fMRI BASICS
The basic principle underlying fMRI is
that it studies physiological processes rather than structural
processes. It is based on the BOLD, blood oxygenation level
dependent, effect. BOLD exploits the fact that
deoxyhemoglobin creates magnetic field gradients which decrease MR
signal. Brain activation is characterized by a drop in the
local oxygen extraction fraction (OEF). So, when brain activation
increases, oxygen levels increase causing the OEF to decrease.
This decrease in OEF leads to decreased levels of deoxyhemoglobin which
cause amplified MR signals. Thus fMRI really studies changes in
the MRI as a result of a particular stimulus or stimuli, etc. It
is used to look at the way in which the brain responds under varying
conditions. Thus, fMRI is not looking for structural markers but
for real time changes in the appearance of the MRI as a result of
varied conditions.