Role of CMR in Ischemic Heart Disease Patients - Essay Example

Role of CMR in Ischemic Heart Disease Patients Role of CMR in Ischemic Heart Disease Patients Introduction Cardiac Magnetic Resonance (CMR) has emerged as a safe, non-invasive clinical imaging to create detailed images of organs and tissues over the past few years. The term non-invasive implies that no surgery is necessary to perform the imaging procedure6, 8, 9. CMR applies the use of radio waves, magnets, and a computer to display pictures of the organs and tissues under test15. CMR can create both moving and still images of the heart and major blood vessels. Doctors apply the CMR imaging to take pictures of the beating heart, which they then utilize to choose the most appropriate method to treat patients suffering from heart problems. In particular, CMR quantifies ventricular function, detects myocardial ischemia and scar, and visualizes myocardial edema and hemorrhage1, 10,22,36,38. Doctors also use CMR to explain the results of other tests such as x-ray and computed tomography. A contrast agent such as gadolinium is infused into a vein during cardiac magnetic resonance imaging. The agent moves in the blood to the heart where it highlights the heart and blood vessels on the CMR pictures2, 5, 10, 15-20, 31, 34. The gadolinium contrast agent is often applied to patients with allergic reactions to dyes used in CT scanning. People with severe kidney or liver problems are tested for heart problems with non-contrast CMR imaging. Multiple modalities of CMR are available for detecting ischemia3-7, 10-15, 25, 28, 35. The tests detect the presence of hemodynamically critical coronary artery stenosis like treadmill testing, stress echocardiography and, nuclear imaging. The treadmill testing does not identify the location of the ischemia31, 33, 40. Echocardiography produces excellent quality images with moderate reproducibility. Nuclear imaging has limited reproducibility, limited image resolution, and suffers from attenuation artifacts. Most CMR testing is performed using a pharmacological stressor such as dobutamine, adenosine, and dipyridamole5, 8, 13,17,19,37. Recent improvements in technology have escalated the image quality of the CMR imaging scans. The latest CMRs in the application provide high-resolution images that are not limited by the aural window7 without the use of ionization or iodine contrast agents. The result is an attractive alternative to the other existing non-invasive modalities. The essay herein presents the role of CMR in the diagnosis of patients with ischemic heart disease and how CMR imaging provides prognostic information in such cases. The article begins with an introduction followed by the various roles of CMR imaging to the different cases it applies and a conclusion. Application of CMR in Cine Imaging Doctors apply CMR imaging to visualize the global and regional left and right ventricular dysfunctions due to ischemia by the addition of dobutamine. The assessment usually base on a cine data set aligned with the true left ventricular short axis covering the heart in 10-20 consecutive 2-dimensional slices3-8, 18, 22, 26. It is also possible for the doctors to acquire the 3-dimensional cine data sets covering the whole heart in a single breath-hold. The main advantages of CMR in cine imaging are high tissue contrast and the possibility to produce freely and reproducibly defined imaging planes1, 3,10,16,40. Moreover, CMR is the most accurate and reproducible imaging modality for testing of global ventricular volumes and function. CMR assesses the regional contractile function either by interpreting the cine loops or by measuring wall motion, thickening and strain using the myocardial tagging methods11. Myocardial tagging during low-dose dobutamine stress is used to measure parameters of diastolic dysfunction such as the time to peak untwists. The measured parameters take part in the identification of coronary stenosis. From a contrary perspective, the high-dose dobutamine stress CMR has a greater diagnostic accuracy to identify the inducible left ventricular wall movement abnormalities. The wall motion abnormalities are indicative of flow-limiting coronary stenosis. The patient requires careful monitoring during the process of measuring the blood pressure and heart rhythm throughout pharmacological stressors infusion1, 13, 15. Application CMR in First Pass Myocardial Perfusion CMR application in the current first pass myocardial perfusion tracks the passage of ball shaped T1-shortening contrast agent infused into a peripheral vein. The method applies in conjunction with ultra-fast cine magnetic resonance sequences2, 4, 6-9, 17. The under perfused heart regions associated with myocardial ischemia show a linear difference from the other uninfected areas. The main advantage of CMR in the first pass myocardial perfusion method is the spatial resolution of the perfusion images. The resolution is of the order of 2 to 3 millimeters3. The high resolution means that the doctors can identify