The BOLD Phenomenon

Mapping eloquent areas by MRI is based on the blood oxygenation level-dependent (BOLD) contrast phenomenon. This is due to the paramagnetic properties of deoxyhaemoglobin. When a pool of neurons pass from a rest state to an activation state, the increased discharge of spikes induces a rise in regional oxygen consumption rate. This event brings two main effects: a regional increase in the absolute number of deoxyhae-moglobin molecules and a local vasodilation, due to the flow autoregulatory mechanism, which is characteristic of the brain circulation. The increase in blood regional flow largely overwhelms the amount of oxygen extracted by the activated neurons and leads to a relative decrease of deoxyhaemoglobin concentration. The resulting net effect is a local rise in oxyhaemoglobin and a local drop in deoxyhaemoglobin. The drop in paramagnetic deoxyhaemoglobin leads to an increased signal intensity in T2*- and T2-weighted images. The mapping of eloquent areas, then, is achieved by acquiring T2*- or T2-weighted images consecutively, while the subject is performing the task or is at rest. Finally, the difference between the performing condition and the resting condition is calculated [16].

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