MAGNETIC WAVE INTELLIGENT TECHNOLOGY

The extracranial magnetic field evoked by the neuronal activity of the brain is very weak, only 50-500 femto-Tesla (1fT=10-15), which is much smaller than 1 in 100 million of earth’s magnetic field. So an extremely sensitive device must be utilized in order to detect and record these fields. In this situation, the conventional MEG scanner SQUID (Superconductive Quantum Interference Device) was born.

The superconducting state of SQUIDs operated at ultra-low temperatures is maintained at massive liquid helium. Helium is a non-renewable natural resource that has great application values in the semiconductor industry, medical science, and defense. Nowadays, the global shortage of helium has pushed the continuous rising of its prices. Some countries such as the United States and Japan have begun to restrict the use of helium in amusement. China imports over 99% of helium. Thus far, the cost of helium has prevented the MEG from being used widely.

As ultra-high sensitivity quantum sensors, optically pumped magnetometers (OPMs) can be used to develop a new generation of more flexible, lower cost, wearable and highly sensitive MEG systems.

History Articles

Technical Foundation

Magnetoencephalography (MEG) and electroencephalography (EEG) are electrophysiological techniques for recording brain activity. Both of them measure the electrochemical activity generated by action potentials produced by neurons in the brain from different perspectives.

Action potentials are the mechanism the neurons use to transmit information. The resting potential of a typical neuron is around -65mV. It is maintained by ion channels that span the membrane. On the other hand, the membrane permeability changed upon synaptic chemical stimulation. Excitatory stimulation of the membrane increases membrane permeability and decreases the potential difference, producing a shock wave that propagates from the dendrites to the cytosol and from the cytosol to the axon, through the neuron. This shock wave resembles an electric current that propagates inside the neuron and is often referred to as a primary (or intracellular) current. The ion channel regains equilibrium after a short time, producing a second shock wave that propagates outside the neuron, often referred to as a secondary (or volumetric) current. These two phenomena (primary and secondary currents) are similar in nature and have the same intensity due to the principle of charge conservation. Primary currents are confined within the neuron and cannot be measured directly in vivo. Secondary currents propagate through the entire conductive volume and can theoretically be measured at any point.

An EEG records the secondary currents of subcortical neurons via electrodes affixed to the scalp. However, the secondary current must propagate through all the different head tissues, following a tortuous path determined by their electrical conductivity in order to reach the scalp. Therefore, the impact of skull-to-brain conductivity ratios must be known for accurate source localization of the EEG signal. According to Ampere's law, the magnetic field created by an electric current is proportional to the size of that electric current with a constant of proportionality equal to the permeability of free space. So are electrophysiological signals. Even if the primary current is confined inside a neuron, it produces a magnetic field that can be measured outside the head. This measurement is called MEG. Secondary currents also produce associated magnetic fields, but they are usually weaker because these currents are distributed over a larger volume. In addition, this magnetic field is not affected by the different head tissues and always propagates in a straight line. Therefore, source localization is relatively easy.

In order to obtain EEG or MEG data, secondary current or magnetic field must propagate away from the neuron. This phenomenon occurs only in open-field neurons, especially in long neurons like pyramidal neurons. In addition, currents produced by one neuron are too weak to be detected outside the head. It is estimated that at least 10,000 neurons must be activated synchronously to properly measure electrophysiological activity. In the human brain, it is the pyramidal neurons in the cortex that meet these requirements. Their dendrites distribute in a parallel grid, which allows space summation, and bundles of these neurons are activated simultaneously, which allows temporal summation. From this perspective, the model suggests that pyramidal neurons are the main contributors to the EEG and MEG signals.

EEG and MEG signals are generated by the same physiological phenomena. They should provide the same information under ideal conditions, assuming similar noise levels and good geometry. But they are slightly different in practice. As we mentioned before, the first difference is that MEG detects primary currents, while EEG measures secondary currents. The second difference is that MEG has higher spatial resolution compared to EEG. Because it is much more complex to do source localization/reconstruction in EEG as secondary currents follow a tortuous path to the electrodes.

Another significant distinction between electroencephalography (EEG) and magnetoencephalography (MEG) is that while MEG measures the magnetic field at a single point, EEG cannot directly measure what is applied in Alzheimer's disease research. Instead, EEG estimates this current by the potential difference between two points on the scalp. According to Ohm's law, if there is a potential difference between two connected points, there must be a current flowing between them, and vice versa. Therefore, EEG is a reference-based measurement, while MEG is reference-free. If the potential distribution of the primary current is estimated, this difference becomes less important. However, if activity is analyzed at the sensor level, reference electrodes will effectively correct the measurement, and this must be taken into account.

A third difference between EEG and MEG relates to sensitivity. EEG measures the secondary currents in the scalp and can detect echoes associated with any developed field neuron while MEG measures the magnetic fields generated by primary and secondary currents. In general, secondary magnetic fields are weaker than primary fields, but in the highly symmetric case, secondary currents can be oriented in such a way that their magnetic fields neutralize the magnetic fields generated by primary currents. These very special structures are called silent sources. A typical example of a silent source is a radial primary current in an (approximately) spherical conductor. As the head could be roughly approximated as a sphere, MEG is often considered insensitive to radioactive sources, whereas EEG is not.