Spherical harmonic based noise rejection and neuronal sampling with multi-axis OPMs

Tierney, Tim M.; Mellor, Stephanie; O'Neill, George; Timms, Ryan C; Barnes, Gareth R.

doi:10.1016/j.neuroimage.2022.119338

Cited by 31 publications

(33 citation statements)

References 36 publications

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“…For such systems, uniform sampling may be sufficient, as long as the sensor spacing is approximately equal to the distance of the sensors to the closest source (Iivanainen et al, 2021). When using triaxial sensors, 75-100 uniformly placed sensors would provide sufficient spatial sampling (Tierney et al, 2022). It is still an open question what the optimal helmet design would be for larger clinical studies.…”

Section: Discussionmentioning

confidence: 99%

“…Six, commercially available Gen2.0 OPMs (QuSpin Inc, Louisville, CO, USA; Zero field magnetometer, 2 nd Generation, dual-axis measurement), with sensitivities of 7-13 fT/√Hz and a dynamic range of ±5 nT (Osborne et al, 2018), were used. We recorded simultaneously along both the radial and a tangential axis to increase the number of measurements and to increase the separability of neuronal and noise signals (Brookes et al, 2021; Nurminen et al, 2013; Rea et al, 2022; Tierney et al, 2021a; Tierney et al, 2022), at a cost of a slight reduction in sensitivity (Osborne et al, 2018). The sensors operate in the spin exchange relaxation-free regime, and the (near) zero-field environment is achieved through ‘on-sensor’ electromagnetic coils wrapped around the vapour cell, that can compensate for remnant fields in the magnetically shielded room (MSR) of up to 50 nT.…”

Section: Methodsmentioning

confidence: 99%

“…Despite the dynamic field compensation, the OPM recordings still contain interference from external (and internal) sources, as well as due to movement of the sensors through the remnant fields when the patients move their heads. Sophisticated spatial filtering techniques for noise removal, such as Signal Space Separation (Taulu et al, 2004), could 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 (Tierney et al, 2022; Wens, 2022). 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 (Hill et al, 2022; Seymour et al, 2022; Tierney et al, 2021a).…”

Section: Methodsmentioning

confidence: 99%

“…could 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 (Tierney et al, 2022;Wens, 2022).…”

Section: Opm Pre-processingmentioning

confidence: 99%

See 3 more Smart Citations

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

Hillebrand

Holmes

Sijsma

et al. 2022

Preprint

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Magneto- and Electroencephalography (MEG/EEG) are important techniques for the diagnosis and pre-surgical evaluation of epilepsy. Yet, in current cryogen-based MEG systems the sensors are offset from the scalp, which limits the signal-to-noise ratio (SNR) and thereby the sensitivity to activity from deep structures such as the hippocampus. This effect is amplified in children, for whom adult-sized fixed-helmet systems are typically too big. Moreover, ictal recordings with fixed-helmet systems are problematic because of limited movement tolerance. Optically Pumped Magnetometers (OPMs) can be placed directly on the scalp, thereby improving SNR and consequently the sensitivity to, and localisation accuracy of, epileptiform activity. In addition, recording during seizures becomes feasible with these wearable sensors. We aimed to demonstrate these advantages of OPMs in a clinical population. Three adults with known weak sources of interictal epileptiform discharges (IEDs), along with three children with focal epilepsy and one adult with frequent seizures underwent MEG recordings using a 12-channel OPM-system and a 306-channel cryogen-based whole-head system. Performance of the two systems was compared in terms of IED-rate and SNR. In one patient the OPMs detected IEDs that were not found with the SQUID-system. In one patient the spike yield was higher for the OPM data (9.00 versus 6.76), with negligible difference in SNR compared to the SQUID data (3.85 versus 3.93; U = -2.86, d = -0.14). This was also the case for a patient with a spike yield that was comparable to that for the SQUID data (after accounting for unilateral coverage with the OPMs; SNR 4.47 versus 4.57; U = -3.81, d = -0.14). For one patient the spike yield (11.03 versus 24.50) and SNR (4.39 versus 4.05; U = 9.53, d = -0.36) were both lower for the OPMs. In two patients no IEDs were found with either system. Importantly, the wearability of OPMs enabled the recording of seizure activity in a patient with hyperkinetic movements during the seizure. The observed ictal onset and semiology were in agreement with previous video- and stereo-EEG recordings. Overall, OPM data were very much comparable to those obtained with a cryogenic system: OPMs outperformed SQUIDs for two of the four patients with IEDs, with either a higher spike yield, or an ability to detect IEDs that were not observable in the SQUID data. For three patients the SNRs of IEDs were (slightly) lower in the OPM data than in the SQUID data, but with negligible effect sizes for two of these patients. The relatively cheap technology, in combination with reduced running and maintenance costs, means that OPM-based MEG could be used more widely than current MEG systems, and may become an affordable alternative to scalp EEG, with the potential benefits of increased spatial accuracy, reduced sensitivity to volume conduction/field spread, and increased sensitivity to deep sources. Wearable MEG thus provides an unprecedented opportunity for epilepsy, and given its patient-friendliness, we envisage that it will not only be used for presurgical evaluation of epilepsy patients, but also for diagnosis after a first seizure.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Opm Pre-processingmentioning

