CT and MRI provide impetus in cardiac imaging revolution

Coronary heart disease is one of the leading causes of morbidity and mortality in developed countries.¹ Accurate detection of early cardiac disease is of utmost importance for the delivery of appropriate treatment.

The top three modalities used by cardiologists to diagnose cardiac disease are echocardiography, SPECT, and cardiac catheterization.² The use of cardiac CT and cardiac MRI, however, is expected to increase significantly over the next three years and alter this ranking.² Both modalities have progressed from being novel research techniques to routine diagnostic tools. They promise to become valuable additions to the radiological armamentarium for the diagnosis of cardiac diseases.

Several publications have demonstrated the high degree of diagnostic accuracy that both CT and MRI can deliver and the possibility of an accurate risk assessment with these modalities.^3-6 Cardiac imaging has never had such a prominent, expansive body of evidence indicating the role of radiology in patient management.⁷

The ability of cardiac CT to evaluate the coronary arteries places it in direct competition with cardiac catheterization. Because cardiac CT is noninvasive, morbidity associated with the procedure is substantially lower than that for catheterization. It is also cheaper and can visualize both the lumen and the arterial wall, improving identification of early plaque formation.

A large number of publications have shown that the diagnostic performance of cardiac CT compares well with that of catheterization.^8-12 The optimal use of cardiac CT is still a matter of some debate, however. The American College of Cardiology published a consensus document in 2006 defining the appropriate uses of both cardiac CT and cardiac MRI.13 In the meantime, two major innovations in CT technology-64-slice and dual-source scanning-have been introduced. The rapid pace of innovation is outstripping the rate at which strategies for clinical application can be decided.

The ACC consensus statement lists the following indications as being appropriate for cardiac CT:

• diagnosis of coronary heart disease in a symptomatic patient with a medium pretest probability of coronary artery disease who has an inconclusive ECG or who is not able to undergo a stress examination;
• evaluation of coronary arteries with new-onset cardiac insufficiency; and
• evaluation of coronary artery anomalies (Figure 1).¹³

In practice, a large number of cardiac CT referrals are for evaluation of coronary artery bypass grafts, exclusion of coronary artery disease in patients with a medium pretest probability of the disease, evaluation of coronary arteries prior to noncoronary cardiac or noncardiac surgery, evaluation of cardiac anatomy prior to minimally invasive cardiac valve or coronary surgery, and resolution of questions raised by other imaging modalities. An example of this last indication would be failure of selective engagement of a coronary artery by catheter angiography.

The most important indication for cardiac CT is the exclusion of coronary artery stenosis in patients suffering from atypical chest pain who have a low to intermediate pretest probability of the disease and an inconclusive stress test (Figure 2). This indication has also been recommended by the European Society of Cardiology

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and the American Heart Association.

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Calcium deposits within the coronary artery wall that are measured by unenhanced low-dose cardiac CT (i.e., calcium scoring) can serve as an independent and powerful prognostic parameter for the subsequent development of major cardiac events.

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MRI offers better contrast resolution than CT and has better temporal and spatial resolution. Today's MRI technology cannot, however, provide a detailed assessment of the coronary artery lumen with resolution equivalent to CT.

Developments in high-field technology and the advent of fast imaging sequences have resulted in cardiac MRI emerging as the new reference modality for assessments of myocardial function, perfusion and viability imaging in ischemic and nonischemic cardiomyopathies, assessment of cardiac tumors (Figure 3), and evaluation of complex congenital heart disease prior to and after surgery.

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Whole-heart acquisitions allow the entire heart to be scanned in a manner similar to cardiac CT, producing 3D data sets that can be reformatted in any desired orientation.

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A NONINVASIVE FUTURE?

Additional technical developments should increase the clinical value of cardiac CT and MRI still further. CT systems with greater detector coverage will permit dynamic volume imaging of the entire heart without table movement. This could enable perfusion imaging and allow practitioners to monitor contrast filling and the direction of flow in coronary vessels. Novel cardiac MRI techniques combining high temporal and spatial resolution may make it possible to produce accurate coronary artery assessments.

Concerns over radiation dose will continue to be addressed. Most initiatives are aimed at reducing dose to a level that is as low as reasonably achievable (the ALARA principle). Implementation of step-and-shoot cardiac CT is one such example. This technique is associated with an effective radiation dose of just 1 to 3 mSv.¹⁷ Another strategy is to lower the tube voltage to 100 kV in patients with a normal body weight or body mass index.¹⁸

Cardiac CT and MRI already provide more information about atherosclerotic plaque than does catheter angiography. Dual-source CT examinations carried out with two beams of x-rays that have distinctly different energies can be used to improve material differentiation. It is possible that this technique may replace methods of CT plaque characterization that rely on Hounsfield units. MRI contrast agents are also being developed that promise to identify vulnerable plaques. These agents work by targeting mediators of neovascularization or macrophages in inflamed plaques.^19,20

Combining noninvasive imaging modalities for assessments of cardiac morphology and function will give new insights into the mechanisms of cardiovascular disease. This approach promises to have a significant impact on clinical decision making throughout the duration of therapy.

RIVALS AND COMPLEMENTS

Neither CT nor MRI will ever replace any of the existing cardiac imaging modalities, regardless of advances in resolution. Echocardiography, for example, can be performed as a bedside examination, which is a considerable advantage for acutely ill patients. Cardiac catheterization offers the option of performing coronary interventions on the basis of imaging results. Cardiac CT and MRI are both purely diagnostic techniques and offer no such therapeutic advantage.

But cardiac CT and MRI have become an integral part of the diagnostic workup for patients suffering from various cardiovascular pathologies. These tools will be used more and more in the future as an alternative to traditional imaging modalities, eventually becoming the methods of choice for a wider range of indications.

References

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www.who.int/mediacentre/factsheets/fs317/en/index.html

]
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