Objective
Optically-pumped magnetometers (OPMs) promise to make MEG technology more clinically accessible, particularly for epilepsy evaluation. However, it is critical that this new technology is validated against the current clinical standard – EEG. Here, we use a 128-channel OPM-MEG system (Cerca Magnetics Ltd, UK) and a whole-head 64-channel EEG (Brain Vision LLC, USA), to test the feasibility of simultaneous OPM-MEG and EEG measurements.
Methods
12 healthy adults (mean age 41 ± 13yrs, 8 female), undertook a right index finger abduction task (50 trials) and an eyes open/eyes closed task (5 trials). Tasks were completed 3 times: with simultaneous EEG/MEG, EEG only, and MEG only, allowing comparison of MEG signal quality in the presence of EEG and vice versa. Before each scanning session, the remnant magnetic field was nulled by combining optical tracking (NaturalPoint Inc, USA) with magnetometer data from sensors in the helmet (Rea et al., 2021).
Results
For MEG and EEG at the sensor level, there is a clear increase in alpha with eyes closed in comparison with eyes open. For the finger abduction task, trials were time locked to the point of movement offset and averaged over trials for each participant, revealing a task-modulated beta response in EEG and MEG. The sensor with the highest SNR was selected separately for each participant and the SNR was calculated for EEG alone, MEG alone, and each in the presence of the other. There was no significant difference in the SNR of MEG signals with and without EEG (p=0.4) or EEG with and without MEG (p=0.5). The presence of EEG also did not affect the quality of residual field nulling.
Conclusions
It is possible to measure whole-head EEG and OPM-MEG together with good quality data. This paves the way for future clinical assessment of patients with epilepsy using the two modalities combined.