Magnetoencephalography examines the magnetic activity of the brain. Together with other methods, it is used to model brain functions. The technique is used mainly in research and to plan difficult neurosurgical procedures on the brain.
What is magnetoencephalography?
Magnetoencephalography studies the magnetic activity of the brain. Along with other methods, it is used to model brain function. Magnetoencephalography, also known as MEG, is an examination method that determines the magnetic activity of the brain. In this process, the measurement is made by external sensors called SQUIDs. SQUIDs work on the basis of superconducting coils and can register the smallest magnetic field changes. The superconductor requires a temperature that is close to absolute zero. This cooling can only be achieved by liquid helium. The magnetoencephalographs are very expensive devices, especially since a monthly input of about 400 liters of liquid helium is necessary for their operation. The main field 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 clarification of the development of a tremor. Furthermore, magnetoencephalography is also used to identify the brain area responsible for a present epilepsy.
Function, effect, and goals
Magnetoencephalography is used to measure the small magnetic field changes produced during neuronal activity of the brain. Electrical currents are excited in neurons during stimulus transmission. Each electric current generates a magnetic field. In this process, an activity pattern is formed by the different activity of the nerve cells. There are typical activity patterns that characterize the function of individual brain areas during different activities. In the presence of diseases, however, deviating patterns can arise. These deviations are detected in magnetoencephalography by slight magnetic field changes. In this process, the magnetic signals from 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 to shield them from the external magnetic fields. Therefore, the magnetoencephalograph is usually set up in an electromagnetically shielded cabin. There, the influence of low-frequency fields from various electrically operated objects is attenuated. 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 do not have such a large spatial dependence as the magnetic fields generated by the brain. Thus, the intensity of the brain’s magnetic signals decreases quadratically with distance. Fields with lower spatial dependence can be suppressed by the coil system of the magnetoencephalograph. This is also true for the magnetic signals of heartbeats. Although the earth’s magnetic field is comparatively strong, it also does not exert a disturbing influence on the measurement. This results from the fact that it is very constant. Only when the magnetoencephalograph is exposed to strong mechanical vibrations, the influence of the earth’s magnetic field becomes noticeable. A magnetoencephalograph is able to record the total activity of the brain without any time delay. Modern magnetoencephalographs contain up to 300 sensors. They have a helmet-like appearance and are placed on the head for measurement. Magnetoencephalographs are divided into magnetometers and gradiometers. While magnetometers have one pickup coil, gradiometers contain two pickup coils spaced 1.5 to 8 cm apart. Like the shielding chamber, the two coils have the effect that magnetic fields with low spatial dependence are suppressed even before the measurement. There are already new developments in the field of sensors. For example, miniature sensors have been developed that can also operate at room temperature and measure magnetic field strengths of up to one picotesla. Important advantages of magnetoencephalography are its high temporal and spatial resolution.Thus, the time resolution is better than one millisecond. Other advantages of magnetoencephalography over EEG (electroencephalography) are its ease of use and numerically simpler modeling.
Risks, side effects, and hazards
No health problems are 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 spatial resolution clearly proves to be an advantage. In addition, it is a non-invasive neurological examination method. However, the biggest disadvantage is the non-uniqueness of the Inverse Problem. In 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 unambiguously assigned to a function or disorder. Only if the previously elaborated model is correct, a successful assignment is possible. However, correct modeling of individual brain functions can only be achieved by coupling magnetoencephalography with the other functional examination methods. These metabolic 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 techniques. The combination of their results leads to an understanding of the processes occurring in individual brain areas. Another disadvantage of MEG is the high cost of the procedure. These costs result from the use of large amounts of liquid helium necessary in magnetoencephalography to maintain superconductivity.
