The role of in-vivo cardiac magnetic resonance (CMR) in the assessment of myocardial tissue ischemia

Coronary artery disease (CAD) with its clinical appearance of stable or unstable angina and acute myocardial infarction is the leading cause of death in developed countries. In view of increasing costs and the rising number of CAD patients, there has been a major interest in reliable non-invasive imaging techniques to identify CAD in an early, asymptomatic stage. Since impaired myocardial perfusion appears very early in the "ischemic cascade", it would be a major breakthrough if the non-invasive quantification of myocardial perfusion before functional impairment could be detected. In CAD, the revascularization strategy is guided by three factors: the evaluation of the coronary anatomy, the functional assessment of the hemodynamic significance of stenosis and the extent and distribution of ischemia. The most widely used techniques in the assessment of myocardial perfusion have significant limitations such as attenuation artifacts susceptibility for SPECT and higher expenses for PET. Both SPECT and PET involve exposure to ionizing radiation. However, Cardiac Magnetic Resonance (CMR) is emerging as an alternative noninvasive approach for the assessment of myocardial perfusion. CMR offers superior spatial resolution, does not involve ionizing radiation and like PET, has the potential to measure myocardial perfusion in absolute terms. Detection of CAD by CMR is based on the evaluation of reversible wall motion abnormalities(RWMA) during dobutamine stress. More recently CAD has also been determined on the visualization of myocardial perfusion by T1-weighted imaging after gadolinium contrast injection. CMR also assesses cardiovascular anatomy and function, infarct size, myocardial strain and metabolism with excellent spatial and temporal resolution. Emerging hybrid positron emission tomography/magnetic resonance imaging (PET/MRI) scanners have considerable potential for cardiovascular imaging. Hybrid imaging combines this information with PET, a highly sensitive and quantitative modality that can follow rare molecular events. Independent of perfusion, CMR can also assess myocardial oxygenation consumption through the application of blood oxygen level-dependent (BOLD) imaging based on magnetic susceptibility differences between oxyhemoglobin and deoxyhemoglobin.