How fMRI Relates to Brain Function
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The "functional" part of fMRI deals with the nature of
blood within the brain. "Blue" blood (deoxygenated
hemoglobin) is more paramagnetic than "red" blood
(oxygenated hemoglobin). This would suggest that the two
kinds of blood would react differently when in a magnetic
field. In fact, the MR signal from the blue blood is
stronger than that of red blood. In the late 1980's it was
shown (using PET, positron emission tomography) that areas
of the brain that are active, performing some function,
require an increased supply of blood but do not actually
utilize the increase in the available oxygen. So, when the
blood leaves regions of active brain tissue it is more
oxygenated than in the rest of the brain. Thus, this blood
interferes less with the local magnetic field than it
would if it were deoxygenated. The MR signal then appears
stronger than it would be in areas where there is not
activation of the brain tissue. So by comparing images,
these areas are then identifiable by their differing
values. This particular way of finding activation is
called Blood Oxygenation Level Dependent imaging, or BOLD
imaging.
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