Exploring the limits of MEG spatial resolution with multipolar expansions

Abstract

The advent of scalp magnetoencephalography (MEG) based on optically-pumped magnetometers (OPMs) may represent a step change in the field of human electrophysiology. Compared to cryogenic MEG based on superconducting quantum interference devices (SQUIDs, placed 2-4 cm above scalp), scalp MEG promises significantly higher spatial resolution imaging but it also comes with numerous challenges regarding how to optimally design OPM arrays. In this context, we sought to provide a systematic description of MEG spatial resolution as a function of the number of sensors (allowing comparison of low-vs. high-density MEG), sensor-to-brain distance (cryogenic SQUIDs vs. scalp OPM), sensor type (magnetometers vs. gradiometers; single-vs. multicomponent sensors), and signal-to-noise ratio. To that aim, we developped an analytical theory based on the physically-grounded technique of MEG multipolar expansions. In a first step, we show that the theory (together with experimental input) enables rigorous, analytical, and quantitative assessment of the limits of MEG spatial resolution in asymptotically high-density MEG. In a second step, we enlarge the theory using simulated data to encompass MEG at lower sensor density, and we build a full description in terms of two qualitatively distinct regimes. The resulting two-regime model of MEG spatial resolution integrates known observations (e.g., the difficulty of improving spatial resolution by increasing sensor density, the gain brought by moving sensors on scalp, or the usefulness of multi-component sensors) and gathers them under a unifying theoretical framework that highlights the underlying physical mechanisms. It also reveals new properties of the limits of MEG spatial resolution, such as their logarithmic divergence at asymptotically high density or their saturation as sensors approach sources of neural activity. We propose that this framework may find useful applications to benchmark the design of future OPM-based scalp MEG systems.

Non-invasive measurements of ictal and interictal epileptiform activity using optically pumped magnetometers

“…Movement-artefacts could not be regressed from the recordings 55,56 since movements were not tracked. Sophisticated spatial filtering techniques for noise removal, such as Signal Space Separation 57 , could also not be applied to the 12 channel OPM-recordings, as the magnetic fields are undersampled and, more fundamentally, because the internal space basis set is invalid as a model of brain signals for on scalp sampling 43,58 . Tierney and colleagues have shown that as a first approximation the magnetic interference can be modelled as a spatially homogenous field, which can subsequently be regressed from the data 42,55,59 .…”