Figure 3

Comparison of tangential MEG using TMR sensors and radial MEG using SQUIDs to measure SEFs in a healthy subject. (a) Photograph of a single channel TMR-MEG system placed on the scalp over the right parietal area. Scalp-attached MEG was performed at 2.63 mm from the scalp to the center of the magnetic sensing point. (b) Isofield map of N20m, the first SEF component for the left median nerve stimulation, measured at 10 different sites with identical sensing axis of 40 degree counterclockwise from superiorly. Note the spatially steep peak of the N20m. (c) Representative SEF waveforms measured by scalp-attached tangential TMR-MEG along three different sensing axes (top) and by the helmet-shaped 200-channel superconducting MEG system (bottom). The 1st and 2nd lines represent two different averaging sessions (N = 9000) to show waveform reproducibility in TMR-MEG for the first (N20m) and second (P30m) components of SEFs comparable to superconducting MEG. (d) The helmet-shaped 200-channel radial SQUID-MEG and the equivalent current dipole of N20m superimposed on sagittal, coronal, and axial magnetic resonance (MR) images of the subject. Note the setback distance between the pickup coils and the scalp due to the Dewar vessel containing liquid helium. A red circle and a bar respectively indicate position (on the central sulcus) and orientation (anterior and superior) of the N20m dipole. (e) Isofield map of N20m measured by a helmet-shaped 200-channel radial SQUID-MEG. N20m dipole is projected to the coil surface.