confidence: 99%

See 2 more Smart Citations

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

Hillebrand

Holmes

Sijsma

et al. 2022

Preprint

Self Cite

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“…We expect that with a larger number of sensors, especially by using triaxial rather than dualaxis devices 51,52 , we will be able to better characterize both the signal and environmental noise space. Also, although OPM sensitivity does begin to decline at 130Hz, it does so only as a first order filter (i.e., a halving of amplitude for every doubling of frequency).…”

Section: Discussionmentioning

confidence: 99%

Concurrent spinal and brain imaging with optically pumped magnetometers

Mardell

O'Neill

Tierney

et al. 2022

Preprint

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The spinal cord and its interactions with the brain are fundamental for movement control and somatosensation. However, brain and spinal cord electrophysiology in humans have largely been treated as distinct enterprises, in part due to the relative inaccessibility of the spinal cord. Consequently, there is a dearth of knowledge on human spinal electrophysiology, including the multiple pathologies of the central nervous system that affect the spinal cord as well as the brain. Here we exploit recent advances in the development of wearable optically pumped magnetometers (OPMs) which can be flexibly arranged to provide coverage of both the spinal cord and the brain concurrently in unconstrained environments. Our system for magnetospinoencephalography (MSEG) measures both spinal and cortical signals simultaneously by employing a custom-made spinal scanning cast. We evidence the utility of such a system by recording simultaneous spinal and cortical evoked responses to median nerve stimulation, demonstrating the novel ability for concurrent non-invasive millisecond imaging of brain and spinal cord.

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Adaptive multipole models of optically pumped magnetometer data

Tierney,

Seedat,

St Pier

et al. 2024

Human Brain Mapping

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Multipole expansions have been used extensively in the Magnetoencephalography (MEG) literature for mitigating environmental interference and modelling brain signal. However, their application to Optically Pumped Magnetometer (OPM) data is challenging due to the wide variety of existing OPM sensor and array designs. We therefore explore how such multipole models can be adapted to provide stable models of brain signal and interference across OPM systems. Firstly, we demonstrate how prolate spheroidal (rather than spherical) harmonics can provide a compact representation of brain signal when sampling on the scalp surface with as few as 100 channels. We then introduce a type of orthogonal projection incorporating this basis set. The Adaptive Multipole Models (AMM), which provides robust interference rejection across systems, even in the presence of spatially structured nonlinearity errors (shielding factor is the reciprocal of the maximum fractional nonlinearity error). Furthermore, this projection is always stable, as it is an orthogonal projection, and will only ever decrease the white noise in the data. However, for array designs that are suboptimal for spatially separating brain signal and interference, this method can remove brain signal components. We contrast these properties with the more typically used multipole expansion, Signal Space Separation (SSS), which never reduces brain signal amplitude but is less robust to the effect of sensor nonlinearity errors on interference rejection and can increase noise in the data if the system is sub‐optimally designed (as it is an oblique projection). We conclude with an empirical example utilizing AMM to maximize signal to noise ratio (SNR) for the stimulus locked neuronal response to a flickering visual checkerboard in a 128‐channel OPM system and demonstrate up to 40 dB software shielding in real data.

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Spherical harmonic based noise rejection and neuronal sampling with multi-axis OPMs

Cited by 31 publications

References 36 publications

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

Concurrent spinal and brain imaging with optically pumped magnetometers

Adaptive multipole models of optically pumped magnetometer data

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