Thursday, July 15, 2010

MEG


Magnetoencephalography (MEG) is an imaging technique sharing components of EEG and fMRI. It uses the fluctuating electrical activity in the brain which induces a magnetic field in the active region which can then be measured using magnetometers. The magnetometers are commonly referred to as SQUIDs (Superconducting Quantum Interference Devices). There are different orientations of the magnetometers – single, axial gradiometer, planar gradiometer – which are positioned above the head.

As with all superconductors, a very cold environment must be provided. SQUIDs are typically cooled with liquid helium which is maintained at its boiling temperature of 4K (for a reference point, liquid nitrogen freezes in liquid helium) and stored in a giant thermos called a dewar.

SQUIDs are sensitive to very small magnetic fields on the order of 5×10−18 T.
The magnetic fields generated in the brain are on the order of 10-15T. The Earth’s magnetic field is about 5.0 × 10-5T. So in order to obtain a signal clean of any ambient noise, the process must take place in a highly shielded room, including a very thick door with a strong vacuum seal (the room is similar to a vault). The walls of the room are composed of layers of aluminium and a ferromagnetic material.

In comparison to the MEG, an EEG measures electrical activity directly and requires electrodes to be applied directly to the head. Also, direct measures of electrical activity can become distorted as the signal passes through ions and the skull, whereas such distortion is not obtained with MEG.

There are fewer safety concerns with an MEG than an MRI since there is no large magnetic field involved. The images obtained can be combined with images obtained in fMRI studies by measuring specific points on the head after the MEG and then overlaying the two images.

Here is a paper that discusses in more detail MEG and EEG.
http://www.bem.fi/bem/research/eegmeg/index.htm

2 comments:

  1. First I've heard of the MEG. Thanks for the description. I know of examples of research applications for the fMRI but how about the MEG? What questions might it be helping researchers answer?

    ReplyDelete
  2. The applications for MEG are similar to those of other imaging techniques. It is frequently used to examine epileptic patients as it can more precisely locate affected brain areas. It can also be used to identify multiple sclerosis, Alzheimer's disease, schizophrenia, Sjögren's syndrome, chronic alcoholism, and some cognitive processes in fetuses.

    ReplyDelete