Magnetoencephalography
The magnetoencephalography (from Greek encephalon brain, gráphein write ), abbreviated MEG, is a measurement of the magnetic activity of the brain, performed by external sensors, the so-called SQUIDs. The magnetic fields are usually first detected by also superconducting coils or coil systems and then measured by the SQUID. MEGs are complex and relatively expensive equipment. For the operation of approximately 400 liters of liquid helium for cooling are needed such as monthly.
Measurement of the fields
The magnetic signals of the brain are only a few femtotesla (1 fT = T ) and must be as completely shielded from external disturbances. For the MEG is usually mounted in an electromagnetic shielding cabin. The shielding attenuates the influence of low-frequency interference as they are caused by cars or elevators and protects against electromagnetic radiation. Frequencies above one kilohertz ( Hz), however, has hardly been investigated using the MEG. Magnetic fields of external disturbances differ from those of the brain by a much smaller spatial dependence of their strength because of the greater distance to the origin. ( The intensity decreases with distance from square. ) Using the above-mentioned coil systems the fields with lower spatial dependence can be greatly suppressed. Therefore, for example, the heartbeat of the person examined in modern MEGs only a small interference effect. The geomagnetic field is indeed about 100 million times stronger than the fields covered by the MEG, but it is very time- constant and only very slightly curved. His influence is only annoying when the entire MEG is exposed to mechanical vibrations.
The magnetic signals from the brain caused by the electric currents of active neurons which induce voltages in the sensing coils of the MEG sensor. Therefore, one can record the particular MEG data reflect the current total activity of the brain without delay. Modern whole-head MEG - equipped with a helmet- like arrangement of up to 300 magnetic field sensors. A distinction is made between so-called magnetometers and gradiometers. Magnetometers have a simple take-up spool. Gradiometer usually have two receiving coils which are arranged at a distance from 1.5 to 8 cm and wound opposes nowadays. Thus electromagnetic interference low spatial dependence can be suppressed before the measurement. The very high time resolution (better than 1 ms ( second) ), the ease of use of the high channel count accurately known sensor positions, and the numerically simpler modeling are the main advantages of MEG in the localization of brain activity compared to the EEG. Probably the biggest disadvantage of MEG localization is the non-uniqueness of the inverse problem. In a nutshell, it means that the localization can be correct only if the underlying model is essentially correct (number of centers and their gross physical arrangement ). Here are the advantages of metabolic functional methods such as fMRI, NIRS, PET or SPECT. Brain research provides through the comparison and the coupling of the different functional methods and more precise knowledge about the correct modeling of individual brain functions.
New developed mini- sensors are able to perform measurements at room temperature and measure field strengths of 1 Picotesla. This opens up new design options and significant price reductions in the operation of equipment.
The MEG is a diagnostic procedure with good spatial and very high temporal resolution, the other method to measure brain activity (functional method ), such as the EEG and functional magnetic resonance imaging (fMRI ) supplements. In medicine, the MEG is used, inter alia, to areas of the brain that cause epileptic seizures, to be able to locate or to schedule complex cranial surgery for example in patients with brain tumors.
History
The first MEG was recorded in 1968 by David Cohen at the Massachusetts Institute of Technology (MIT).