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As neurons fire, they metabolyse oxygen from the surrounding blood. Approximately 6 seconds after a burst of neural activity, a haemodynamic response[?] occurs in which that region of the brain is infused with oxygen-rich blood.
Because oxygenated haemoglobin is diamagnetic, while deoxygenated blood is paramagnetic, MRI is able to detect a small difference (a signal of the order of 3%) between the two. This is called a blood-oxygen level dependent, or "BOLD" signal. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research.
BOLD effects are measured using a T2[?] imaging process, which is different from the T1[?] scan taken in ordinary structural MRI images (the former measures the rate of change of spin phases, while the later detects the half-life of inverted spins). T2 images can be aquired with moderately good spatial and temporal resolution; scans are usually repeated every 2-5 seconds, and the voxels in the resulting image tend to be around 0.25 cubic centimeters. Other non-invasive functional medical imaging techniques can improve on one of these figures, but not both.
The science of applying fMRI is quite complicated and multi-disciplinary. It involves:
Aside from BOLD fMRI there are other ways to probe the brain activity with MRI:
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