Wednesday, June 10, 2009

Functional MRI

Functional MRI is an exciting idea: it allows us to create a map of how the brain responds when the person being imaged is subjected to some particular stimuli (say being shown a picture or told to perform a task). Functional MRI (fMRI) involves taking a measure of the amount of oxygen in a particular location of brain tissue. When a person’s mind activates with a particular thought/concept/action the activation can be measured by observing an increase in the amount of oxygen present (oxygen is delivered to activating regions by hemoglobin).

However, oxygen is only measured indirectly, we actually sample hydrogen protons many many times to detect small changes in the overall amount of oxygen present. It would seem sensible to perform these functional MRI analyses after having the subject breathe air with a significant concentration of oxygen-17 (which can be imaged in MRI), then image the oxygen-17 concentration instead of the concentration of hydrogen protons bonded to oxygen. This would make the fMRI measurements more direct to oxygen measurements as opposed to the present indirect method - it may also circumvent the present requirement of acquiring a ton of samples just to determine that the oxygen concentration has changed. Oxygen-17 based MR imaging has been proposed [1,2], however, I have never seen it used in an fMRI type analysis.

[1] J. Pekar, L. Ligetti, Z. Ruttner, R. Lyon, T. Sinnwell, P. van Gelderen, D. Fiat, C. T. Moonen and A. McLaughlin, In vivo measurement of cerebral oxygen consumption and blood flow using 17O magnetic resonance imaging, Magn. Reson. Med., 21, 313-319 (1991).