitus, hypertension, hypercholesterolemia, inactivity, genetic factors, and also elevated body mass index. Numerous studies have shown
[00:05:30] that coronary artery calcification is an exceptionally powerful prognostic marker for future cardiac events. It represents the most powerful cardiovascular risk marker that cardiologists have, and currently, it outperforms all other risk scoring methods. The greater the degree of coronary artery calcification, the greater the risk for future cardiovascular events. Indeed, it can be seen as we take in a snapshot of an individual's coronary arteries, at a single point in time and seeing what effect the sum of all of their cardiovascular
[00:06:00] risk factors, in their lifetime, have had on their coronary arteries. I would now like you to comment on the coronary artery calcification seen on the current scan. Let us look through this case together. Were you able to see the calcification at the left main stem,
[00:06:30] the proximal LAD segment, and the middle LAD segment? And also coronary calcification in the left circumflex artery, the proximal right coronary artery, and also the mid right coronary artery segment. Let us do one together. On the left-hand side, you can see the coronary calcium scan. And on the right-hand side, you can see how coronary artery calcium scoring is performed in real-time,
[00:07:00] in a patient with significant calcification. You can plain note, that this particular software color codes the calcium, once selected, into a different color for each vessel. Note, how the distal LAD is being marked with an orange color, for each calcified plaque. And now, as we move to the right coronary artery, the right coronary artery calcified plaque is being color-coded as green. As we move down the right coronary artery, each
[00:07:30] calcified plaque is selected and color-coded in the same color schematic. We are now approaching the inferior part of the heart and looking at the distal calcified plaque within the right coronary artery. Once all of the coronary calcium has been marked and color-coded appropriately, we're able to receive an output of the total coronary artery calcium. In this particular case, we can see that the total coronary artery calcium score for all
[00:08:00] of the coronary arteries, was 1,061 Agatston units. As well as being an exceptionally useful technique for evaluating cardiac risk, coronary artery calcium scoring is often routinely performed just prior to a coronary CT angiogram. This has a number of benefits that include better planning of the scan range for the coronary CT angiogram and also giving a clinician an idea as to what the burden of disease is likely to be, before the angiogram component of the scan has been performed.