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Functional MRI

Author: Dr Simon Moss

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Evolution of fMRI

MRI

Nuclear magnetic resonance imaging (NMRI or simply MRI) applies magnetic fields and radio waves to induces changes in water molecules-which can then be monitored. Images can then be constructed that represent the concentration of these molecules across the head. Hence, MRI was initially used to provide structural, not functional, imaging-that is, to characterize the structure of brain regions not represent which regions are active in various contexts.

Functional MRI (fMRI)

fMRI is an extension of MRI, enabling functional scans. fMRI measures oxygen in the blood to represent cerebral blood flow, which tends to correlate with minute changes in cerebral activity. That is, active nerve cells consume oxygen, which is transferred from the hemoglobin in red blood cells. After a delay of one second or so, and then peaking after about 4 seconds, this consumption of oxygen increases blood flow to these regions, process called the hemodynamic response.

Active regions of the brain are more likely to be engorged with oxygenated, rather than deoxygenated, blood. The magnetic properties of oxygenated blood differ from the magnetic properties of deoxygenated blood. Hence, MRI scans can gauge the level of oxygenated blood to characterize neural activity.

Utility of fMRI

Limitations of fMRI

Other factors, in addition to neural activity, might also affect the relative levels of oygenated and deoxygenated blood. Attempts to ascertain the extent to which oxyhemoglobin in the blood has been converted to deoxyhemoglobin, intended to overcome this problem, are currently being considered.

Merits and drawbacks relative to PET (see PET scans)

In general, fMRI affords a temporal resolution of a few seconds, which is slightly more precise than PET. In addition, fMRI affords a spatial resolution of 1 mm or so, which is also slightly more precise than PET.

Nevertheless, fMRI does pose a few difficulties. First, even marginal head movements compromise the utility of this approach. Like PET, fMRI can only measure changes in neural activity---called activation or deactivation-rather than absolute levels.

References

D'Esposito, M. (2006). Functional MRI: Applications in clinical neurology and psychiatry. London: Informa Healthcare.

Faro, S. H., & Mohamed, F. B. (2006). Functional MRI: Basic principles and clinical applications. New York: Springer.

Springer, S. P., & Deutsch, G. (1998). Left brain, right brain (5th ed). New York: Freeman and Company.



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Last Update: 5/28/2016