the subendocardial ischemia with relative reliability and ease. The recent technological advancements have enhanced the spatial resolution to around 1mm in the imaging plane. The signal to noise ratio and diagnostic yield has also improved because of the technological improvements with the acquisition at 3-Tesla. Both developments enhance the value of CMR perfusion assessment. In typical cases, the clinical practitioners visually interpret the CMR myocardial perfusion results, however, there exists quantitative methods of measuring the characteristics of myocardial signal intensity profiles. The quantitative approaches3, 4, 7 have been validated against the x-ray angiography, SPECT and PET29-31 for the interpretation of CMR results. Recent MR-IMPACT studies reported an improved detection of coronary stenosis by CMR compared with SPECT in the first multicentre, multivendor comparison. Myocardial perfusion CMR imaging is, therefore, useful in the delineation of the micro vascular obstruction and ischemia. Perfusion studies are either performed during resting conditions or the administration of a vasodilator agent, usually adenosine or dipyridamole13. Applications of CMR in Early and Late Gadolinium Enhancement The presence of sarcolemmal disintegration and abdominal washout kinetics increases the distribution of extracellular gadolinium-based contrast agents following an acute ischemic injury. The presence of a fibrotic tissue in chronic myocardial infarction also increases the distribution volume of the contrast agents33-36, 40. T1-sensitive inversion-recovery CMR methods are applied to delineate the resulting differences in contrast distribution between normal and injured myocardium. Imaging within the first few minutes after application of the contrast agents delineates micro vascular obstruction. The resulting effect is the prevention of contrast delivery to the infarct core3, 5, 9 and hence weak signal in T1-weighted imaging11. Consequently, the acutely and chronically infarcted tissue without the micro vascular obstruction retains the contrast agent, therefore, appearing bright. Research reveals the preferred imaging time to be 10 to 20 minutes33, 35 after contrast agent administration when the differences between the scars, normal myocardium and blood pool are greater. In current terms, the literature refers to the method as late contrast-enhanced, delayed contrast-enhanced or hyper- enhancement CMR22. The method has become the reference standard for the in-vivo evaluation of myocardial viability because its very high contrast and spatial definition. These qualities allow for a detailed assessment of the spatial distribution of a scar15. Late gadolinium-enhanced CMR can detect infarction in as little as 1mm of tissue due to the high spatial resolution capacity. The first validation of the technique applied to animal models shows excellent agreement with histology. In comparison, research shows that CMR is more sensitive in detecting subendocardial MI, SPECT or PET and in chronic CAD. In chronic CAD, the extent of CMR envisages the possibility of functional recovery after revascularization17. Application of CMR in T2-Weighted Imaging CMR application in T2-weighted imaging tests the presence of myocardial edema. Edema is a characteristic of the many forms of acute myocardial injury associated with inflammation of the myocardial tissues resulting into increased free water in the infarcted myocardium tissues10, 14. Edema infections alter the myocardial T2-relaxation as it prolongs proton T1- and T2-relaxation and can, therefore, be detected with the T2-weightded CMR imaging. T2-weighted CMR is used to delineate the ischemic risk area following an acute myocardial infarction23, 29, 31. The ischemic risk region usually extends beyond the scar. Edema is visible as bright areas on the T2-weighted Magnetic Resonance Sequences in infarcted myocardium. Edema imaging using the T2-weighted CMR imaging illustrates acute ischemic injury within the first 30 minutes after the start of ischemia before the onset of myocardial injury. For emergency departments, the T2-weighted imaging is added to the MI exam in an unstable angina or MI patients1. The addition is necessary because the development of myocardial edema is an early feature in the acute coronary syndromes. However, the T2-weighted CMR imaging suffers one major drawback of a relatively small contrast to noise ratio between the edematous and normal myocardium3. The limitation combined with slow flowing blood at the subendocardial border makes interpretation of the T2-weighted images harder than the other CMR methods. Fortunately enough, recent methodological research developments promise improvement in the limitations mentioned above20. Application of CMR in Coronary magnetic resonance angiography CMR imaging helps in the delineation of coronary morphology and hence detects proximal coronary stenosis. However, Coronary Magnetic Resonance (Coronary MR) rarely applies in acute cardiac syndrome (ACS) cases where invasive