MEG is a method used to measure the magnetic field of the brain. The basis for EEG is a dipole that appears when a group of neurons becomes active. With this dipole, a magnetic field appears (after all, a dipole is little more than an electric current). This magnetic field is measured with MEG.
The strength of a magnetic field depends on the strength of the current around which the field runs. An advantage of measuring magnetic fields is that these signals are not disturbed by other brain structures. These structures all have a magnetic permeability of practically zero, so as if it were empty space without obstacles.
The only currents in the brain that cause a magnetic field that can be measured around the head are the postsynaptic potentials. This is because the action potential is so short that the different magnetic fields created in this process are not synchronous, and therefore cannot be added together, and in some cases even cancel each other out.
Magnetic fields appear on the skull as circular fields with a magnetic value. These signals can be picked up here by magnetometers. These are a special kind of sensor placed parallel to the skull. Such a magnetometer should be seen as a circle and the magnetic field can then enter such a circle. When this happens, an inductive current is generated in the magnetometer. The strength of this current is determined by the strength of the magnetic field, and in this way the strength of the magnetic field can be calculated. When there are many magnetometers next to each other that pick up signals, a precise picture can be obtained of how the magnetic fields are distributed across the skull.
The magnetic fields created in the brain are more than a million times smaller than the magnetic field of the earth. It is therefore also very difficult to pick them up. The magnetometers must have very little resistance in order to pick up the signals, so they must be super-conductive. The resistance of the magnetometers can be reduced by making them colder, so that the temperature goes toward zero degrees Kelvin (-273º Celcius). This can be accomplished by putting the magnetometers in a very well insulated environment called the dewar. The dewar is filled with liquid helium, which has a temperature of four degrees Kelvin.
Because the magnetic fields in the brain are so small, the induction currents in the magnetometer are also very small. So, these signals must be amplified in order to analyse them. This is done by the SQUIDS (superconductive quantum interference devices).
The line of each magnetic flux can be split into a horizontal and a vertical component. Only the vertical component affects the induction current produced in the magnetometer. So, there are two points where a magnetic field generates a lot of current in the magnetometer, and that is at the points where the field is perpendicular to the skull. Where the fields come out parallel to the skull, no signal is picked up.
The result is that the signal is not greatest at the spot directly above the origin of the magnetic field. The signal is better captured in spots around the source. However, because this is further away from the source, the signal is weaker. We then say that the signal has a bipolar distribution (there are two points at which the signal is greatest). We call these points the maximum extrema (where the wave exits the head) and minimum extrema (where the wave enters the head again). After all, the magnetic field always has a direction. The direction of the magnetic field is easy to determine with the right thumb rule. When you hold the thumb of your right hand in the direction of the electric current, the curl of your other fingers forms the direction of the magnetic field.
Author: Myrthe Princen (translated by Melanie Smekal)