On the Potential of a New Generation of Magnetometers for MEG: A Beamformer Simulation Study

Abstract

Magnetoencephalography (MEG) is a sophisticated tool which yields rich information on the spatial, spectral and temporal signatures of human brain function. Despite unique potential, MEG is limited by a low signal-to-noise ratio (SNR) which is caused by both the inherently small magnetic fields generated by the brain, and the scalp-to-sensor distance. The latter is limited in current systems due to a requirement for pickup coils to be cryogenically cooled. Recent work suggests that optically-pumped magnetometers (OPMs) might be a viable alternative to superconducting detectors for MEG measurement. They have the advantage that sensors can be brought to within ~4 mm of the scalp, thus offering increased sensitivity. Here, using simulations, we quantify the advantages of hypothetical OPM systems in terms of sensitivity, reconstruction accuracy and spatial resolution. Our results show that a multi-channel whole-head OPM system offers (on average) a fivefold improvement in sensitivity for an adult brain, as well as clear improvements in reconstruction accuracy and spatial resolution. However, we also show that such improvements depend critically on accurate forward models; indeed, the reconstruction accuracy of our simulated OPM system only outperformed that of a simulated superconducting system in cases where forward field error was less than 5%. Overall, our results imply that the realisation of a viable whole-head multi-channel OPM system could generate a step change in the utility of MEG as a means to assess brain electrophysiological activity in health and disease. However in practice, this will require both improved hardware and modelling algorithms.

Optimising experimental design for MEG resting state functional connectivity measurement

“…Perhaps counter-intuitively, as the signal to noise ratio of MEG measurements has increased (i.e. as system architecture has improved), the sensitivity of source reconstruction, particularly using adaptive methods such as beamforming, to coregistration accuracy has also increased (Boto et al, 2016). This means that in modern multi-channel systems, even modest errors in knowledge of sensor location and orientation relative to brain anatomy generate a significant degradation in reconstruction accuracy.…”

Measuring MEG closer to the brain: Performance of on-scalp sensor arrays

“…Boto and colleagues (2016) reported a fivefold SNR (amplitude) improvement with an OPM compared to a SQUID array, which was obtained by assuming equivalent sensor noise levels for the two arrays which both comprised axial gradiometers with 5-cm baselines. The authors computed the magnetic field with local-spheres head model (Huang et al, 1999) in a single head geometry.…”

“…Recently, Boto and colleagues (2016) reported a simulation study where they investigated the performance of on-scalp MEG sensor arrays. In the simulations, they employed a SQUID sensor configuration of 275-channel axial gradiometer system, that measured the gradient of the field component normal to the MEG helmet, and determined the OPM sensor positions by projecting the SQUID locations to 4 mm from scalp.…”

“…OPMs could also be placed within few millimetres from scalp, permitting EEG-cap-like MEG arrays. Certain aspects of performance of such on-scalp MEG sensor arrays has been recently evaluated by Boto and colleagues (2016); they showed that multichannel whole-head OPM system that measures the magnetic field component normal to the local scalp surface offers a fivefold improvement in sensitivity and clear improvements in reconstruction accuracy and spatial resolution of beamforming compared to traditional SQUID arrays. Our simulations go beyond that work by evaluating also OPM arrays that measure tangential field components, by investigating how the different aspects of MEG signal generation are reflected in the field components and by introducing measures that can be used to quantify and benchmark performance of MEG arrays and which also serve as means to understand the performance differences between arrays.…”

Data‐driven model optimization for optically pumped magnetometer sensor arrays

“…Optically pumped magnetometers (OPMs) provide a sensitive, flexible and low‐cost alternative to superconducting quantum interference devices (SQUIDs) for measuring magnetoencephalography (MEG) data (Boto et al, ). OPMs work by measuring the transmission of laser light through a cell containing a vapour of spin‐polarised alkali atoms, providing a highly sensitive measure of the local magnetic field inside the cell.…”