High spatial resolutional measurement of biomagnetic fields

A high spatial resolution superconducting quantum interference device (SQUID) system was developed. This SQUID system enabled measurement and discrimination of the magnetic field pattern under 800-μm resolution. Magnetic fields produced by the compound nerve action current of the frog sciatic nerve were measured. The compound action potential and compound action magnetic fields when the stimulus current changes from 0.2 to 1 mA were measured. It was possible to observe that several components of the compound action magnetic fields have different conduction velocities in different kinds of fibers of the nerve bundle within 4 ms after stimulation. When the stimulus intensity increased, the waveform of the compound action potential changed and the duration of the late component of the compound action potential became longer. We found that this change was caused by the excitation of the nerve which has slow conduction velocity. We also succeeded in measurement of the auditory evoked magnetic fields of mice with high spatial resolution. The polarity change of peak magnetic fields appear in the 10-mm area. It is difficult to detect these signals using a SQUID magnetometer with a large size pickup coil or measurements of action potential.