Electron beam computed tomography (EBCT) (C150XP Imatron) is used to determine coronary calcium. The scanner operates in the high resolution volume mode (630 Ma, 130kV, scanning 100 ms, 40 slices @ 3mm, 512 matrix, 26-cm field of view, sharp kernel . Prospective electrocardiographic triggering in mid-diastole (80% of the RR interval) is applied during a single inspiratory breath-hold. The calcium score is calculated with Imatron's implemented software, according to the method of Agatston et al. A calcified plaque was defined as a lesion of at least two adjacent pixels (>0.51mm2) with a signal density >130 Houndsfield units. EBCT can accurately detect coronary calcium, which indicates the presence of coronary atherosclerosis.

Coronary calcium, as seen in the early stage of coronary artery (CAD), is closely correlated with fatty "soft" plaque, which may rupture and cause myocardial infarction. But the correlation between arterial mineralization and the probability of plaque rupture is unknown. In symptomatic patients, EBCT calcium screening has been used to estimate the severity of stenosis. No close correlation, however, could be found between the amount of calcium and arterial lumen narrowing.

A recent study of 1,764 symptomatic patients with suspected CAD showed that calcium screening with EBCT is a highly sensitive and moderately specific test to predict stenotic disease. Exclusion of coronary calcium defines a substantial subgroup of patients, albeit symptomatic, with a very low probability of significant stenoses.

The calcium scores for men and women shown in table 1 and table 2 indicate that:
1) the mean score in men was significantly higher than that in women at each age class;
2) for both genders, there was a significant increase in calcification with age;
3) the variability of scores within each age class was high; this was caused by a minority of patients with excessive calcification; and
4) the difference between patients with and without significant CAD was highly significant for the total group of patients and within each age group.

Exclusion of calcium in this study: No calcium was found in 128 (23.7%) of men and in 116 (40.8%) of 284 women without significant CAD, as compared with 5 (0.7%) of 685 men and 0 of 255 women with coronary stenoses approximately 50% (table 3). Thus, exclusion of calcium was associated with an extremely low probability of significant stenosis in men and women.

In contrast, there was a considerable number fo patients without stenoses who, although symptomatic, did not reveal any calcification: 55% of symptomatic men and women< 40 years of age had no calcification. In this group, the risk of stensis was 0%.

Clinical useful score cutpoints for management decisions:

In figure 158a (for men) and figure 158b (for women), the scores are divided into three levels for each age group: the lower range defines 95% of patients without stenoses. Thus, up to the lower score threshold, significant CAD in a symptomatic person is very unlikely. The upper range includes 90%of patients with significant stenoses. Calcium levels beyond the upper score threshold are associated with obstructed disease in the majority of patients. In the white zone, the diagnosis is uncertain. The white zone consisted of 98 men (8%) and 40 women (7.40 %). A narrow white area indicates an excellent test variable, with only a few patients in the indefinite zone (approaching a "yes" or a "no" decision). According to this, EBCT calcium screening was especially helpful in men<50 years old and in women<60 years old.

Mechanism of calcification:

Coronary calcium predominantly consists of calcium phosphate, which is not only absorbed or deposited passively, but also forms in situ by organized, regulated process.

It is strongly associated with the total plaque burden, but it is not a direct marker of the vulnerable plaque; but the greater the calcium score, the greater the potential for increased numbers of potentially lipid-rich plaques, which are widely thought to be the culprit lesions of acute coronary syndromes.

Lesser obstructive plaques are associated with a higher incidence of coronary occlusion, as compared with severely obstructed plaques. This occurs not because of the higher risk of such plaques in and of themselves, but because of their much greater number.

Clinical implications:

In patients with typical angina (provoked by physical or emotional stress) and/or a definite diagnosis of myocardial ischemia on conventional stress tests, there is no indication for EBCT calcium screening because of the extremely high pretest likelihood of significant stenosis.

Coronary angiography is required for the definite diagnosis and treatment in these patients.

There is an indication for calcium screening with EBCT in patients with atypical chest pain, especially if noninvasive tests are not feasible or inconclusive. Calcium screening has potential as a filter in symptomatic patients to reduce the number of invasive procedures which do not lead to intervention.

A recent study shows that EBCT is an efficient and safe test to use in patients presenting to the Emergency Department with acute chest pain, allowing rapid discharge with a negative scan.It also confirms that the absence of calcium implies a very low (0.6% annual) incidence of coronary events,whereas the presence of coronary calcium is an independent predictor of future cardiac events,supporting prior reports showing that coronary calcium to be a strong predictor of future cardiovascular events.

Reference: Haber, R. et als., Correlation of Coronary Calcification and Angiographically Documented Stenoses in Patients With Coronary Artery Disease:Results of 1,764 Patients,JACC2001;37:451-7.

Reference:Georgiou,D. and Others,Screening Patients With Chest Pain in the Emergency Department using Electron Beam Tomography:AFollowupStudy,JAMCC,Vol.38,No.1,2001,pp.104-109.