Spatiotemporal signal space separation method for rejecting nearby interference in MEG measurements

TLDR

Traditional MEG is limited by interference from nearby sources, excluding patients with vagus nerve stimulators, magnetic particles, or dental materials, because conventional rejection methods cannot suppress artefacts generated within 20–40 cm of the sensor array. This study proposes a spatiotemporal signal space separation method that removes both external interference and scalp‑generated artefacts. The method first separates brain and external signals using signal space separation grounded in sensor geometry and Maxwell’s equations, then removes nearby artefacts by time‑domain statistical analysis and projection. Tests with artificial dipoles and patient data confirm that the method eliminates artefacts while preserving the brain signal field patterns.

Abstract

Limitations of traditional magnetoencephalography (MEG) exclude some important patient groups from MEG examinations, such as epilepsy patients with a vagus nerve stimulator, patients with magnetic particles on the head or having magnetic dental materials that cause severe movement-related artefact signals. Conventional interference rejection methods are not able to remove the artefacts originating this close to the MEG sensor array. For example, the reference array method is unable to suppress interference generated by sources closer to the sensors than the reference array, about 20-40 cm. The spatiotemporal signal space separation method proposed in this paper recognizes and removes both external interference and the artefacts produced by these nearby sources, even on the scalp. First, the basic separation into brain-related and external interference signals is accomplished with signal space separation based on sensor geometry and Maxwell's equations only. After this, the artefacts from nearby sources are extracted by a simple statistical analysis in the time domain, and projected out. Practical examples with artificial current dipoles and interference sources as well as data from real patients demonstrate that the method removes the artefacts without altering the field patterns of the brain signals.