Magnetoencephalography

The Magnetoencephalography studies the magnetic activity of the brain. Together with other methods, it is used to model brain functions. This technique is mainly used in research and for planning difficult neurosurgical interventions on the brain.

What is magnetoencephalography?

Magnetoencephalography, also called MEG is an examination method that determines the magnetic activity of the brain. The measurement is carried out by external sensors, the so-called SQUIDs. SQUIDs work on the basis of superconducting coils and can register the smallest changes in the magnetic field. The superconductor requires a temperature that is almost absolute zero.

This cooling can only be achieved with liquid helium. The magnetoencephalographs are very expensive devices, especially since around 400 liters of liquid helium are required to operate each month. The main area of ​​application for this technology is research. Research topics are, for example, the clarification of the synchronization of different brain areas during movement sequences or the elucidation of the development of a tremor. Magnetoencephalography is also used to identify the area of ​​the brain responsible for an existing epilepsy.

Function, effect & goals

Magnetoencephalography is used to measure the small changes in the magnetic field that are generated during the neuronal activity of the brain. Electrical currents are stimulated in the nerve cells when the stimulus is transmitted.

Every electric current creates a magnetic field. The different activity of the nerve cells creates an activity pattern. There are typical activity patterns that characterize the function of individual brain areas in different activities. In the presence of diseases, however, deviating patterns can arise. In magnetoencephalography, these deviations are detected by slight changes in the magnetic field.

The magnetic signals of the brain generate electrical voltages in the coils of the magnetoencephalograph, which are recorded as measurement data. The magnetic signals in the brain are extremely small compared to external magnetic fields. They are in the range of a few femtotesla. The earth's magnetic field is already 100 million times stronger than the fields generated by brain waves.

This shows the challenges of the magnetoencephalograph in shielding them from external magnetic fields. As a rule, the magnetoencephalograph is therefore installed in an electromagnetically shielded cabin. There, the influence of low-frequency fields from various electrically operated objects is dampened. In addition, this shielding chamber protects against electromagnetic radiation.

The physical principle of shielding is also based on the fact that the external magnetic fields are not as dependent on location as the magnetic fields generated by the brain. The intensity of the brain’s magnetic signals decreases quadratically with distance. Fields that are less dependent on location can be suppressed by the coil system of the magnetoencephalograph. This also applies to the magnetic signals from heartbeats. Although the earth's magnetic field is comparatively strong, it does not interfere with the measurement.

That results from the fact that it is very constant. The influence of the earth's magnetic field is only noticeable when the magnetoencephalograph is exposed to strong mechanical vibrations. A magnetoencephalograph is able to record the total activity of the brain without delay. Modern magnetic encephalographs contain up to 300 sensors.

They have a helmet-like appearance and are placed on the head for measurement. In magnetoencephalographs, a distinction is made between magnetometers and gradiometers. While magnetometers have one pick-up coil, gradiometers contain two pick-up coils at a distance of 1.5 to 8 cm. The two coils, like the shielding chamber, have the effect that magnetic fields with little spatial dependence are suppressed even before the measurement.

There are already new developments in the field of sensors. So mini-sensors were developed that also work at room temperature and can measure magnetic field strengths of up to a picotesla. Important advantages of magnetoencephalography are its high temporal and spatial resolution. The time resolution is better than a millisecond. Further advantages of magnetoencephalography over EEG (electroencephalography) are its ease of use and numerically simpler modeling.

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Risks, side effects & dangers

No health problems are to be expected when using magnetoencephalography. The procedure can be used without risk. However, it should be noted that metal parts on the body or tattoos with metal-containing color pigments could influence the measurement results during the measurement.

In addition to some advantages over EEG (electroencephalography) and other methods for examining brain function, it also has disadvantages. The high time and space resolution clearly proves to be an advantage. It is also a non-invasive neurological examination. The main disadvantage, however, is the ambiguity of the inverse problem. In the case of the inverse problem, the result is known. However, the cause that led to this result is largely unknown.

With regard to magnetoencephalography, this fact means that the measured activity of brain areas cannot be clearly assigned to a function or disorder. A successful assignment is only possible if the previously worked out model applies.Correct modeling of the individual brain functions can only be achieved by coupling magnetoencephalography with the other functional examination methods.

These metabolically functional methods are functional magnetic resonance imaging (fMRI), near infrared spectroscopy (NIRS), positron emission tomography (PET) or single-photon emission computed tomography (SPECT). These are imaging or spectroscopic methods. The combination of their results leads to an understanding of the processes taking place in the individual brain areas. Another disadvantage of the MEG is the high cost factor of the process. These costs result from the use of large amounts of liquid helium, which is necessary in magnetoencephalography, to maintain superconductivity.