Measurement of Brainstem Auditory Evoked Magnetic Fields Using a Highly Sensitive SQUID Magnetometer with a Variable Baseline

This study focuses on the detection of ABR (auditory evoked brainstem response) magnetic fields using a SQUID magnetometer. The ABR magnetic field is a very weak signal with an intensity of less than 10-14 T which is near the limits of sensitivity of the SQUID magnetometer. We developed a highly sensitive dc-SQUID magnetometer. It has a first-order gradiometer with a variable baseline, i.e., the distance between two co-axial coils which are wound in opposite directions to reduce the environmental magnetic noise. In this system, one can choose the best pick-up coil to reduce the environmental magnetic noise. We tried to measure ABR magnetic fields using three gradiometers. It was possible to detect the ABR magnetic fields using a 50-mm baseline gradiometer. We also studied the optimum baseline to measure magnetic fields by computer simulation.