Although many markers for atherosclerosis have been proposed, the challenge is to identify one that may be applied across broad spectrums of the population, at a reasonable price, and is highly effective at defining low- to high-risk patients. Computed tomographic measurements of the extent of coronary artery calcification (CAC) fulfill these criteria as an effective marker of atherosclerosis.1 It is important to note that arterial calcification occurs as a stabilizing force within an atherosclerotic lesion.1 As such, it remains controversial as to whether CAC is a marker of disease burden or a coexisting factor. The balance of evidence suggests the latter, whereby CAC is present along with non-calcified plaque that may be more vulnerable to rupture and advancing, progressive disease states.1 But, importantly, CAC coexists and as its extent increases, so does the global burden of disease. Understanding this concept of CAC as it relates to other forms of plaque remains key to comprehending why it is such an effective risk stratifier.
Within the past decade, there has been an explosion of evidence as to the role of CAC scanning in identifying risk in an array of patient cohorts as well as in population-based registries.2–9 Understanding the difference between these two bodies of evidence remains important in the application of this modality in everyday clinical practice. First, the large patient series, including patients that are generally higher risk than the general population, can aid clinicians in using CAC scanning in their at-risk patients.5 Moreover, the population series may aid clinicians who are considering embarking on screening at-risk individuals within a community.5,8 In this chapter, we will examine each of these types of evidence separately and put forth strategies for their utility as an effective means to define at-risk individuals.
What is clear from both patient and population-based series is that the more extensive CAC, the higher the risk in that patient. Generally, computed tomographic measurement of CAC is calculated by examining the Agatston score.10–12 An example of calculation of the Agatston score is noted in Fig. 24-1. There are other methods of scoring CAC but this is the most common and will be largely discussed in this chapter. Of recent, investigators from the NIH–NHLBI-sponsored Multi-ethnic Study of Atherosclerosis (MESA) registry published an alternative approach that includes the coverage of calcification.13 This approach may gain more popularity as it provides a similar estimation of disease burden that is commonly applied in the angiographic literature. However, current practice largely utilizes the easy-to-calculate Agatston score.
How to calculate the Agatston score? The Agatston score is calculated as the product of the area (A) by the density coefficient (Dcoef) of the calcification, as measured by Hounsfield units (HU). Only HU >130 m2 is included in these calculations and the range of HU is noted in ...