Development of an on-scalp MEG system using optically-pumped magnetometers

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School of Science | Doctoral thesis (article-based) | Defence date: 2021-01-15
Degree programme
82 + app. 64
Aalto University publication series DOCTORAL DISSERTATIONS, 222/2020
Magnetoencephalography (MEG) is a noninvasive functional neuroimaging method in which the magnetic field due to the neuronal currents that constitute brain activity is measured using sensors outside the head. These neuromagnetic measurements can be used to make inference about brain function within neuroscientific research and clinical medicine. Current state-of-the-art MEG systems utilize ultra-sensitive superconducting quantum interference device (SQUID) sensors that require cryogenic cooling and thermal insulation between the sensors and the subject's head. The thermal insulation necessitates a relatively large brain–sensor separation, leading to loss of signal as well as spatial resolution. Recent developments to another type of sensor, called optically-pumped magnetometer (OPM), has resulted in sensors with sufficient performance for MEG use. OPMs do not require cryogenic temperatures to operate and can thus be placed much closer to, or even directly in contact with the subject's scalp. Such on-scalp MEG systems will significantly improve signal amplitude as well as increase spatial resolution, and may thus enable the detection of phenomena so far only detectable in invasive measurements. In addition, the improved adaptability of OPMs will enable new types of MEG systems, including wearable MEG devices allowing for subject movement and smaller, low-cost systems. This Thesis contributes to the development of an on-scalp MEG system based on OPMs and explores what such a system can or should consist of. In particular, the requirements for how well the sensor positions must be known in order to estimate the MEG signal source within the brain are determined using simulations. Following the determination of these requirements, an optical method to localize the sensors that fulfills those requirements is developed and validated. Further, the performance of a first-generation on-scalp MEG system capable of source estimation is demonstrated and its performance compared to that of a state-of-the-art SQUID-based MEG system. Finally, a method and software package for magnetic field modeling and coil design is developed, which can be utilized for the next generation of on-scalp MEG systems.Through these works, this Thesis provides a stepping stone for the continued development of on-scalp MEG as an imaging modality and demonstrates its future prospects.
Supervising professor
Parkkonen, Lauri, Prof., Aalto University, Department of Neuroscience and Biomedical Engineering, Finland
Thesis advisor
Parkkonen, Lauri, Prof., Aalto University, Finland
magnetoencephalography, optically-pumped magnetometer, on scalp, neuroimaging, co-registration, stream function, coil design, magnetic field modeling
Other note
  • [Publication 1]: Rasmus Zetter, Joonas Iivanainen, Matti Stenroos and Lauri Parkkonen. Requirements for Coregistration Accuracy in On-Scalp MEG. Brain Topography, 31:931–948, June 2018.
    Full text in Acris/Aaltodoc:
    DOI: 10.1007/s10548-018-0656-5 View at publisher
  • [Publication 2]: Rasmus Zetter, Joonas Iivanainen and Lauri Parkkonen. Optical Co-registration of MRI and on-scalp MEG. Scientific Reports, 9:5490, April 2019.
    Full text in Acris/Aaltodoc:
    DOI: 10.1038/s41598-019-41763-4 View at publisher
  • [Publication 3]: Joonas Iivanainen, Rasmus Zetter and Lauri Parkkonen. Potential of on- scalp MEG: Robust detection of human visual gamma-band responses. Human Brain Mapping, 41:150–161, September 2019.
    Full text in Acris/Aaltodoc:
    DOI: 10.1002/hbm.24795 View at publisher
  • [Publication 4]: Rasmus Zetter, Antti J Mäkinen, Joonas Iivanainen, Koos C J Zevenhoven, Risto J Ilmoniemi and Lauri Parkkonen. Magnetic field modeling with surface currents. Part II. Implementation and usage of bfieldtools. Journal of Applied Physics, 128:063905, August 2020.
    Full text in Acris/Aaltodoc:
    DOI: 10.1063/5.0016087 View at publisher