The number of coronary CT angiographies performed in Europe has already overtaken the practice of calcium scoring, according to a 2007 European Society of Radiology survey on cardiac radiology. Is calcium scoring, then, a technique of the past? To answer this question, it is important to examine the clinical applications for both techniques.
The coronary calcium score (CCS) is an independent predictor of an individual's risk of coronary artery disease. The predictive value for all-cause mortality is improved when the CCS is added to the Framingham risk score, compared with Framingham risk scoring alone.1
Calcium scoring is advocated for asymptomatic patients with an intermediate risk of coronary artery disease but not for those individuals with very low or high risk. Its usefulness for repeated examinations is questionable, given that variability is approximately 20%.2 Natural variations according to age, sex, and ethnicity should be noted, as should the tendency of diabetic patients to have, on average, twofold higher calcium scores. These factors should all be taken into account when interpreting the CCS. Patients with end-stage renal disease may also exhibit extensive media layer calcification, recording scores of up to 4000 Agatston units.3
A review of 35,765 patients, pooled from multiple studies conducted between 1975 and 2007, showed that a zero calcium score excluded cardiac events over a 10-year period with an excellent negative predictive value of 99.9%.4 The cardiac event rate was very low (0.03%) in the 16,106 patients who had zero calcium scores, but it was not zero.
This finding illustrates a limitation of calcium scoring: CCS can provide a gross estimate of plaque burden, but it cannot detect noncalcified plaque.
Coronary CTA, on the other hand, allows noncalcified plaques to be visualized directly (Figure 1).5 These deposits represent 80% of the total plaque burden. New data indicate that the correlation between calcified and noncalcified plaque is not strictly linear and that it may depend on various individual factors involved in the pathogenesis of atherosclerosis, possibly including sex and age. A study of patients with low calcium scores, in which 56% of individuals had noncalcified plaque with mild (< 50%) stenosis, showed the prevalence of significant coronary stenosis to be a worrisome 8.7%.6 This finding highlights the fact that coronary CTA may be a more accurate tool for evaluating overall plaque burden, compared with the calcium score.
VULNERABLE PLAQUE
Plaque burden, and specific noncalcified plaques, could be potential risk factors for acute coronary syndrome. Prospective data are lacking, but retrospective studies have shown that acute coronary syndromes are associated with noncalcified and mixed plaques.7 A subset of noncalcified plaques with specific criteria, such as a lipid-rich necrotic core, are considered to be at increased risk of rupture (vulnerable plaques), which, in turn, could cause an acute coronary event. The lower the CT density of a noncalcified plaque, the higher may be the chance of that plaque having a lipidrich component.5 Still, the spatial resolution of coronary CTA is too low to directly image a lipid-rich core and thus a vulnerable plaque.
Another sign linked with plaque vulnerability is positive remodeling of the vessel wall. This “plaque eccentricity” often can be visualized well on coronary CTA (Figure 1).
