EEG and MEG  




What is functional Magnetic Resonance Imaging? 

       The fMRI technique differs from MRI in that it localizes brain activity rather than only structures.   It  produces images of activated brain regions by detecting the indirect effects of neural activity on local blood volume, flow, and oxygen saturation.  fMRI was developed in the early 1990s, when MRI equipment was fitted with devices that sped up the recording process.  The equipment is then connected to a computer which rapidly produces two-dimentional images.  The recording time can be as little as 40 milliseconds, and the resolution 1x1 mm.  Because the image acquisition and processing times are similar to those for structural MRI, this allow for fMRI to be readily used into existing radiological practice. 

       The most common fMRI method yields images of oxygen consumption of different parts of the brain.   It is called blood oxygen level dependent imaging, or BOLD.  These changes in tissues blood volume have been directly correlated with evoked brain activity.  These conclusions have been made by the differences in results obtained during both stimulated and nonstimulated states.  Another method is dynamic susceptibility contrast, DSC, which produces images of a contrast agent flowing through the blood vessels of the brain.  

       The fMRI technique is mostly used in research on normal brain functioning because it has yet to be proved that consistent results could be obtained in patients with pathological conditions.  However, a major research advantage to fMRI is that there is a natural correspondance to MRI structural images.  This provides an anatomic basis to the functional localizations. It can also easily identify multiple regions of activation without the modeling or registration complications of other imaging techniques.  Another advantage of fMRI over other techonology, such as PET, is that the BOLD method does not require injection or ingestion of any substances prior to the scan.  It does, however, require the subject to remain still during the procedure in order to prevent the images from being blurred.