Optical Co-registration of MRI and On-scalp MEG

Abstract

Objective: To estimate the neural current distribution underlying magnetoencephalographic (MEG) signals and to link such estimates to brain anatomy, MEG data have to be co-registered with an anatomical image, typically an MR image. Optically-pumped magnetometers (OPMs) enable the construction of on-scalp MEG systems providing higher sensitivity and spatial resolution than conventional SQUID-based MEG systems. Here, we present a co-registration method that can be applied to on-scalp MEG systems, regardless of the number of channels. Methods: We apply a structured-light 3D scanner to create a surface mesh of the subject's head and the sensor array, which we fit to the MR image. To assess accuracy, we quantified the reproducibility of the surface mesh and localized current dipoles embedded in a phantom. Finally, we measured somatosensory evoked fields (SEF) to median nerve stimulation and compared their source estimates with those obtained with a SQUID-based MEG system. Results: The structured-light scanner reproduced the head surface with < 1 mm error. Phantom dipoles were localized with a mean error of 2.14 mm. The difference in SEF dipole positions between OPMs and SQUIDs were 5.0, 0.9, and 1.6 mm for N20m, P35m and P60m response peaks, respectively. Conclusion: The developed co-registration is inexpensive, fast and can easily be applied to on-scalp MEG. It is also more convenient to use than traditional co-registration methods while also being more accurate. Significance: We developed and validated a co-registration method that can be applied to on-scalp MEG systems. The method enables accurate source estimation with these novel MEG systems.