Magnetoencephalography (MEG) is a noninvasive neuroimaging technique in which the magnetic fields of electrically active neuron populations in the brain are measured outside the head. The neuromagnetic field contains temporal and spatial information about the underlying neuronal sources. Measurements of this field can be used to make inference about brain function. The temporal resolution of MEG is excellent as the magnetic field gives an instantaneous measure of the neuronal activity at the physiologically relevant frequency range. However, the spatial resolution, or the amount of spatial information, is limited in the state-of-the-art MEG systems as the current superconducting sensor technology does not allow the field measurement as close to the neural sources as noninvasively possible but limits the measurement distance to a couple of centimeters from the subject's scalp. At that distance, the field amplitude as well as the number of spatial degrees of freedom in the field have decayed considerably. Recent developments in quantum optics have enabled sensors suitable for measuring the neuromagnetic field within millimetres from the scalp. With these optically-pumped magnetometers (OPMs), on-scalp sensor arrays can be constructed increasing the spatial resolution of MEG. In this Thesis, improvement in signal amplitude as well as in spatial resolution due to on-scalp field sensing is quantified both with simulations and measurements. Requirements for spatial sampling of the neuromagnetic field to achieve those improvements are theoretically investigated. To sense the neuromagnetic field on scalp, an OPM-based MEG system is constructed. The system uses active magnetic shielding with external coils to reduce interference in the measurements. Neuromagnetic responses to visual stimulation are measured using the OPM system and are compared to those obtained with a state-of-the-art cryogenic MEG system. The Thesis shows the potential of on-scalp sensor arrays implemented with OPMs to increase the spatial resolution and applicability of MEG.
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- optically-pumped magnetometer
- on scalp
- magnetic shielding