Abstract
To estimate the neural generators of magnetoencephalographic (MEG) signals, 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. We present a co-registration method that can be applied to on-scalp MEG systems, regardless of the number of sensors. We apply a structured-light scanner to create a surface mesh of the subject’s head and the sensor array, which we fit to the MR image. We quantified the reproducibility of the mesh and localised current dipoles with a phantom. Additionally, we measured somatosensory evoked fields (SEFs) to median nerve stimulation and compared the dipole positions between on-scalp and SQUID-based systems. The scanner reproduced the head surface with <1 mm error. Phantom dipoles were localised with 2.1 mm mean error. SEF dipoles corresponding to the P35m response for OPMs were well localised to the somatosensory cortex, while SQUID dipoles for two subjects were erroneously localised to the motor cortex. The developed co-registration method is inexpensive, fast and can easily be applied to on-scalp MEG. It is more convenient than traditional co-registration methods while also being more accurate.
Highlights
Magnetoencephalography (MEG) is a non-invasive functional neuroimaging method for investigating electric neuronal activity of the the living human brain[1]
Since sensitivity and spatial resolution are related to the distance between the sources and the sensors, the need of cryogenics eventually results in a considerable loss of signal amplitude and spatial resolution[2,3]
To localise the head position indicator (HPI) coils with respect to the MEG sensors, known currents are driven into the coils either sequentially or at different frequencies prior to or continuously during MEG measurements and a magnetic dipole model representing each coil is fitted to the acquired MEG sensor signals
Summary
Magnetoencephalography (MEG) is a non-invasive functional neuroimaging method for investigating electric neuronal activity of the the living human brain[1]. MEG systems measure the magnetic field produced by neural currents in the brain using sensors positioned around the head. New sensor technologies with sensitivity high enough for MEG have emerged recently; optically-pumped magnetometers (OPMs)[4,5] and high-Tc SQUIDs6 hold promise as alternatives to low-Tc SQUIDs. New sensor technologies with sensitivity high enough for MEG have emerged recently; optically-pumped magnetometers (OPMs)[4,5] and high-Tc SQUIDs6 hold promise as alternatives to low-Tc SQUIDs These new sensor types do not require the same degree of thermal insulation as low-Tc SQUIDs and can be placed almost directly on the scalp, considerably boosting both the sensitivity to neural sources as well as spatial resolution. The current standard co-registration method in SQUID-based MEG relies on the combination of head position indicator (HPI) coils attached to the participant’s head and a pen-like electromagnetic 3D digitiser. Prior to MEG measurements, the positions of the HPI coils as well as a set of anatomical landmarks on the head are digitised. The actual co-registration is performed by aligning the HPI-coil locations as determined by the MEG system with those determined by digitisation, and aligning the digitised anatomical landmarks with the same landmarks in the MR image
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