Noninvasive cardiovascular imaging: Cardiac CT, Cardiac MRI

CT and MRI are both being used more frequently to help diagnose disease of the heart and great vessels.
High-speed helical (spiral), non-contrast enhanced Cardiac CT scans are being used more frequently to detect calcium deposits found in atherosclerotic plaque in the coronary arteries before symptoms develop (Calcium-Score Screening Heart Scan). Although the predictive value of coronary calcium score screening is not fully defined, more coronary calcium indicates more atherosclerosis, and a greater likelihood of arterial narrowing and future cardiovascular events. Advanced multiple detectors CT (MDCT) is also being used more and more to allow the identification of both calcified and non-calcified plaque within the coronary artery walls or to exclude the possibility of significant stenosis due to atherosclerosis. When contrast-enhanced 3-dimensional versions of this type of scanning are performed, early stages of atherosclerosis of the coronary arteries can be seen before the development of arterial narrowing. It allows early detection of potentially unstable "soft plaque" and non-calcified plaque during a potentially reversible phase of development, making it possible to non-invasively monitor plaque regression with various therapies. MDCT showed a high negative predictive value for coronary stenosis, but it had a relatively variable degree of positive predictive value in several studies. Cardiac CT have a additative role in diagnosis in coronary artery disease combined with coronary angiography. In addition, post-bypass surgery evaluation, left ventricular function, and valvular morphology and function were also able to be evaluated.
Cardiac MRI has many potential applications, such as evaluation of coronary artery anatomy and flow and myocardial perfusion. In the future, it may be possible to use MRI to track progression of atherosclerosis in both the coronary and carotid arteries. Because of its non-invasive nature, repeated MRI examinations could be used to identify those at increased risk for heart attack. MRI is already being considered to evaluate myocardial viability, as an alternative or adjunct to a stress echocardiogram and/or nuclear heart examination, in order to provide additional valuable information, such as the condition of the myocardium and wall motion. With high-speed MRI, the physician can evaluate left ventricular wall motion during physical stress. Last, studies looking at the ability of the MRI to assess myocardial metabolism are also underway.
Techniques are also being developed to combine the data acquired by Cardiac CT angiography of the coronary arteries and the data regarding myocardial viability acquired by Cardiac MRI. Based on the co-registration of the CT Angiography and MRI data, a spatial relationship can be directly established between the diseased coronary artery distribution and the myocardium at risk. With the advancement of these imaging modalities, patients may expect to realize improved pre-revascularization planning and reduced invasiveness of the diagnostic process.
Despite its advantages, these tests are not a substitute for other imaging techniques in all cardiovascular conditions. Unlike an echocardiogram machine, the MRI and CT scanners cannot be brought to the bedside of an acutely ill patient. Nonetheless, as previously mentioned, Cardiac MRI is already being used more and more as the examination of choice in certain settings (e.g. assessment of myocardial viability). In addition, early success with non-invasive angiography of coronary arteries based on advanced Cardiac CT techniques has led to its routine application in certain settings (e.g. coronary artery bypass graft patency evaluation, detection of anomalous coronary arteries, and clarification of equivocal results from other screening exams for coronary disease). These tests have already become an integral part of the evaluation of patients with various forms of cardiovascular disease as promising new versions continue to develop. Careful further clinical studies of appropriate size and design will be needed to clarify the role of coronary CT and cardiac MR angiography, especially in the context of stable and acute coronary syndrome.

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