Another method that provides images that may be used as indicators for neuronal activity is that of blood oxygenation dependent (BOLD) contrast. The underlying mechanism in BOLD contrast imaging is similar to that of blood volume mapping by intravascular paramagnetic contrast agents described above. The major difference is that blood itself is used as the intravascular contrast agent, or, more specifically, the change in the blood deoxyhemoglobin concentration provides the magnetic signal. Oxygenated hemoglobin has a magnetic susceptibility close to that of tissue, whereas deoxyhemoglobin has a susceptibility higher than that of oxygenated hemoglobin. The difference in the susceptibility is high enough to cause blood vessels to show a measurable signal when the basal oxygenation level falls. During neuronal stimulation it has been shown that blood flow increases substantially, whereas oxygen consumption is not increased as much. This leads to an increase in the concentration of oxygenated blood in capillaries and venules close to areas where neurons are active, relative to their resting state. Since the susceptibility of oxygenated hemoglobin is closer to that of brain tissue, a decrease in the strength of the microscopic gradients and a commensurate increase in MRI signal intensity is noted during neuronal activity. This is the basis of the postulated mechanism for BOLD image contrast.
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