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

Fuzhi Cao,Nan An,Min Xiang,Yang Gao,Weinan Xu,Xiaolin Ning,Yanfei Yang,Wenli Wang

doi:10.3389/fnins.2021.706785

Abstract

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.

Highlights

Magnetoencephalography (MEG) can directly measure the external magnetic field generated from pyramidal neurons synchronously activated in the brain
In the reference phantom experiment, five groups of data for each device were used for co-registration with pseudo magnetic resonance imaging (MRI)
When the co-registered positions and orientations of the laser scanner were used as the sensor configuration, the source localization error was 0.23 mm under the boundary element method (BEM) model, reflecting the error caused by the inverse solution

Summary

Introduction

Magnetoencephalography (MEG) can directly measure the external magnetic field generated from pyramidal neurons synchronously activated in the brain. Low-Tc superconducting quantum interference devices (SQUID)-MEG has become a reliable technology after 30 years of development. It operates at the temperature of liquid helium (4 K), resulting in expensive. 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. On-scalp MEG constructed with OPMs increases the SNR by approximately 3–5-fold (Tierney et al, 2020). By customizing a personalized helmet, OPM-based on-scalp MEG is wearable and suitable for people with different head circumferences, especially developing children (Boto et al, 2018, 2021; Hill et al, 2019)

Methods

Results

Conclusion