Cardiac CT Angiography: An Overview of Atherosclerotic Plaque Definition

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Cardiac CT Angiography: An Overview John A. Rumberger, PhD, MD, FACC, FSCCT Version: December 2019 Overview: Cardiologists and Radiologists have been performing and teaching the use of Electron Beam and Multidetector CT ‘Virtual’ Cardiac/Coronary Angiography [CCTA] since the early 1990’s. I established myself as an early innovator during my years at The University of Iowa, Iowa City, Iowa and The Mayo Clinic, Rochester, Minnesota. I co-edited and authored the first textbook on Cardiac CT [Ultrafast Computed Tomography in Cardiac Imaging: Principles and Practice, 1992]. Later I was involved with preventive Imaging Centers in Ohio, Florida, Maryland and New Jersey. I have lectured worldwide on Cardiac CT, have published hundreds of peer reviewed scientific articles, and been involved with the personal education in the application and interpretation of Cardiac CT for thousands of physicians. The non-contrast CT ‘HeartScan’ remains the standard of care for initial evaluation of personalized cardiac ‘risk’, defining the extent of calcified coronary artery atherosclerotic plaque and providing both diagnostic and prognostic information that can be used to follow changes in plaque burden over time and most importantly responses to therapy in terms of atherosclerotic plaque progression. You can find individual lectures on the use of non-contrast CT and cardiac risk on my YouTube channel. However, the uniqueness of contrast enhanced CCTA as a powerful cardiac diagnostic tool has been established in multiple published studies world-wide. For instance, it is now the ‘go-to’ imaging for patients suspected of having advanced cardiovascular disease in many metropolitan Emergency Departments and it’s utilization in the outpatient areas is increasing exponentially for diagnostics, as opposed to conventional stress testing. CCTA involves injecting iodinated contrast into an arm vein; the imaging is timed to the appearance of the contrast into your heart and the entire scanning, gated to your heart beat, takes generally about 10 seconds. There is more radiation exposure than with a conventional noncontrast HeartScan, but practicing modern day radiation limitation imaging and new image reconstruction algorithms one can lower the radiation dose now to be comparable to a diagnostic mammogram. CCTA is timed to your heart beat and allows us to not only assess native heart or coronary arteries but also define heart size/ function and evaluate heart valves. In patients with known heart disease CCTA also can be used to visualize coronary artery stents as well a coronary artery bypass grafts. CCTA is the ONLY clinically available imaging modality that can display the heart in three-dimensions, without distortion, in an infinite number of views. A unique advantage of virtual coronary angiography using CCTA is that we can identify the physical characteristics of the coronary artery plaque; not just measure ‘stenosis’ or the degree of coronary narrowing.

What is Atherosclerotic ‘Plaque’? What we mean by plaque is the development and accumulation of white blood cells, inflammatory proteins, and byproducts of inflammation that appears in the WALL of the blood vessel years before it can develop into a heart attack or a true blockage of blood flow. There is a very significant ‘silent’ period as the plaque develops in the wall, accumulating and expanding, actually making the wall bulge but not narrowing the vessel ‘lumen’ [i.e. the tube part of the artery through which blood flows]. Below is an artist’s rendering of the process of

atherosclerotic plaque development over YEARS!

The problem is that the development of plaque produces no symptoms until it may be too late. The first and last event in 50% of the people who develop a heart attack is death or major disability. Thus, finding the plaque earlier is vitally important as, if found early, we can then do effective means of intervention from diet/exercise, to medications, or even ‘interventions’ that would extent longevity and overall health. What Unique Features of Plaque is Found Using CCTA? However, it is the anatomic features that the atherosclerotic plaque assumes which can provide vital information. As noted in the figure above, plaque ‘instability’ can result in rupturing the plaque through the blood vessel wall [sort of like a volcano exploding] and exposing the flowing blood to this debris. The debris is highly ‘thrombogenic’ meaning that it is likely, when mixed

with the blood, to develop a local blood clot or ‘thrombus’. One of the reasons we often suggest aspirin therapy is that this can act as a mild ‘anticoagulant’ and allow the repairs to the ruptured wall segment to clot off WITHOUT ALSO clotting off the entire vessel lumen [resulting then in STOPPING a heart attack]. CCTA is unique for non-invasive imaging technologies in that it can be used to discern the characteristics of the atherosclerotic plaque. 1. Calcified Plaque I always say that ‘…calcification of the atherosclerotic plaque is a GOOD thing, but for a BAD reason.’ What I mean is that the calcification or ‘hardening of the arteries’ [to use an old fashion but well known description] is an attempt by the body to contain the areas of blood cells and other debris in a ‘calcified shell’ [see figure above]. In this way, over time, it can help ‘stabilize’ the plaque. Although the presence and amount [i.e. the coronary artery calcium ‘score’] is a useful surrogate to the overall atherosclerotic plaque burden] the calcified plaque, for the most part is stable, although marking the progress of the healing. Calcified plaque in general is confined to the vessel wall and can be seen in situations in which there has NOT been a rupture of the plaque through the single cell layer [called the endothelium] into the vessel lumen. For an example from CCTA consider the figure below. Note in particular, the adjacent cartoon indicates that the vessel wall, which contains the calcified plaque, has compensated for the space occupied by calcification by thickening, but there is no compromise to the vessel lumen diameter itself. This is then considered ‘stable’ plaque.

