Coronary artery calcium measurement with cardiac CT has been used for cardiovascular risk assessment since 1990.1 The American College of Cardiology (ACC) and American Heart Association (AHA) describe calcium scoring as an independent predictor of coronary events in asymptomatic subjects. 2 Coronary artery calcium measurements can improve risk stratification by reclassifying asymptomatic individuals considered to be at intermediate risk of coronary artery disease (i.e., they have a 10% to 20% risk of a coronary event during the next 10 years) into higher or lower risk categories. Patient management can then be modified as necessary; for example, through encouragement to adopt a healthier lifestyle or introduction of medical therapy.
Recommendations published by the European Society of Cardiac Radiology and the North American Society for Cardiovascular Imaging note that a calcium score of zero will exclude most clinically relevant coronary artery disease in asymptomatic individuals.3 A zero score is associated with a low chance of relevant (≥50% reduction in diameter) coronary artery stenosis in symptomatic patients, such as those with chest pain. This is not always the case, however, because soft plaques could be causing the symptoms. Coronary CT angiography should not be ruled out in these patients.
A high coronary calcium score—for example, greater than 400 in a 50-yearold man—is associated with a very high (>90%) probability of relevant coronary artery stenosis. Immediate conventional coronary angiography should be considered in such cases.2
Several factors limit the use of calcium scoring in clinical practice. Coronary calcium is a sensitive indicator of coronary atherosclerosis, but it is not specific for the detection of relevant coronary artery stenosis (91.8% sensitivity, 55% specificity).4,5 The presence of calcium in a plaque also cannot be considered as an indicator of that plaque's biological behavior: for instance, its clinical stability or vulnerability. Many studies have additionally reported wide differences among ethnic groups in terms of the prevalence of coronary artery calcium. The clinical implications of coronary calcium scores should consequently be interpreted with caution.2,3
Studies reporting the clinical usefulness of calcium scoring are based mainly on electron-beam CT (EBCT). This modality has traditionally generated the most reproducible coronary calcium measurements. The growing availability of multislice CT in radiology departments means that this modality is increasingly replacing EBCT for the evaluation of coronary heart disease. Published reports now show that MSCT is comparable to EBCT for coronary calcium measurements.6,7 MSCT data acquisition techniques are not standardized, however.
The accuracy and reproducibility of calcium measurements made with MSCT need to be guaranteed. ACC/ AHA training guidelines have established that calcium scoring should be a formal part of cardiac MSCT training programs.8 Care must still be taken to avoid some of the pitfalls that can alter the accuracy of coronary calcium scoring with MSCT.
SCAN TECHNIQUE
We perform MSCT coronary calcium scoring with a 64-slice scanner (Sensation 64, Siemens Medical Solutions). Details of the protocol and parameters are summarized in the table (above). The parameters meet minimum requirements for coronary artery calcium scanning proposed by the AHA (Figure 1).9
Beta blockade is mandatory in patients with heart rates higher than 65 to 70 bpm to minimize the effects of cardiac motion. If the prospective gating technique (step-and-shoot) is used, data reconstruction in an early diastolic phase (40% of R-R interval) could be helpful. It can reduce interscan variability in calcium score comparing to the phase of 80% of the R-R interval, commonly used in EBCT.10
Mild abnormalities to the heart rate that occur during scanning, such as premature beats, can be compensated for manually using a spiral technique with retrospective ECG gating. Retrospective ECG gating is associated with higher radiation exposure, however, and x-ray tube modulation should be used to lower this.
