OPM MEG

Optically pumped magnetometer Magnetoencephalography (OPM-MEG) 

The goal of this device is to record the magnetic field related to the electrical currents produced by the brain (postsynaptic potentials). 

OPM-MEG uses a technique called optical pumping, where light is used to polarize the atoms of a vapor. The interaction between the polarized atoms and the external magnetic fields change the properties of the light passing through the vapor and can therefore be used to detect magnetic fields. The sensitivity is down to femtoteslas (fT), and is achievable at room temperature, making it more practical than the MED devices with SQUIDs (which use cryogenics and therefore need very low temperatures, as He has to stay at around 0K). 

The application of MEG is mostly to study functional brain activity, particularly the location of origin in electrical signals. 

At zero magnetic field, each of these atoms have a tiny magnetic moment and a random orientation. If the lasers are tuned at the right frequency, the photons in the laser will be absorbed by the atoms, making all the iridium atoms have aligned magnetic moments. If they’re now all aligned in the right direction, the atoms are considered "saturated" and can no longer absorb photons and allow the laser to pass through. The photodetector measures the intensity of this transmitted laser light, which reflects the atoms’ state.

When a brain is nearby, the neural activity generates weak magnetic fields known as neuromagnetic fields. This neuromagnetic field, will also cross through the glass cell and interact with the magnetic fields of the atoms. This interaction alters the alignment of the atoms, which in turn changes the laser light intensity detected by the photodetector. By precisely measuring these changes, OPM sensors can detect the brain’s magnetic field with high precision. This results in a very sensitive magnetometer. 

Background field response 

As the head moves, there are several changes in the magnetic field artefacts due to the different magnetic fields in each position. The best way would be to reduce external magnetic field to zero, however this is mostly not possible.

In research and clinical settings, one can add to the walls of the study room multiple independent controlled coils. This allows adaptation of current patterns depending on head location. Inside each coil square, one can adjust the magnetic field to zero. As the helmet moves, one can change the current: the image shows that it is possible to change the position of the currents if the helmet should move to the edge of the array. This results in a background magnetic field always close to zero, independent of one’s movements. 

OPM MEG advantages 

The major improvements have been to be able to study moving patients and children up to 2 year olds. Also, it was possible to measure data from epilepsy with a child during a seizure, as the fact that he was moving wasn’t a problem.

In conclusion, quantum sensors can be positioned closer to the brain than conventional cryogenic sensors, meaning higher sensitivity and improvements in spatial precision. The range of people possible to scan is now higher and it is possible to scan patients whilst moving. Furthermore, the costs are less than 50% compared to a conventional system.