Next-gen technology for brain research
Transcranial magnetic stimulation (TMS) is a non-invasive procedure for stimulating nerve cells with the use of magnetic fields. It has been used in research, and to treat psychiatric disorders and depression. It offers high spatiotemporal resolution via the application of a very strong magnetic field pulse – 10 000 times the Earth’s magnetic field – to temporarily disrupt brain activity. Μagnetoencephalography (MEG) is the most precise technique for measuring the magnetic fields generated by the electrical activity in the brain. MEG uses ultrasensitive sensors capable of detecting fields typically less than a hundred-millionth of the Earth’s magnetic field. Although TMS and MEG could be combined for studying the functional connectivity between different regions of the brain, they are incompatible.
Combining TMS and MEG
Undertaken with the support of the Marie Skłodowska-Curie Actions (MSCA) programme, the STIMUSURE project aims to overcome this limitation by developing the first device to combine TMS and MEG. “The magnetic fields from TMS disrupt the ultra-sensitive MEG sensors, making simultaneous use of these techniques impossible,” explains MSCA research fellow Lari Koponen. Following software and hardware manufacturing advances, STIMUSURE constructed an innovative sensor that could withstand the peak magnetic fields generated by TMS and still record the weak magnetic fields produced by the brain. The sensor operates on the principle of optical pumping, a process where light is used to manipulate the magnetic properties of atoms. It essentially measures the changes in the light absorption and emission patterns which are proportional to the intensity of the magnetic field. Optically pumped magnetometer (OPM) sensors are ideal for weak magnetic fields and applications in biomagnetism, such as measuring the magnetic fields produced by neural activity in the brain or the heart. Importantly, they can operate at room temperature. The STIMUSURE sensor was technically validated following TMS pulses, proving it is possible to combine MEG with simultaneous TMS.
Technological advancements
The project also produced the first intrinsic optically pumped magnetic gradiometer for human use. This constitutes an advanced type of magnetometer designed to measure the gradient of a magnetic field, rather than just the field's absolute strength. This magnetic gradiometer is particularly useful for detecting small changes in magnetic fields across space, which can provide more detailed information about the source of the magnetic fields, such as neural activity in the brain or electrical activity in the heart. Importantly, it was shown to reduce within-sensor interference.
Future directions
While STIMUSURE did not secure further funding to transition the sensor from a proof-of-concept to a research-grade tool, the project's achievements pave the way for future research. “The advancements in sensor technology and the creation of computational models provide a strong foundation for overcoming the remaining technical challenges,” highlights Koponen. Future efforts will need to focus on refining TMS hardware and securing the necessary support to bring these innovations to full fruition. The rationale of combining TMS and MEG marks an important step towards more precise functional brain mapping and deeper understanding of brain connectivity.
Keywords
STIMUSURE, brain, sensor, TMS, MEG, transcranial magnetic stimulation, magnetoencephalography
