Transforming and comparing data between standard SQUID and OPM-MEG systems

Marhl, Urban; Jodko-Władzińska, Anna; Brühl, Rüdiger; Sander, Tilmann; Jazbinsek, V.

doi:10.1371/journal.pone.0262669

Cited by 29 publications

(19 citation statements)

References 52 publications

(59 reference statements)

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“…This allows for an alternate optimal sensor design that is tailored based on actual data, which may be more effective than utilizing a universal, uniformly-spaced sensor design. Recently, there have been discussions that suggest that since some OPM sensor arrays allow for flexible sensor position placement, it may allow for fewer but more effectively-placed sensors [19,7]. Our results indicate this to be true.…”

Section: Introductionsupporting

confidence: 60%

Effcient magnetometer sensor array selection for signal reconstruction and brain source localization

Yeo¹,

Taulu²,

Kutz³

2022

Preprint

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Magnetoencephalography (MEG) is a noninvasive method for measuring magnetic flux signals caused by brain activity using sensor arrays located on or above the scalp. A common strategy for monitoring brain activity is to place sensors on a nearly uniform grid, or sensor array, around the head. By increasing the total number of sensors, higher spatial-frequency components of brain activity can be resolved as dictated by Nyquist sampling theory. Currently, there are few principled mathematical architectures for sensor placement aside from Nyquist considerations. However, global brain activity often exhibits low-dimensional patterns of spatio-temporal dynamics. The low-dimensional global patterns can be computed from the singular value decomposition and can be leveraged to select a small number of sensors optimized for reconstructing brain signals and localizing brain sources. Moreover, a smaller number of sensors which are systematically chosen can outperform the entire sensor array when considering noisy measurements. We propose a greedy selection algorithm based upon the QR decomposition that is computationally efficient to implement for MEG. We demonstrate the performance of the sensor selection algorithm for the tasks of signal reconstruction and localization. The performance is dependent upon source localization, with shallow sources easily identified and reconstructed, and deep sources more difficult to locate. Our findings suggest that principled methods for sensor selection can improve MEG capabilities and potentially add cost savings for monitoring brain-wide activity.

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Section: Introductionsupporting

confidence: 60%

Effcient magnetometer sensor array selection for signal reconstruction and brain source localization

Yeo¹,

Taulu²,

Kutz³

2022

Preprint

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“…( d ) Digitised helmet points (red dots) aligned with helmet-model, and co-registered to the anatomy (head surface from MRI). ( e ) Magnetometer field pattern for an IED (at the time point of maximum SNR) recorded with the SQUID-based system (left), as well as field pattern for an IED recorded with the OPMs, projected onto the SQUID-sensor layout (using inverse/forward projection with minimum norm 52 , 53 ). Note the good agreement between the IED field patterns, despite the limited sampling with the OPMs, suggesting that both systems recorded similar phenomena.…”

Section: Methodsmentioning

confidence: 99%

“…The DAiSS toolbox in SPM (version 12) was used to reconstruct the time-series of neuronal activity (so-called virtual electrodes) for the centroids 63 of 246 regions of the Brainnetome atlas (BNA) 64 . Broadband (0.5-48 Hz) beamformer weights were constructed, for which the data covariance matrix was filtered using a discrete-cosine-transform after applying a Hanning taper, 5% Tikhonov Magnetometer field pattern for an IED (at the time point of maximum SNR) recorded with the SQUID-based system (left), as well as field pattern for an IED recorded with the OPMs, projected onto the SQUID-sensor layout (using inverse/forward projection with minimum norm 52,53 ). Note the good agreement between the IED field patterns, despite the limited sampling with the OPMs, suggesting that both systems recorded similar phenomena.…”

mentioning

confidence: 99%

Non-invasive measurements of ictal and interictal epileptiform activity using optically pumped magnetometers

Hillebrand

Holmes

Sijsma

et al. 2023

Sci Rep

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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 and/or logistical considerations. Optically Pumped Magnetometers (OPMs) can be placed directly on the scalp, thereby improving SNR and enabling recordings during seizures. We aimed to demonstrate the performance of OPMs in a clinical population. Seven patients with challenging cases of epilepsy underwent MEG recordings using a 12-channel OPM-system and a 306-channel cryogen-based whole-head system: three adults with known deep or weak (low SNR) sources of interictal epileptiform discharges (IEDs), along with three children with focal epilepsy and one adult with frequent seizures. The consistency of the recorded IEDs across the two systems was assessed. In one patient the OPMs detected IEDs that were not found with the SQUID-system, and in two patients no IEDs were found with either system. For the other patients the OPM data were remarkably consistent with the data from the cryogenic system, noting that these were recorded in different sessions, with comparable SNRs and IED-yields overall. 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. The relatively affordable 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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“…d) Digitised helmet points aligned with helmet-model, and co-registered to the anatomy (head surface from MRI). e ) Field pattern for an IED recorded with the SQUID-based system (left), as well as field pattern for an IED recorded with the OPMs, projected onto the SQUID-sensor layout (after inverse/forward projection with minimum norm (Knösche, 2002; Marhl et al, 2022)). Note the good agreement between the IED field patterns, despite the limited sampling with the OPMs, suggesting that both systems recorded the same phenomenon.…”

Section: Methodsmentioning

confidence: 99%

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

Hillebrand

Holmes

Sijsma

et al. 2022

Preprint

View full text Add to dashboard Cite

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.

show abstract

Transforming and comparing data between standard SQUID and OPM-MEG systems

Cited by 29 publications

References 52 publications

Effcient magnetometer sensor array selection for signal reconstruction and brain source localization

Effcient magnetometer sensor array selection for signal reconstruction and brain source localization

Non-invasive measurements of ictal and interictal epileptiform activity using optically pumped magnetometers

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

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