angiography is the preferred test process31. The non-invasive coronary imaging is of little importance in the diagnostic process. CMR offers a broad range of tools that assist in the detection, differential diagnosis and treatment of patients suffering from acute and chronic signs and symptoms of CAD. The recent reduction of data acquisition times for most CMR methods allow for multi-parametric assessment in a single imaging session. The clinical applications of the CMR in ACS detection are continually evolving34, 36. Patients with suspected ACS are managed with the early interventional strategies. However, an initial non-invasive functional test is preferred in low -risk patients or in the presence of related medical problems that increase the risk of complications from cardiac catheterization. In the latter context, the CMR is a more attractive alternative to the established diagnostic methods. CMR is also important in differentiating acute from chronic MI by combining late gadolinium enhancement with T2-weighted imaging3. The result will delineate edema associated with acute infarction. The CMR is, therefore, useful in detecting the early changes following ACS. CMR also sets delayed presentations where the abnormalities on a T2-weighted CMR lasts for up to several weeks as opposed cardiac markers that return to normal after a few weeks16. Application of CMR in Chronic Ischemic Cardiomyopathy and Myocardial Viability Assessment Patients suffering from chronic ischemic cardiomyopathy have single or multi-vessel CAD together with dilated, dysfunctional ventricles containing a mixture of different ischemic substrates in the same or different perfusion territory19. The substrates exist in the form of stunned, ischemic, hibernating, necrotic or scarred myocardium. The need for viability assessment is, therefore, of significance as it helps in deciding whether a patient would benefit from a revascularization procedure or not25. CMR has emerged as the most preferred tool for the viability assessment procedure. CMR viability assessment combines information about wall thickness and myocardial contractility reserve. The process also applies in non-invasive echocardiography since it can visualize even the restrained amounts of myocardial scar formation. The first approach to this assessment procedure is the measurement of end-diastolic wall thickness28. The extent of wall thinning secondary to MI healing and scar formation is closely related to the degree of infarct transmurality. The process has proved to be very sensitive but not specific for prediction of functional recovery. The procedure furthermore indicates that thinned myocardium(less than 6mm)18,27 has a low likelihood of function improvement after revascularization and accurately reflects a scar tissue. The presence of subendocardial infarctions reduces the possibility of function improvement following revascularization. However, thinned myocardial segments may undergo a reverse remodeling process after successful myocardial revascularization with the recovery of function and, therefore, improve in regional wall thickness3, 5, 10. The second approach is contractile reserve assessment during low-dose dobutamine stress. Hibernating myocardial segments usually show biphasic response in patients with chronic left ventricular dysfunction. The state is accompanied by an improved contractility during the low-dose dobutamine infusion, followed by a worsening of function6. Dobutamine stress CMR viability testing process has a high specificity and moderate sensitivity. The reliability values are in line with dobutamine echocardiography. The application of integrated use of CMR imaging techniques significantly improves diagnostic accuracy of the above approaches 14. The first step depicts myocardial scarring to determine the transmural extent. Then dobutamine stress CMR imaging differentiates between the areas with or without contractility reserve in the areas in which the likelihood of recovery is uncertain28. Application of CMR in Prognosis Assessment and Management CMR is evidently a useful tool in the management of cardiovascular diseases. A negative stress perfusion CMR in patients with suspected CAD yields a high negative predictive value for future adverse cardiac events39. Conversely, stress perfusion events predict subsequent undesirable cardiac events. Patients without a history of MI but with clinical suspicion of CAD, who show a presence of myocardial enhancement stands high chances of adverse cardiac events as a result of ischemia-related myocardial scarring risks12, 16, 18. Conclusion CMR imaging is a versatile diagnostic tool for the detection of ischemic heart disease. The tool provides additional information in comparison to other clinical tests for the detection, differential diagnosis and prognostication of ACS. CMR has high accuracy and reproducibility and therefore applies as a reference in the quantitative measurements of left ventricular function. 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