A conformal array of microfabricated optically-pumped first-order gradiometers for magnetoencephalography

Abstract

An array of 21 first-order gradiometers based on zero-field optically-pumped magnetometers is demonstrated for use in magnetoencephalography. Sensors are oriented radially with respect to the head and housed in a helmet with moveable holders which conform to the shape of a scalp. Our axial gradiometers have a baseline of 2 cm and reject laser and vibrational noise as well as common-mode environmental magnetic noise. The median sensitivity of the array is 15.4 fT/Hz1/2, measured in a human-sized magnetic shield. All magnetometers are operated independently with negative feedback to maintain atoms at zero magnetic field. This yields higher signal linearity and operating range than open-loop operation and a measurement system that is less sensitive to systematic and ambient magnetic fields. All of the system electronics and lasers are compacted into one equipment rack which offers a favorable outlook for use in clinical settings.

Measuring functional connectivity with wearable MEG

“…Recent work has shown that inherent OPM-gradiometers (i.e. OPMs where the same light is passed through two separate vapour cells, eliminating the need for digital subtraction and consequently the noise increase) are possible to construct ( Nardelli et al, 2020 ). However, axial gradiometers would require cells to be stacked on top of one another (i.e.…”

First experiences of whole-head OP-MEG recordings from a patient with Epilepsy

“…This is arguably more of a problem for OP-MEG than for SQUID-MEG since SQUID-MEG typically uses gradiometers which provide inherent noise reduction (Hämäläinen et al ., 1993). While there are gradiometer OP-MEG designs (Nardelli et al ., 2020; Zhang et al ., 2020), our array was based on magnetometers. It has been shown that high SNR OP-MEG data can be achieved even when there is large participant movement (Boto et al ., 2018; Seymour et al ., 2021), but in these scenarios, the timing of the trials is set by an external stimulus.…”

Modelling optically pumped magnetometer interference as a mean (magnetic) field

“…when someone rotates their head the magnetic field designed to keep zero field in one orientation will be incorrectly applied to a different orientation). We do not investigate this effect here but note that it could be mitigated by operating sensors in a closed loop mode (Nardelli et al, 2020), learning this field profile directly from the data itself or by utilizing active shielding so as to keep these values to a minimum (Holmes et al, 2018(Holmes et al, , 2019Iivanainen et al, 2019).…”

“…We note that when a participant moves, one could incorporate linear gradient terms. These would be quite beneficial for very low frequency studies (<3Hz), designing a model informed closed loop OPM systems (Nardelli et al, 2020) or for informing coil currents in active shielding systems (Holmes et al, 2019). However, for simplicity we will ignore these higher order terms here and just focus on the benefits of using a very simple mean field approximation.…”

“…Another hardware option is to configure the sensors as gradiometers which have excellent noise suppression (Colombo et al, 2016; Nardelli, Perry, Krzyzewski, & Knappe, 2020; Sheng et al, 2017) and are capable of unshielded neural recordings (Limes et al, 2020). In contrast, atomic magnetometers (Osborne, Orton, Alem, & Shah, 2018) typically require considerable active and passive shielding for successful MEG recordings (Barry et al, 2019; Lin et al, 2019; Tierney et al, 2018, 2020).…”