an fMRI is basically like a regular MRI machine. The difference is that a regular MRI scan maps structures where the fMRI maps the activity of specific brain regions during the scan. It does this by measuring the difference in blood flow to specific areas of the brain. Brain cells that are more active need more oxygen, which means more blood. It turns out that blood that is full of oxygen and blood that is depleted of oxygen have different magnetic properties that the MRI machine can measure, so by looking at where the oxygenated blood is going, the machine can see which brain regions are more active.
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