Some Basic Notions From Applied Physics to Biophysics

After the pioneer measurement of MEG signals by Cohen (1968) using a simple induction coil magnetometer, this method became practical only after new technologies based on the principle of superconduction became available. In the course of the last decades this led to the construction of whole-head devices that allow measuring simultaneously from more than 150 sensors. In Amsterdam we have a CTF/VSM apparatus, manufactured in Vancouver (Canada). To give a rough idea of the financial aspects, an MEG apparatus costs about 2.5 x 106€, while the yearly running costs, including salaries, are in the order of 6 x 105€. It is most important that a small staff consisting of physicists, software specialists and technicians (at least positions), will give the necessary support and collaboration in the development of acquisition protocols and signal analysis. I should add that besides the exploration of magnetic signals produced by the brain, the same technique has proved valuable in the study of the heart, both of adults and foetuses. This can be illustrated by two recent studies: it was shown that recordings of the adult heart magnetic field (Magnetocardiogram or MCG) in the depolarization process has the potential to detect subtle myocardial ischemia induced by exercise (Kanzaki et al. 2003). The analysis of foetal MCG (FMCG) recordings may also contribute significantly to a better understanding of the heart function of the foetuses, and thus may help improve perinatal morbidity and mortality (Anastasiadis et al. 2003).

A few basic notions of applied physics and biophysics may be useful as a short technical introduction into the realm of MEG.

First, we may ponder about some basic notions of applied physics. In general, magnetic fluxes can be measured using induction coils. However, very weak magnetic fields are not measurable using normal wires since the induced currents dissipate as heat by the electrical resistance of the wires. The discovery of the principle of superconduction, i.e. of materials that have essentially no electrical resistance at extremely low temperatures, opened up the possibility of measuring tiny magnetic fields as those produced by the brain. These superconductors are usually known by the abbreviation SQUID (superconducting quantum interference devices). To assure that SQUIDs work properly they have to be maintained at a very low temperature. This is achieved by immersing the SQUIDs in liquid helium contained in an insulated vessel known as a Dewar (—269 °C). The SQUID may be considered a device for transforming a time-varying magnetic field to a time-varying voltage, since a magnetic flux passing perpendicular to a superconducting coil induces an electrical current in the coil. The latter can be further amplified using appropriate electronics. Thus the field of 'low temperature physics' created new devices that allowed measuring very weak magnetic fields such as those produced by neurons.

Second, we may review some basic principles of biophysics. Neurons generate time-varying electrical currents when activated. Longitudinal intra-cellular currents flowing along dendrites or axons generate magnetic fields around them, just as it happens in a wire, according to the well known right-hand rule of electromagnetism. Pyramidal neurons of the cortex, with their long apical dendrites oriented perpendicular to the cortical surface, if activated with a certain degree of synchrony, generate coherent magnetic fields. In this way we may say that these neurons behave as 'current dipoles', the activity of which can be detected by SQUIDs placed at a small distance from the skull. We should note that the resulting MEG signals depend on the orientation of the neurons with respect to the skull.The MEG 'sees' only those magnetic fields that are perpendicular to the skull. These magnetic fields are generated by neuronal currents that are oriented tangentially to the skull. In contrast, those that are oriented radially to the skull do not generate a magnetic field outside the head.

Third, we have to note that in order to record MEG signals it is necessary to use specialized recording conditions, since these signals are very weak. Even when thousands of cortical pyramidal neurons are synchronously active the resulting magnetic field at the head surface has a very small magnitude, in the order of 10—12 Tesla (1 fT = 10—15 Tesla), which is much smaller than the earth magnetic field and urban magnetic noise fields. This poses a hard detection problem, equivalent to that of detecting, at a distance, the voice of one single individual in a large noisy crowd. To accomplish this strenuous task the MEG is generally recorded in a magnetic and radiofrequency shielded room with walls made of mu-metal and aluminium, what adds appreciably to the cost of the system.

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