A Spherical Coil Array for the Calibration of Whole-Head Magnetoencephalograph Systems.

We proposed a spherical coil array for determining the positions, orientations, and sensitivities of the magnetometers for whole-head magnetoencephalograph (MEG) systems. The coil array comprises 16 concentric and symmetrically oriented circular coils with a diameter of 150 mm, which is larger than those of conventional calibration coil arrays, such as 5 or 60 mm. The accuracy of the calibration was expected to improve because the relative mechanical errors in machining and assembly could be reduced when the coil diameter was increased. Moreover, we also proposed a method to estimate the uncertainties in calibration, and it was demonstrated that the sensor parameters of each sensor were successfully obtained with uncertainties using a spherical calibration coil array. Additionally, the accuracy of the calibration was evaluated using a dry-type MEG phantom, and as expected, it was found that the accuracy was improved from that of a conventional calibration coil array composed of an assembly of multiple three-axis bobbins.