Coronary Artery Calcium

Fig. 11.1

This graph, from the SCORE (Systematic COronary Risk Evaluation) database, illustrates a key feature of prevention strategies. Known as the “Rose prevention paradox,” the figure illustrates the seemingly contradictory situation that the majority of events from a disease occur in members of the population at low or moderate event risk, while only a minority of events occur in the highest-risk segment of the population. See text for additional details. CVD cardiovascular disease, SCORE Systematic COronary Risk Evaluation (With permission from Cooney et al., European Journal of Preventive Cardiology 2009)

11.2 Clinical Use

11.2.1 Measurement of Coronary Calcium

In the coronary arteries, calcifications almost always occur in the setting of atherosclerosis. The only known exception is renal failure, which may be associated with medial (nonatherosclerotic) calcification of the coronary artery wall along with atherosclerotic calcification. Even in renal failure patients, extensive intimal calcification is also present, contributing to the exceptionally high cardiovascular mortality. In fact, the coronary calcium score is considered to provide an estimate of vascular age in patients without renal failure. It has been shown that the amount of coronary artery calcium reflects the total atherosclerotic burden, including both calcified and noncalcified plaques. The quantitative relationship between calcified and noncalcified plaque components, however, is not uniform and, in some individuals, only noncalcified plaques are present.

Coronary artery calcium can be detected on noncontrast cardiac CT (Figs. 11.2 and 11.3). To quantify coronary artery calcium, the area and density, volume, or mass of calcified deposits is measured. Several quantification methods exist: (1) the Agatston score (Tables 11.1 and 11.2), (2) the volume score, (3) the derived calcium mass, and (4) the calcium coverage score (Table 11.2).

Fig. 11.2

Considerable information on cardiac structures can be garnered from noncontrast CT. The left anterior descending (LAD), left circumflex (LCX), right coronary artery (RCA), and the ascending aorta (Ao) can be seen, as labeled. The four chambers of the heart are also seen and labeled as RA (right atrium), RV (right ventricular outflow tract), LA (left atrium), and LV (left ventricle). Relative chamber sizes can be estimated from noncontrast CT. The pericardium is visible as a thin line (arrowhead). The inter-atrial septum is also seen (asterisk)

Fig. 11.3

Chronic apical infarction seen on a noncontrast scan as an incidental finding in a 50-year-old female patient with a calcium score of zero. Panel A shows the apical infarction with thinning and fatty degeneration of the myocardium (arrow), while a nearby calcification is also visible on a four-chamber view reconstructed from the non-contrast acquisition (arrowhead). Panel B shows a 5-mm average reconstruction in the four-chamber view from contrast-enhanced CT angiography in the same patient, delineating the thinned fatty myocardium (arrows) and an apical thrombus (arrowhead) that contains the calcification

Table 11.1

Classification of calcium scores using the Agatston method

Agatston units	Ranking
0	Absent
>0 – 10	Minimal
>10 – 100	Mild
>100 – 400	Moderate
>400 – 1,000	Severe
>1,000	Extensive

The thresholds shown here are empirical

Table 11.2

Comparison of four methods for coronary artery calcium (CAC) scoring

	Agatston score^a	Volume score	Derived calcium mass	Calcium coverage score
Calculation of calcium score	CS^b = W^c × A^d	VS^e = Vnox^f × Nvox^g	Cac^h × Vcpⁱ	Affected 5-mm segments/total number of 5-mm segments
Advantages	Most commonly used method for which substantial reference data exist	High reproducibility	Accurate and little variability	Helpful in conveying to the patient the percentage of his/her arteries that have calcified plaque
Advantages		Useful for assessing progression of coronary calcium	Accurate and little variability
Disadvantages	Low interscanner agreement	Partial volume effects may impair accuracy	Difficult to measure	Requires accurate tracing of the coronary arteries, which adds more reading time
	Nonlinearity with respect to amount of calcium	Less clinical data available	Requires a phantom to be placed under the patient for calibration	Not well established clinically
	Dependence on noise		Less clinical data
	Score does not correspond to a physical measure		Less clinical data
Interscan variability	15–20%	10–12%	10–12%	No data
Unit	mm²	mm³	mg of calcium hydroxyapatite equivalence	Percentage (%)
Clinical use	Most commonly used	Can be used to measure progression of atherosclerosis	Best measure of the amount of calcium in coronary arteries	Too tedious for clinical practice

^a The Agatston score was first described by Agatston et al. in 1990. The Agatston score is the traditional method of quantifying coronary artery calcium

^b Calcium score by Agatston method for a single lesion, Agatston scores for each artery, each calcification, or the entire heart—sometimes called total calcium score (TCS)—are calculated by summing the respective values for the regions of interest

^c Weighing factor, where W = 1 if 130 HU ≤ CT max <200 HU, W = 2 if 200 HU ≤ CT max <300 HU, W = 3 if 300 HU ≤ CT max <400 HU and W = 4 if 400 HU ≤ CT max

^d Area of lesion in mm²

^e Volume score

^f Volume of one voxel

^g Number of voxels

^h Calcium concentration

ⁱ Volume of calcified plaque

The prevalence and extent of coronary artery calcium increases with age, and both are higher in men compared with women at any age. The prognostic value of absolute coronary artery calcium scores and calcium ‘percentiles’ – relative to age and gender – has been compared in the Multi-Ethnic Study of Atherosclerosis. In this study, although both scoring methods yielded effective risk stratification, the absolute measurement approach performed better than the percentile approach. Therefore, reporting the absolute coronary artery calcium score is currently recommended for clinical practice.

In cardiac CT imaging, certain noncardiac incidental findings may be seen such as thoracic and abdominal diseases including liver cysts, calcified lymph nodes, emphysema, and lung nodules. Other calcified tissues including cardiac valve leaflets can resemble coronary artery calcium and must be interpreted correctly in order not to falsely increase the total “coronary” calcium score (Fig. 11.4). Misinterpretation is highly unlikely in experienced centers, as these different entities are easy to distinguish from each other. Importantly, aortic valve calcification has independent prognostic value for risk stratification. It is important to distinguish all of these noncoronary findings so that they can be properly diagnosed and managed.