Actually - also to clarify a subtle point -
fMRI as it seems it was defined by the discussion, points to the BOLD
effect - Blood Oxygenation Level Dependent. This technique, of course
specifically relates to susceptibility differences due to deoxyhemoglobin.
However, in terms of FUNCTIONAL activation, MRI can be utilized in
different ways. For example, a recent paper presented at ISMRM this year
demonstrated diffusion weighted imaging being utilized to determine
functional activation. (I'm sure there are better references) A CBF based
technique could also be used, as I'm sure other techniques are possible.
-Yueh