Calibration of a Coil Array Geometry Using an X-Ray Computed Tomography

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

“…This set of coils is called a spherical coil array. The 150-mm diameter of the proposed spherical calibration coil is much larger than those of the conventional calibration coil arrays, which are 5-60 mm [3][4][5][6][7]. The larger the diameter of the coil, the smaller the relative error of the coil shape from the design value even when it was machined with the same tolerance.…”

“…Here, we demonstrate the calibration of a whole-head MEG system using a spherical coil array, applying it to the one developed and installed at Kanazawa Institute of Technology, which was composed of 160 axial-type SQUID gradiometers with a baseline length of 50 mm [15]. The MEG system was installed in an MSR in our laboratory and was originally calibrated using a conventional calibration coil array [7]. The spherical coil array was positioned in the helmet-shaped sensor array so that all SQUID sensors could capture the reference magnetic fields from the coils, as illustrated in Fig.…”

“…Multiple circular coils with the diameter of 150 mm are wound around the spherical coil bobbin. Moreover, we demonstrate that the structurally simple spherical coil array with large diameter coils allows for a more accurate calibration by the evaluation using a dry-type MEG phantom [13] than the one using our conventional calibration coil array composed of multiple 3-axis coil bobbins [7], which is briefly described in Appendix A.…”

“…An array of coils that fits into the helmet-shaped sensor array of a whole-head MEG system is often applied to generate reference magnetic fields to determine the effective position and orientation of SQUID sensors after cooling [3][4][5][6][7]. The reference magnetic field signal can be theoretically calculated by assuming that the SQUID sensor is placed at a specific point.…”

“…The precision of the coil array is a crucial factor in accurate calibration. In our previous study, we utilized an array of coils composed of multiple 3-axis coil bobbins to calibrate the MEG sensor array and demonstrated that the calibration accuracy was enhanced by precise measurement of the coil geometry using an X-ray computational tomography (CT) [7]. When the diameter of the coil is increased, the relative error of the coil shape from the designed values can be smaller than that of the conventional calibration coil array, and a more accurate calibration is expected.…”