Calcium scoring, coronary CTA display strengths

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 it justified, then, to label calcium scoring 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 to 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 lipid-rich 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).

The real strength of coronary CTA is its high sensitivity in detecting vessel stenosis (>50%)(Figure 2) and its excellent negative predictive value (~98 to 99%) in excluding stenosis.

Coronary CTA is best used in patients with clinically suspected coronary stenosis. These will mainly be symptomatic patients with intermediate pretest probability of coronary artery disease. Calcium scoring is best suited to asymptomatic patients at intermediate risk of coronary artery disease for general risk stratification.

Joint guidelines from the American College of Cardiology Foundation (ACCF) and the American College of Radiology⁸ and guidelines from the European Society of Cardiology⁹ state that coronary CTA mainly benefits patients with intermediate pretest probability and acute or atypical chest pain syndrome who have equivocal or uninterpretable ECG stress test findings. The high accuracy of coronary CTA may even justify its use as a firstline noninvasive test in the near future. Current clinical indications for coronary CTA are summarized in the table.

A limitation of coronary CTA relates to the evaluation of patients with severe coronary calcification. Related artifacts, including motion blurring, and subsequent overestimation of stenosis, can lead to false-positive findings, particularly once the calcium score rises above 400 Agatston units.

Some researchers have proposed that calcium score act as a gatekeeper and that only patients without severe calcification go on to have coronary CTA. This approach remains controversial owing to the implications regarding radiation exposure, particularly in younger patients and women.

Coronary CTA has shown excellent performance in the evaluation of bypass graft patency.¹⁰ Bypass graft assessment can be limited, however, by surgical clips, particularly at the distal anastomosis, and in the presence of heavily calcified native coronary arteries. Coronary CTA is consequently useful in patients who are not primary candidates for invasive angiography, such as those with atypical chest pain or acute chest pain but negative serial enzymes and nonspecific ECG abnormalities. The technique's ability to evaluate other extracardiac thoracic pathologies causing chest complaints is an advantage in this setting.

Functional 4D cine imaging of valves can be performed quickly and easily with coronary CTA using the same data set acquired for evaluation of the coronary arteries. Suspected aortic stenosis can be assessed on planimetry of the systolic aortic valve area. This can be useful in patients presenting with aortic valve calcification. ¹¹ Coronary CTA can also visualize incomplete diastolic aortic valve closure, which indicates aortic regurgitation (Figure 3).¹²

Patients with infective endocarditis may benefit twofold from coronary CTA. Coronary artery disease can be excluded noninvasively, avoiding the risk of embolization due to mobile valvular vegetations during cardiac catheter manipulation. Paravalvular involvement, such as abscess or pseudoaneurysm, may also be evaluated better on 3D CTA images. This information may be of relevance during preoperative planning.¹³

Cardiac CTA can also provide estimates of left ventricular function.¹⁴ Imaging of myocardial perfusion defects, wall motion abnormalities, and viable versus nonviable myocardium have all shown promising initial results.¹⁵ A number of ongoing studies are focusing on these topics. Technical developments, such as 320-slice CT systems and dual-source CT, may speed up improvements in functional imaging. The role of PET/CT (Figure 4) is being explored as well, and this could complement myocardial perfusion and viability imaging.

PRACTICAL POINTERS

Several issues should be taken into account when launching a cardiac CT program. More intense collaboration with clinical partners will be necessary if the program is to be successful. Good patient preparation is important for obtaining high-quality images.

Good quality CT data sets are an essential prerequisite for accurate interpretation by physicians. The patient's heart rate should be as regular as possible and as low as possible (< 65 bpm) prior to imaging. This can be achieved most effectively by using beta blockers. Patient preparation with nitro can help to improve the interpretation of coronary arteries because of its vasodilatatory effects.

Rapid communication of coronary CTA findings to the cardiologist may sometimes be necessary, and mechanisms and communication systems must be in place to allow this.

Practitioners dealing with symptomatic chest pain patients should review the patient's current symptoms and latest test results. This review may include an interview with the patient to obtain accurate information, particularly in cases of unstable coronary artery disease.

European Society of Urogenital Radiology guidelines state that patients should be monitored for 30 minutes after administration of iodinebased contrast in case of possible allergic reactions. This is a good time to do an initial read of the coronary CTA images and if necessary, communicate findings rapidly to a cardiologist.

High-quality, dedicated physician training is a major factor in the success of a cardiac CT program. Didactic material must be mastered as well as the manual skills of image acquisition and manipulation of images on the workstation. At least one basic threeday training course, including the interpretation of 50 cardiac CT scans, is recommended for ACCF/American Heart Association (AHA) level one accreditation.¹⁶

In practice, however, level-one accreditation does not provide sufficient confidence to establish a diagnosis in difficult cases. We recommend additional training to reach ACCF/AHA level two competency, which requires interpretation of 150 cardiac CT cases. Other training methods are available, but there is no substitute for hands-on experience and practice working through cases.

The most important skill to be mastered is the interpretation of coronary CT angiograms. This requires individualized instruction on 3D workstations, including the manipulation of data sets. This skill is obtained most effectively in small groups, ideally with one student per workstation (maximum two students per workstation) and with a high faculty-to-student ratio.

It is crucial to learn how to handle different postprocessing techniques, especially interactive multiplanar reconstructions, maximum intensity projections, and the typical views and angulations of each coronary artery (Figure 1). Training courses should have dedicated and experienced faculty who are present in the classroom during instruction. Cardiac CT acquisition should be learned by participating in live cases at specialized cardiac imaging centers.

In conclusion, coronary CTA is a robust and promising modality that is complementary to calcium scoring in many respects. Coronary CTA is clearly superior to calcium scoring in a number of ways, and this distinction remains an area of active research. The calcium score is the preferred tool for estimating the risk of coronary artery disease risk, but coronary CTA has created several new clinical applications. These include the noninvasive diagnosis of coronary artery stenosis (>50%) and plaque and the functional evaluation of valves and the myocardium.

The full potential of cardiac CT remains to be explored. More data will doubtless become available over the next few years. Rapid advances in CT scanner technology are also creating new opportunities and applications for this modality. The higher temporal resolution of dual-source CT and the increased volume coverage of 320-slice CT systems promise to yield further applications, possibly including myocardial perfusion imaging.