Magnetoencephalography (MEG)

The magnetoencephalography investigated 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 studies the magnetic activity of the brain. Together with other methods, it is used to model brain functions.

Magnetoencephalography, also known as 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 magnetoencephalography is very expensive devices, especially since around 400 litres 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 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. In the nerve cells, electrical currents are stimulated when the stimuli are 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 magnetoencephalography, 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 magnetoencephalography in shielding them from external magnetic fields. As a rule, the magnetoencephalography 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 magnetoencephalography. 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 only becomes noticeable when the magnetoencephalography is exposed to strong mechanical vibrations. A magnetoencephalography 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 magnetoencephalography, 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. Mini-sensors have been developed that also works 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.

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. With 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 quantities of liquid helium, which is necessary in magnetoencephalography, to maintain superconductivity. 

MEG (magnetoencephalography) vs EEG

It is a non-invasive medical diagnostic technique for recording the electrical activity of the brain. Unlike electroencephalography (EEG), universally used in the medical field and which measures the electrical potentials induced on the scalp by the electrical activity of the brain, GCM records the magnetic field generated by intracranial currents. To do this, extremely sensitive magnetic sensors are used, made with superconducting circuits. These sensors are known by the acronym SQUID (Superconducting quantum interference device), are capable of measuring very small variations of the magnetic field, of the order of some femtoTesla (10 −15T), or one hundred billionths of the Earth's magnetic field. This allows you to register at a certain distance from the patient and without contact with him.

To work, the SQUIDs require very low temperatures, of the order of 4 K which requires the use of suitable thermally insulating containers. The advantage of MEG over EEG is mainly due to the fact that the human body is essentially transparent to magnetic fields, while the electrical conductivity of the tissues tends to confuse the electrical signals generated in the brain. With the MEG, it is, therefore, possible to localize the sources of magnetic signals with high precision within the brain. This technique has been used successfully for the identification of epileptic foci (the points in the brain where epileptic seizures occur). A MEG machine is however very complex, as it uses a large number of SQUID sensors (several hundred), all cooled to 4 K with liquid helium. The very high sensitivity of the SQUIDs also requires the use of extremely sophisticated and expensive shielding systems from external noise sources. All this means that this technique is used in a few specialized centers, the use of extremely sophisticated and expensive shielding systems from external noise sources. 

What is magnetoencephalography, when it is needed and contraindications?

It is a very special examination that serves to monitor the correct functioning of the brain. This is how magnetoencephalography works. The magnetoencephalography is a very sophisticated non-invasive examination and which serves for the study of brain function. We must remember to understand how it works, that the activity of the brain is? The electrical impulses that travel between nerve cells (neurons) and these pulses produce magnetic fields. The magnetic fields generated by the neuroelectric activity of the brain are recorded by the MEG; this is the abbreviation that identifies the equipment.

How does the exam take place? The patient sits in an armchair, and impulses are detected through a kind of helmet with sensors. The information acquired is used to study the normal functioning of the brain, and in particular on some aspects of language, vision, movement, memory and emotions. 

MEG is not an invasive examination and therefore has no particular contraindications. How long does it last? Generally about 2 hours: consider about 30 minutes of preparation, and the rest instead concerns the execution of the MEG. You also need to pay attention to some things in the previous 12 hours: avoid drinking too much, wear comfortable clothes and ask if you can be accompanied. Who can't do the MEG? All people with a heart pacemaker, metal clips in the brain, cochlear implants, permanent face tattoos, metal splinters and fragments in the body. Why? These materials or designs worsen the quality of the MEG survey.

Author: Vicki Lezama