On electromagnetic head-surface digitization in MEG and EEG

Abstract In magnetoencephalographic (MEG) and electroencephalographic (EEG) studies, the accuracy of the head digitization impacts the co-registration between MEG/EEG and structural magnetic resonance imaging (MRI) data. The co-registration accuracy in turn affects the spatial accuracy in neural source imaging. Precisely digitized head-surface (scalp) points do not only improve the co-registration but can also be used to deform a template MRI to create an individualized-template MRI that can be used for conductivity modeling in MEG/EEG source imaging if the individual’s structural MRI is unavailable. Although optical scanners have been introduced for digitization, especially for dense EEG electrode sets, electromagnetic tracking (EMT) systems (particularly Fastrak®, Polhemus Inc., Colchester, VT, USA) have been the most often applied solution for digitization in MEG and EEG acquisition. EMT systems may occasionally suffer from ambient electromagnetic interference which makes it challenging to achieve (sub-)millimeter digitization accuracy. The current study i) evaluated the performance of the Fastrak® EMT system under different conditions in MEG/EEG digitization, and ii) explores the usability of two alternative EMT systems (Aurora®, NDI, Waterloo, ON, Canada; Fastrak® with a short-range transmitter) for digitization. Tracking fluctuation, digitization accuracy, and robustness of the systems were evaluated in several test cases using test frames and human head models. The digitization performance of the two alternative systems was compared against the Fastrak® system. The results showed that the Fastrak® system is accurate and robust for MEG/EEG digitization if the recommended operating conditions are met. The Fastrak® with the short-range transmitter shows comparatively higher digitization error if digitization is not carried out very close to the transmitter. The study also evinces that the Aurora® system can be used for MEG/EEG digitization. Its real-time error estimation feature can potentially improve digitization accuracy, albeit some modifications would be required to make the system a practical and easy-to-use digitizer.