Magnetoencephalography ( Neuromagnetism )

The same fluctuating electrical currents from the brain that produce the electroencephalogram (EEG) also produce a magnetic field over the head, called a neuromagnetic field. A measurement of this field is called a magnetoencephalogram (MEG). Because the MEG is produced by the same currents that produce the EEG, some types of MEG recordings resemble EEG recordings. For example, a single MEG trace (a recording of the fluctuating magnetic field versus time at one location on the head) roughly resembles an EEG trace recorded at a related location. However, an MEG spatial map over the head is quite different from the corresponding EEG spatial map, where each map shows the magnetic field or EEG potential over a large region of the head, frozen at one instant in time. They are different because the MEG spatially samples the currents differently than does the EEG. This difference in sampling allows the MEG map to provide some different information about electrical sources in the brain than does the EEG map. In that sense the MEG is complementary to the EEG, and both are necessary to obtain maximum information about the electrical sources in the brain. Thus, the MEG is now grouped together with the EEG among the noninvasive techniques for looking at the human brain; the MEG and EEG are unique in that they look at fast electrophysiological brain events (milliseconds). This is in contrast to functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and single photon emission tomography (SPECT) which look at events in seconds or minutes, and magnetic resonance imaging (MRI), and computer assisted tomography (CAT) which only look at anatomic structures. The fluctuating neuromagnetic field is very weak, with an amplitude typically below 10 tesla, or 10 gauss in the older magnetic units; this is much weaker than the urban fluctuating magnetic noise background of about 10 tesla or 10 gauss, and weaker yet than the earth's steady field of about 0.5x10 tesla, or one-half gauss. Therefore the two main requirements for measuring the MEG are a magnetic detector of high sensitivity, and the suppression of the fluctuating magnetic background (the steady background usually presents no problem). The sensitive detector used is the SQUID (Superconducting Quantum Interference Device) which operates at cryogenic temperatures; a SQUID sensitive enough to measure the brain requires the low temperature of 4 K, and is contained in a liquid helium dewar. The recent MEG dewars are helmet-shaped and enclose most of the head; a dewar of this type contains many SQUIDs, mostly arrayed on a spherical section over the head, typically at grid points 2 or 3 cm apart. This spherical section is at an average distance of about 2 cm from the scalp, where some of the separation is due to the vacuum space in the dewar. The many SQUIDs allow simultaneous magnetic field measurements at the many corresponding points over the head. The fluctuating background is suppressed in two ways. In the first way, a magnetically shielded room is used, in which the measurements are made; the room excludes almost all of the fluctuating external fields. Examples are shown in Figs 1 and 2; the degree of shielding varies with the number of wall layers, in the range of 2-8 layers. In the second way of excluding background, the gradient of the magnetic field is measured instead of the field itself, where the gradient is the difference in field measurements between two different locations. These can be neighboring locations along the scalp, or they can be radially-separated locations, one close to the scalp and the other some distance away. The background is suppressed because the gradient of the background is much reduced compared with that of the brain; the background source is much further away, and the gradient falls off very rapidly with distance from the source. Advanced gradiometers can yield further improvements by taking Athe gradient of the gradient@, involving a double radial separation. A gradiometer system of this type is used in Fig. 2. A good, multi-layer room can eliminate the background almost completely but adds expense, while the gradiometer method, although less expensive, cannot eliminate background as well. A combination of a gradiometer MEG system in a modest two or three layer room is now usually used. The cost of the entire MEG system, including shielded room, is in the range of $2-$3 million (US). MEG data and instantaneous maps are produced in the following way. First, in preparation for the recording session, all magnetic material on the subject’s body is removed or demagnetized, and he or she changes into magnetic-free clothes. Fiducial locations on the head are accurately noted. Then the subject, seated or in the prone position, is placed with their head in the helmet dewar, as in Fig. 2. A bite-bar is occasionally used to minimize head motion. Next, the raw recordings are made, which consist of MEG traces on all channels, due either to evoked neural activity or to spontaneous activity, depending on the purpose of the session. Then, either online or offline, a sequence of spatial maps are