What is MEG ?

Magnetoencephalography (MEG) is completely noninvasive, non-hazardous technology for functional brain mapping, providing spatial discremination of 2 mm and an excellent temporal resolution on the order of 1 ms, localizing and characterizing the electrical activity of the central nervous system by measuring the associated magnetic fields emanating from the brain. Every current generates a magnetic field according to the right hand rule of physics. This same principle is applied in the nervous system whereby the longitudinal neuronal current flow generates an associated magnetic field. MEG measures the intercellular currents of the neurons in the brain giving a direct information on the brains activity, spontaneously or to a given stimulus. Measurement preparation and collection times are relatively short and can be performed by a technician with a minimum of training.

The magnetic field generated by a single neuron is almost negligible; thus, when several thousands of nearby cells are synchronously active, the summated extracranial magnetic field typically achieves a magnitude of only a few hundred femto Tesla (1 fT = 10^-15 Tesla). Even the strongest neuromagnetic signals, those associated with epileptic spikes, are only a thousand femto Tesla in the order of 10^-13 Tesla in magnitude. This is still more than one billion times smaller than the earth's steady magnetic field and the noise fields generated by even distant moving metal objects (e.g., cars and elevators) and power lines. The detection and isolation of neuromagnetic signals is a challenging problem akin to listening for the footsteps of an ant in the middle of a rock concert, the signals are very small and the background noise is nearly overwhelming. To reduce the amount of magnetic noise reaching the biomagnetometer, the system is operated in a magnetic and radiofrequency shielded room made of mu-metal and aluminum. The recording dewar contains magnetic detection coils which are continuously bathed in liquid helium to superconducting temperatures of -269(4.2K) degrees Celsius.

Magnetic signals are most readily measured using induction coils composed of loops of wire. The spontaneous or evoked magnetic fields emanating from the brain induce a current in these coils, which in turn produce a magnetic field in a special device called a superconducting quantum interference device (SQUID). When a time-varying magnetic flux passes perpendicular to the coil, it induces a time-varying electrical current within the wire. For typical metal wires, this current is quickly dissipated as heat by the electrical resistance of the wire. Clinical biomagnetometers therefore use special induction coils made of superconducting wire. Superconducting coils have essentially no electrical resistance; thus, the amount of current induced within the coil instantaneously tracks even very small changes in the magnitude of the impinging magnetic flux. In clinical biomagnetometers, the induction coil is coupled to a second superconducting device. For simplicity, the SQUID can be thought of as a very low noise device for transducting magnetic fields or currents into a voltage. A SQUID acts as a low-noise, high-gain, current-to-voltage converter that provides the system with sufficient sensitivity to detect neuromagnetic signals of only a few femto Tesla in magnetitude. The SQUID voltage is a periodic function of the magnetic flux. The period of this voltage change is one magnetic flux. The noise density of the modern thin film de-SQUID is a few millionths of a flux quantum per root Hz. A SQUID can be used as a magnetometer by operating it within a flux-locked loop; the flux-locked loop outputs a voltage that is linearly (rather than periodically) proportional to magnetic flux sensed by the SQUID. The sensitivity of the SQUID to magnetic fields may be enhanced further by coupling it to a superconducting pickup coil having greater area and number of turns than the SQUID inductor, alone. This pickup coil is termed a "flux" transformer". The SQUID and induction coils of biomagnetometers are generally maintained in a superconducting state by immersion within a liquid helium bath contained in an insulated cryogenic vessel known as a dewar.

The information provided by MEG is entirely different from that provided by Computed Tomography (CT) or Magnetic Resonance (MR) imaging. Unlike the latter two which provide structural/anatomical information, MEG provides functional mapping information. MEG is a functional imaging capability complementary to the anatomical imaging capabilities of MRI, CT. That is, whereas MRI and CT are capable of imaging anatomy, MEG is able to image neurological function. Using MEG, we are measuring the activity of the brain in real time. The brain can be observed "in action" rather than just viewing a still MR image. MEG data can be used to identify both normal and abnormal functions of brain structures which are anatomically so crisply seen in the static MRI scans. The two modalities can then be fused into a composite image of function and anatomy. As a result, the combination of MEG and MRI techniques has considerable clinical potential.

MEG combines many of the advantages of various other new functional imaging modalities, such as Positron Emission Tomography (PET) and functional MRI (fMRI), which are weakly invasive and measure signals caused by changes of blood flow. In PET radioactive marker substances are injected into the subject's bloodstream, and in fMRI the patient is exposed to a high static magnetic field and a small alternating radiofrequency field, with no health hazards reported. The temporal resolution of PET is tens of seconds, and although fMRI can be collected at 50- to 100-ms intervals. the intrinsic inertia of changes in blood flow limits the temporal resolution of fMRI to 1 s. However, MEG's temporal resolution of 1 ms is far superior than the others, while having an equivalent spatial resolution.

Up to now, MEG studies have emphasized the ability to locate the sources of evoked responses. However, with the unique possibility to record signals quickly over the whole cortex, the focus will certainly move towards studies of spontaneous activity and its changes during various tasks. These magnetic measurements on the brain (MEG), measurements on the magnetic field of the heart (MCG), and measurements on the magnetic field of the fetal (fMEG) are carried out at CTF Systems Inc.

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