What is the principle of memory? How do emotions generate? What is the pathological basis of various mental and brain diseases? In order to understand these major scientific issues related to the operation mechanism of the human brain and our self-awareness, scientists have developed technologies such as electroencephalography and functional magnetic resonance imaging for brain science research. Compared to them, MEG can non-invasive and real-time detect the process of neural activity in the brain by measuring the extremely weak magnetic field signals generated by neural electrical activity outside the skull.
MEG combines high temporal resolution (<1ms) and relatively high spatial resolution (3-5mm), allowing for non-invasive and accurate recording of neural electrical activity information in the brain that can only be recorded using invasive methods such as cortical electroencephalography (EEG). This provides the largest theoretical transmission bandwidth in non-invasive brain imaging technology and provides richer information for studying the working principles of the brain, and making it the most promising technological means in the field of brain computer interface and artificial intelligence research, there is huge development space in the future field of brain computer fusion.
The core of traditional MEG: superconducting magnetic quantum detectors
The core of traditional MEG is an ultra-sensitive magnetic detector. Due to the extremely weak extracranial magnetic field generated by neural electrical activity in the brain, it is only 50-500fT (ft, 10-15), which is only one millionth of the strength of the Earth's magnetic field. Therefore, an extremely sensitive magnetic detector is needed.
In 1962, Josephson predicted and demonstrated that superconducting current can penetrate the insulation in a "weak connection" manner from one superconductor without attenuation to another superconductor. This process is the tunneling effect of superconducting Cooper pairs, which was ultimately confirmed by experiments and won the 1973 Nobel Prize in Physics. Based on a pair of Josephine junctions in a superconducting state, Superconductor Quantum Interference Devices (SQUID) have been born and used in a range of fields including MEG, exploration, and anti-submarine warfare.
In 1986, William Gibson described it as a "terrifying" squid that could extract information from brain memory chips in his famous short story 《Johnny's Memory》.
The Matrix Guard, who was concretized as an electronic squid in the movie 《The Matrix》 (1999), is a tribute to this classic image. The information that needs to be stored is infused through a series of microsurgical procedures, I mechanically spit out this already familiar promotional slogan, The customer's password is stored on a specially designed chip. Except for SQUID (squid, someone in our industry who doesn't want to bring up this topic), there is no way to extract information. Medication cannot be obtained, a head can't be cut open, and torture cannot be obtained. I have no knowledge of the content of the information myself, I never know.
Squid? A thing with many tentacles and crawling around? "We drilled out of the subway passage, and on the street was an abandoned market. There is also a piece of land that can be considered a square, with rotten fish heads and rotten fruits everywhere on the ground. In the dark area across from the square, several dark shadows stared at us.
Quantum perturbation superconducting detector. During war, it was used to search submarines and search for enemy Cyber weapon systems.
Oh? Navy stuff? Used during battles? So, squid can read what's stored on your brain chip? "She stopped. I feel like her eyes, hidden behind those two mirrors, are fixed on me.
When it comes to detecting magnetic fields, even the lowest level squid is billions of times stronger than past magnetic detectors, it's like hearing someone whispering amidst the cheers in the stadium
———Fragments from William Gibson's 《Johnny's Memorie》
The process of non-invasive and precise detection of neural electrical activity in the brain from outside the brain is just as difficult as hearing someone whispering in the cheers of a sports stadium. Although MEG technology was born thanks to the SQUID detector, during its 50 year history, a series of inherent problems of SQUID itself have greatly limited the development of MEG technology.
SQUID needs to operate at ultra-low temperatures with a large amount of liquid helium to maintain its superconducting state. Helium is a scarce and non renewable resource on Earth, and has important applications in fields such as semiconductors, medicine, and national defense. Currently, with the global shortage of helium resources, the price of liquid helium is constantly rising. Some countries such as the United States and Japan have begun to restrict the application of helium in the entertainment industry, and more than 99% of helium in China relies on imports. The operation of SQUID magnetoencephalography requires a large amount of liquid helium, which is extremely expensive, which greatly limits the widespread application of magnetoencephalography.
