THE BOLD EFFECT

The previous chapters described magnetic resonance imaging (MRI) techniques for measuring cerebral blood flow and blood volume. By introducing contrast agents or manipulating the magnetization of arterial blood before it arrives in a tissue voxel, the MR signal becomes sensitive to aspects of local tissue perfusion. Such techniques are clinically valuable for investigating disorders characterized by perfusion abnormalities, such as stroke and tumors, and these techniques have also seen limited use in investigations of normal brain function. But the functional magnetic resonance imaging (fMRI) technique that has created a revolution in research on the basic functions of the healthy human brain is based on an intrinsic sensitivity of the magnetic resonance (MR) signal to local changes in perfusion and metabolism. When neural activity increases in a region of the brain, the local MR signal produced in that part of the brain increases by a small amount due to changes in blood oxygenation. This Blood Oxygenation Level Dependent (BOLD) effect is the basis for most of the fMRI studies done today to map patterns of activation in the working human brain.

The BOLD effect is most pronounced on gradient echo (GRE) images, indicating that the effect is primarily an increase of the local value of T. The fact that the oxygenation of the blood has a measurable effect on the MR signal from the surrounding tissue was discovered by Ogawa and co-workers imaging a rat model at 7T (Ogawa et al., 1990). They found that the MR signal around veins decreased when the oxygen content of the inspired air was reduced, and the effect was reversed when the oxygen was returned to normal values.