Blood-oxygen-level dependent (BOLD) MRI for myocardial ischemia evaluation

Blood-oxygen-level dependent (BOLD) magnetic resonance imaging (MRI) is technique useful in detecting myocardial ischemia. Oxyhaemoglobin and deoxyhaemoglobin differ in their magnetic properties – the former in diamagnetic while the latter is paramagnetic. This difference in magnetic properties can be used as an endogenous contrast to visualize tissue oxygenation. This technique has been termed BOLD MRI. Deoxygenation of hemoglobin causes inhomogeneity in the local magnetic field and hence a change in T2 weighted MRI images. BOLD MRI can detect changes in deoxyhemoglobin levels as a result of changes in blood flow and oxygen consumption. When coronary vasodilation is induced by giving dipyridamole, the levels of deoxyhemoglobin in the coronary venous blood falls as the blood flow increase is disproportionate to the oxygen demand. Occlusion of a coronary artery on the other hand increases the myocardial venous blood deoxyhemoglobin concentration due to myocardial ischemia. In areas supplied by a stenotic coronary artery, there will be no vasodilatory reserve as the capillaries are already maximally dilated. Hence dipyridamole can be used to identify ischemic myocardium by BOLD MRI as these regions will show decreased signal on MRI suggesting hypoperfusion.