In addition, SQUID is very sensitive to external vibrations, electromagnetic pulses (EMP), and is prone to damage (EMP is used to eliminate electronic squid in the movie "Matrix"). It requires a strictly shielded working environment and regular heating maintenance, as well as replacing damaged detectors. The cost of equipment maintenance is also high. Due to the sensitivity of SQUID and the requirement for ultra-low temperature, traditional magnetoencephalography requires the detector to be fixed in a huge liquid helium dewar, which makes magnetoencephalography work only in a stationary state and cannot effectively adapt to different sizes of heads, especially in children and adolescents. This further restricts the application range of magnetoencephalography.
OPM, New Dawn
OPM is a technology that utilizes the magnetic effect of atomic spin to measure weak magnetic fields by polarizing atomic gases with a beam of light. The spin-exchange collision relaxation-free (SERF) OPM is a new type of OPM that has achieved rapid development in recent years. By eliminating the atomic tracking delay caused by spin-exchange collision, the SERF magnetometer has two orders of magnitude higher sensitivity than traditional OPM under the same physical conditions. The SERF mechanism was first discovered by Professor Happer and others at Princeton University in 1973, and in 2002, a group led by Professor Romalis at Princeton University first demonstrated OPM based on the SERF principle, with a single channel sensitivity of 7fT/Hz-1/2 and currently reaching 0.16fT/Hz-1/2, surpassing the level that the best SQUID magnetometers can achieve (0.91fT/Hz-1/2). Currently, research in this field by the United States and China is at the highest level in the world. OPM can work at room temperature without the need for liquid helium cooling. They are small in size and light in weight, and can be produced in low-cost mass production through semiconductor processes, bringing new dawn to the field of MEG.
New Type of MEG Based on OPM
With the maturity and miniaturization of the SERF OPM, based on 3D printed customized helmets and optical high-precision spatial calibration technology, the Sir Peter Mansfield Imaging Center at the University of Nottingham, UK (2003 Nobel Prize in Physiology and Medicine, birthplace of magnetic resonance imaging), for the first time in 2017, achieved full temperature OPM based MEG signal recording and spatial traceability localization, and verified the feasibility of wearable MEG, This work was published in Nature magazine in 2018 and received widespread attention. Compared to traditional devices using SQUID detectors, this new type of MEG has the following advantages:
1.The detector operates near room temperature and does not require liquid helium or other consumables. Compared to the millions of yuan of liquid helium consumed annually in traditional MEG the cost of use is almost zero.
2.The detector is closer to the surface of the scalp and has a higher signal-to-noise ratio for detecting neural electrical activity.
3.The detector can be flexibly and wearable, suitable for all populations from infants to adults, and can also be recorded while in motion.
4.The equipment has a smaller volume and lower theoretical production cost, which can be used in smaller magnetic shielding environments, which is conducive to application popularization.
This work has also caused a huge response in the fields of neuroscience and clinical neurology:
Professor Andrew Welchman, Chairman of the Neuroscience Department of the Wellcome Foundation in the UK, commented:“This technology makes traditional scanning methods that require the head to be fixed look like outdated hairstyles.”
Professor Timothy Roberts, rotating chairman of the International Association of MEG at Children's Hospital in Philadelphia, said: "Wearable MEG has great application prospects in the early diagnosis of a series of neurological diseases, including Parkinson's disease and childhood autism.
OPM MEG as an emerging brain imaging technology, is expected to significantly reduce the comprehensive use cost of MEG, reducing it by more than 2/3 from the current ¥10000 yuan per hour, thereby better benefiting patients and providing highly cost-effective research tools for brain science. It can also provide high spatiotemporal accuracy electrical activity diagnosis of the whole brain that traditional technology cannot provide for children, Parkinson's patients, and others.
In this emerging technology field, China started early and is not far behind the international advanced level, even leading in some indicators. It is expected to become a breakthrough in China's research and development of brain imaging and high-value medical imaging equipment.
In addition, due to the maximum transmission bandwidth provided by OPM MEG in the field of non-invasive brain computer interfaces, its theoretical accuracy can approach or even exceed invasive brain computer interface technologies such as cortical electroencephalography（EEG）. This technology can also be combined with neural intervention techniques such as transcranial magnetic stimulation (TMS), which has great practical prospects in the field of brain computer interfaces and is expected to play an immeasurable role in future brain computer fusion fields. Is it possible in the future to use OPM MEG to make the human brain directly connected to computers in science fiction works such as the Ghost in The Shell and the Matrix a reality? With the miniaturization or even chipping of detectors, and the continuous development of signal acquisition and processing technology, it may be possible.
