Promise of MR and CT in ischemic heart disease

WIDESPREAD CLINICAL ACCEPTANCE of new cardiac imaging techniques traditionally has been slow. This notion is particularly so for MR and computed tomography (CT). It might be surprising for many readers that the first publication on the application of MR in ischemic heart disease was nearly 20 years ago (1). Even more surprising is the realization that the feasibility and potential value of CT in ischemic heart disease can be traced to papers published in the late 1970s (2, 4). Indeed, the concept of delayed contrast enhancement of myocardial necrosis (infarction) was already known using prototype electrocardiograph (ECG)-gated CT in 1979 (2-4). Finally, it seems that MR and CT will have important roles in the imaging of coronary vascular disease. So, with cardiac imaging the future takes a long time. A number of technological innovations have now rendered MR and CT as attractive clinical cardiac imaging modalities. Moreover, the multiple capabilities of cardiac MR and CT render them as attractive for future needs of noninvasive imaging for coronary artery disease (CAD), as well as ischemic heart disease (IHD). For more than three decades, noninvasive imaging techniques have solely addressed the detection of hemodynamically significant coronary arterial stenoses; the extent of ischemia caused by such lesions; and the identification of the presence and extent of myocardial infarction. These objectives have for the most part been addressed by echocardiography and nuclear imaging. Actual demonstration of the responsible coronary arterial stenosis(es) or occlusion has required selective X-ray coronary arteriography. Cardiac MR and CT have the capability to evaluate not only the presence and consequences of hemodynamically significant stenoses and occlusions, but can additionally provide evaluation of the atherosclerotic plaque, arterial wall, and endothelial dysfunction. Genetic profiling of patients has the capability of very specifically identifying patients who will develop progressive CAD and eventually suffer from IHD. A collection of genes associated with myocardial infarction has been identified (5). With genetic profiling at an early age, MR and CT might be used to detect CAD at early and intermediate stages rather than merely near the end of its natural history, typified by advanced atherosclerotic lesions causing hemodynamically significant stenoses or occlusions. The natural history of CAD extends for several decades, starting with endothelial dysfunction in the second or third decade; endothelial thickening in the third and fourth decade; and simple to complex plaque formation along with positive remodeling (arterial expansion) in the fourth to fifth decade. Plaque rupture and healing expands the plaque, causing progressive stenosis by thrombosis, leading to abrupt occlusion with acute myocardial infarction. While the timing of this atherosclerotic process is far from precise, symptomatic disease, morbidity, and mortality can be expected by the sixth to seventh decade. For more than 30 years, this process has mostly been interrogated by echocardiography, nuclear imaging, and X-ray coronary arteriography at the stage of late complex flow-limiting plaque or occlusion in the sixth to eighth decade of life. A special issue of JMRI in 1999 was devoted to MRI in cardiovascular disease (6). The widespread clinical use of MRI and in addition CT has occurred in the past few years. It seems appropriate for an update on new cardiac imaging techniques at this time. The next few issues of JMRI will contain review articles on cardiac imaging. The first two in the current issue are focused on CAD. One article reviews the current status of MRI and the second article reviews the current status of CT. Subsequent issues will feature reviews of MRI in Nonischemic Acquired Heart Disease and MRI in Congenital Heart Disease. The two review articles to follow will demonstrate the multiple capabilities of MR and CT for addressing IHD, but also for evaluating the earlier stages of CAD, such as plaque and mural imaging. While further technological advances and research are required, coronary computed tomographic angiography (CTA) and MR angiography (MRA) can be used to assess the coronary wall as well as providing traditional coronary arterial luminology. Perhaps the earliest stage of CAD is endothelial dysfunction manifested by arterial coronary arterial dysfunction–diminished vasodilator responsiveness. It has already been shown in several publications that MR flow (phase contrast) techniques applied to the coronary sinus can document diminished coronary arterial responsiveness to vasodilators in patients with normal coronary arteriograms (7, 8). There is no need to further elaborate the multiple capabilities of MR and CT; this is done superbly by the authors of the following two articles. It is more important to emphasize the need for futuristic thinking in order to exploit the capabilities of these techniques to more effectively interrogate the early stages of CAD. The future may continue to take a long time if staid thinking merely sees MR and CT as competitors to echocardiography and nuclear techniques for imaging of end-stage CAD (IHD).