Blood Oxygen Level Dependent (BOLD)
Blood–oxygen-level-dependent (BOLD) imaging is a technique used to generate images in functional MRI (fMRI) studies. The goal of this technique is to discern regional differences in cerebral blood flow in an effort to delineate more specific regional activity. This version of magnetic resonance imaging depends on the different magnetic properties of oxygenated versus deoxygenated hemoglobin and thus, indirectly, on variations in local tissue perfusion. The utility of BOLD imaging for functional magnetic resonance imaging (fMRI) also depends on the physiological phenomenon by which metabolically active cerebral tissue “demands” more perfusion than less-active tissue. Thus, populations of neurons that are particularly active during a cognitive or motor task actually elicit a relative surplus of perfusion, which, in turn, results in an increase in the ratio of oxygenated to deoxygenated hemoglobin, detectable as a change in the BOLD signal.