2. Non-Calcified Plaque Non-calcified plaque is another matter. This plaque is in transition and in many instances it has already eroded through the endothelium and is creating an isolated area of blood clot or thrombus in the vessel lumen. This type of plaque cannot be identified on the standard ‘HeartScan’ but is readily defined by CCTA. This plaque is considered by most experts to be

‘potentially unstable’ – that is, it can possibly go on to form more local blood clots and increase the risk of a heart attack. When identified by CCTA we insist that the patient take aggressive actions to stabilize the plaque by starting an aspirin regiment and taking cholesterol medications [in particular ‘statins’ that actually can help stabilize the plaque]. Any other sources for generalized inflammation are also addressed. The goal here is to allow the non-calcified plaque to be ultimately covered by the endothelium thus sealing the rupture and to allow the time for the body to calcify the plaque. An example of isolated non-calcified plaque is shown in the figure below.

Note, in this instance there is very mild encroachment of the non-calcified plaque into the vessel lumen, but no current compromise to arterial blood flow. The adjacent cartoon shows that the endothelium barrier has been broached and there is now an isolated area of blood clot or thrombosis protruding into the vessel lumen.

3. Mixed Plaque ‘Mixed’ plaque quite simply means the presence of BOTH isolated calcified and non-calcified plaque in adjacent areas of the blood vessel. It is usually observed at the same time that the coronary artery calcium score is in the ‘moderate’ or ‘extensive’ range, again showing the power of the non-contrast CAC score as the more ‘hard/stable’ plaque, the more likely there is also to be ‘non-calcified/potentially unstable/transitioning’ plaque. The presence of mixed plaque further encourages very aggressive interventions with respect to modifiable risk factors. Also in such individuals there is an increased likelihood of actual ‘obstructive’ disease that may require conventional/diagnostic/invasive coronary angiography and possibly mechanical interventions such as angioplasty/placement of stents/open-heart bypass surgery. Below is an example of a CCTA showing ‘mixed plaque’.

Note at the arrow there is an area of what appears to be ‘significantly narrowed lumen diameter’ suggesting a potentially important narrowing that may require further investigation.

4. Ulcerated/Dissected Plaque The presence of plaque ‘ulceration’ is exceeding uncommon in an otherwise stable patient; but regardless of the clinical situation, is an important and ominous observation. To get a general idea of what ulceration means – think of something like a sudden pothole appearing on the road – the support structure of the road has totally disrupted – a sinkhole is another physical example. The ulceration can also be part of a ‘dissection’ or tearing apart or weakening so much that splitting the vessel apart may be eminent. Say you are driving down the road and one of your tires begins to shred – the layers that make up the tire wall are spitting or separating from each other. Most of you are familiar with the term ‘aortic dissection’ – such as happened with TV star John Ritter and Actor Alan Thicke. Also, many of you may know the term ‘aneurysm’ referring to a bulge in the wall of a large artery [such as the aorta] that with time, as it enlarges and bulges more will burst suddenly and without warning. Shown below is an example of dissected/ulcerated plaque occurring in a coronary artery as diagnosed using CCTA. Ulcerated plaque may or may not be seen with any severe obstruction of the lumen, so there may be no symptoms initially. The contrast given to the patient intravenously has now ‘infiltrated’ into the vessel wall via the ulceration/dissection. This is considered a highly unstable situation that

should be treated as a medial urgency

Summary: CCTA has become established in most major and minor medical centers in terms of a variety of applications [including imaging other than the coronary arteries for dual chamber pacemaker placement, to assist with anatomic information prior to ablation procedures, assessment before and after mitral valve clip and TAVR, etc]. The above has been an attempt to provide an overview of a single facet of the value of CCTA in defining coronary artery disease. For more information check out Twitter [@DrRumberger], and check out my YouTube Channel for case presentations and videos of many of my lectures; if you are a physician consider subscribing to my Cardiac CT Workshop, featuring 50 case histories and diagnoses using cardiac CT at Acknowledgment: Many of the Figures Shown Here Were Done in Conjunction with MedMastery.

John A. Rumberger, PhD, MD, FACC, FSCCT

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