A whole-head magnetoencephalography system with compact axial gradiometerstructure

Abstract

We have fabricated a whole-head superconducting quantum interference device (SQUID)gradiometer system for measuring the magnetoencephalography (MEG) of the humanbrain. Major technical features of the system are the compact structure of the gradiometerand compact readout electronics. As the gradiometers, first-order gradiometers of 50 mmbaseline were used to reduce environmental noises. To simplify the fabrication process ofthe gradiometers, and to increase the refill interval of liquid He, the superconductiveconnection between the pickup coil wires and input coil pads was done by direct bonding ofNb wires. Therefore, bulky superconducting blocks or superconducting screwswere not used for the superconductive connection, and superconducting shieldingwas not used around the SQUID module, resulting in no distortion of externalfield uniformity. The distance between the compensation coil of the gradiometerand SQUID input coil pad was reduced to 10 mm, and the total length of thegradiometer is 70 mm. A sensor helmet having 128 gradiometers was cooled inside ahelmet-shape liquid He dewar. The average boil-off rate of the MEG system is10 l d−1 and the refill interval is 7 days when the 128-channel system is in operation every day. Tosimplify the readout electronics of the SQUID system, double relaxation oscillationSQUIDs (DROSs) having large flux-to-voltage transfer coefficient were used. Themagnetically shielded room (MSR) has a wall thickness of 200 mm, and consists of twolayers of Permalloy and one layer of aluminum. When the 128-channel system was operatedinside the MSR, the average magnetic field noise level of the 128 channels was about3.5 fTrms Hz−1/2 at 100 Hz. Spontaneous and evoked brain magnetic fields were measured using thedeveloped system.