Dry Phantoms With Deep Signal Sources for Magnetoencephalography

We developed dry phantoms with signal source depth of 2.5, 12.5, and 22.5 mm for magnetoencephalography (MEG) systems. Their calibration was based on three-dimensional measurements and numerical calculations. The positions of the effective equivalent current dipoles (ECDs) were shifted 0.1 to 0.9 mm from their designed positions, and uncertainties in calibration were about 0.17 mm. An experimental evaluation of an MEG system gave displacements of the measured ECDs of 0.6±0.3, 0.8±0.3, and 0.9±0.3 mm for the three signal source depths. Thus, the signal source localization accuracy of our MEG system was within about 1 mm for deep signal sources.