A whole-head magnetoencephalography system with compact axial gradiometer structure

We have fabricated a whole-head superconducting quantum interference device (SQUID) gradiometer system for measuring the magnetoencephalography (MEG) of the human brain. Major technical features of the system are the compact structure of the gradiometer and compact readout electronics. As the gradiometers, first-order gradiometers of 50mm baseline were used to reduce environmental noises. To simplify the fabrication process of the gradiometers, and to increase the refill interval of liquid He, the superconductive connection between the pickup coil wires and input coil pads was done by direct bonding of Nb wires. Therefore, bulky superconducting blocks or superconducting screws were not used for the superconductive connection, and superconducting shielding was not used around the SQUID module, resulting in no distortion of external field uniformity. The distance between the compensation coil of the gradiometer and SQUID input coil pad was reduced to 10mm, and the total length of the gradiometer is 70mm. A sensor helmet having 128 gradiometers was cooled inside a helmet-shape liquid He dewar. The average boil-off rate of the MEG system is 10ld^-1 and the refill interval is 7 days when the 128-channel system is in operation every day. To simplify the readout electronics of the SQUID system, double relaxation oscillation SQUIDs (DROSs) having large flux-to-voltage transfer coefficient were used. The magnetically shielded room (MSR) has a wall thickness of 200mm, and consists of two layers of Permalloy and one layer of aluminum. When the 128-channel system was operated inside the MSR, the average magnetic field noise level of the 128 channels was about 3.5fT _rmsHz^-1/2 at 100Hz. Spontaneous and evoked brain magnetic fields were measured using the developed system.