Automatic coregistration of MRI and on-scalp MEG

Gu, Wenyu; Ru, Xingyu; Li, Dongxu; He, Kun; Cui, Yunyang; Sheng, Jin; Gao, Jia‐Hong

doi:10.1016/j.jneumeth.2021.109181

Cited by 15 publications

(10 citation statements)

References 28 publications

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“…A compromise may be the use of a limited number of different-sized rigid helmets, with holders in which OPMs can be pressed onto the scalp (Zetter et al, 2019). The depth of the OPMs within their holder could be measured manually, or sensor positions and orientations could be determined automatically, using either an optical approach (Gu et al, 2021) or utilising the field-nulling coils (Iivanainen et al, 2022). The latter approach has the advantage that any changes in the orientations of the OPM’s sensitive axes due to crosstalk from neighbouring OPMs are already taken into account.…”

Section: Discussionmentioning

confidence: 99%

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%

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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“…Thus, the structured-light scanner is recommended over Polhemus. Due to the lack of the information on the true sensor positions and orientations in clinical practice, previous studies mainly evaluated the co-registration error through surface fit or repeatability errors (Zetter et al, 2019;Hill et al, 2020;Gu et al, 2021). The present study considered the final co-registration error as the standard to quantify the co-registration.…”

Section: Discussionmentioning

confidence: 99%

“…However, it operates at the temperature of liquid helium (4 K), resulting in expensive maintenance costs. In addition, the requirement for temperature insulation between sensors and the scalp increases the distance between them to approximately 2 cm (Zetter et al, 2019;Gu et al, 2021), reducing the signal-to-noise ratio (SNR) of the brain signal. New technologies have emerged to overcome the low SNR, including optically pumped magnetometers (OPMs) (Tierney et al, 2018;Vivekananda et al, 2020) and high-temperature SQUIDs (Pfeiffer et al, 2019;Schneiderman et al, 2019), which can be placed very close to the scalp.…”

Section: Introductionmentioning

confidence: 99%

“…Therefore, Hill et al suggest the use of a rigid helmet. The commonly used co-registration devices for on-scalp MEG include the electromagnetic digitization system (Polhemus) (Hill et al, 2019), structured-light scanner (Zetter et al, 2019), and laser scanner (Gu et al, 2021). The co-registration accuracies of these devices require comprehensive evaluation.…”

Section: Introductionmentioning

confidence: 99%

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Co-registration Comparison of On-Scalp Magnetoencephalography and Magnetic Resonance Imaging

Cao

et al. 2021

Front. Neurosci.

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Magnetoencephalography (MEG) can non-invasively measure the electromagnetic activity of the brain. A new type of MEG, on-scalp MEG, has attracted the attention of researchers recently. Compared to the conventional SQUID-MEG, on-scalp MEG constructed with optically pumped magnetometers is wearable and has a high signal-to-noise ratio. While the co-registration between MEG and magnetic resonance imaging (MRI) significantly influences the source localization accuracy, co-registration error requires assessment, and quantification. Recent studies have evaluated the co-registration error of on-scalp MEG mainly based on the surface fit error or the repeatability error of different measurements, which do not reflect the true co-registration error. In this study, a three-dimensional-printed reference phantom was constructed to provide the ground truth of MEG sensor locations and orientations relative to MRI. The co-registration performances of commonly used three devices—electromagnetic digitization system, structured-light scanner, and laser scanner—were compared and quantified by the indices of final co-registration errors in the reference phantom and human experiments. Furthermore, the influence of the co-registration error on the performance of source localization was analyzed via simulations. The laser scanner had the best co-registration accuracy (rotation error of 0.23° and translation error of 0.76 mm based on the phantom experiment), whereas the structured-light scanner had the best cost performance. The results of this study provide recommendations and precautions for researchers regarding selecting and using an appropriate device for the co-registration of on-scalp MEG and MRI.

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“…In OPM-MEG, the co-registration becomes more complex, owing to the flexible sensor configurations of the OPMs. Several co-registration methods have been developed for OPM-MEG based on different devices 10,11 , including the electromagnetic digitizing system 12 and optical scanners (i.e., the structured-light scanner and the laser scanner) 13,14 . The co-registration using the electromagnetic digital system must sequentially digitize hundreds of head points and several reference points on the helmet, which is time-consuming.…”

Section: Introductionmentioning

confidence: 99%

Depth-detectable optical co-registration of optically pumped magnetometer-magnetoencephalography and magnetic resonance imaging

Cao¹,

An²,

Wang³

et al. 2022

5th International Conference on Computer Information Science and Application Technology (CISAT 2022)

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Automatic coregistration of MRI and on-scalp MEG

Cited by 15 publications

References 28 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

Co-registration Comparison of On-Scalp Magnetoencephalography and Magnetic Resonance Imaging

Depth-detectable optical co-registration of optically pumped magnetometer-magnetoencephalography and magnetic resonance imaging

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