The blood oxygen level dependent signal that serves as the basis for current methods in fMRI. First used to generate functional images of the human brain in the early 1990s, BOLD fMRI uses the intrinsic paramagnetic signal ofdeoxyhemoglobin (dHB) to detect local hemodynamic changes in blood flow associated with neuronal activity. Because it is strongly paramagnetic and is present in the draining veins after oxygen has been unloaded in the tissues, dHB can be used to depict changes in blood oxygenation, unlike oxyhemoglobin, the main haemoglobin in arterial blood, that has no substantial magnetic properties. Put another way, dHB exerts magnetic susceptibility effects on local tissues, which are detected by T2 signal-weighted images as decreased signal intensity. In contrast, oxyhemoglobin is diamagnetic, and thus has little effect on such images. With neuronal activity, the disproportionate increase in local blood flow in a sense dilutes the amount of dHB, which in turn leads to a positive signal on T2 signal-weighted images. Thus, for example, during cognitive tasks, regional cerebral blood flow, and therefore the rate of oxygen delivery, has a proportionally higher increase than does local oxygen consumption, resulting in a decrease of paramagnetic dHB content in the draining capillaries and veins. As dHB increases magnetic field strength, it creates intra- and perivascular field gradients, spin-phase coherence loss on T2 signal-weighted pulse sequences, and a consecutive attenuation of MR signal intensity. Correlations between the time course of the BOLD signal and performance on the task identifies those brain regions showing task-related changes in the signal. Given that the BOLD effect is due to the deoxygenated blood in the draining veins, the spatial localisation of the region where there is increased blood flow resulting in decreased oxygen extraction is not as precisely defined as the morphological features in MRI. Instead, there is a physiological blurring such that it is estimated that the linear dimensions of the physiological spatial resolution of the BOLD signal are around 3 mm at best.
See Brain (neuro-) imaging, Deoxyhemoglobin (dHB), Diffusion tensor magnetic resonance imaging, Functional magnetic imaging (fMRI), Oxyhemoglobin, T2 signal-weighted (T2-W) technique