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Functional MRI
Functional MRI (fMRI) measures signal changes in the brain that
are due to changing neural activity. The brain is scanned at low
resolution but at a rapid rate (typically once every 2-3 seconds).
Increases in neural activity cause changes in the MR signal via
a mechanism called the BOLD (blood oxygen level-dependent) effect.
Increased neural activity causes an increased demand for oxygen,
and the vascular system actually overcompensates for this, increasing
the amount of oxygenated hemoglobin ("haemoglobin" in
British English) relative to deoxygenated hemoglobin. Because deoxygenated
hemoglobin reduces MR signal, the vascular response leads to a signal
increase that is related to the neural activity. The precise nature
of the relationship between neural activity and the BOLD signal
is a subject of current research. The BOLD effect also allows for
the generation of high resolution 3D maps of the venous vasculature
within neural tissue.
While BOLD signal is the most common method employed
for neuroscience studies in human subjects, the flexible nature
of MR imaging provides means to sensitize the signal to other aspects
of the blood supply. Alternative techniques weight the MRI signal
by cerebral blood flow (CBF) and cerebral blood volume (CBV). The
CBV method requires injection of a class of MRI contrast agents
that are now in human clinical trials. Because this method has been
shown to be far more sensitive than the BOLD technique in pre-clinical
studies, it may potentially expand the role of fMRI in clinical
applications. The CBF method provides more quantitative information
than BOLD signal, albeit at a significant loss of detection sensitivity.
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