Design and Demonstration of Novel Magnetoencephalogram Detectors

Abstract

Brain activities have attracted considerable attention in medical and healthcare areas and engineering applications. Magnetoencephalogram (MEG) is a non-invasive technique for mapping and evaluating the functional activity of the brain. It is commonly measured by superconducting quantum interference devices (SQUIDs) which is the most reliable sensitive magnetometer for MEG measurement. However, the SQUIDs are still threatened by the need of cryogenic cooling liquids and a shielding room. For recording non-invasive cranial nerve activity, the technology with no liquid helium and high spatial resolution has been required for about 30 years. Sensitive micro-magnetic sensors referred to as magnetoimpedance (MI) sensors based on MI effect in thin amorphous magnetic wires are utilized as an electronics compass in a cellar phone. We have developed a gradiometer based on MI sensor for the purpose of biomagnetic field measurement. The gradiometer is composed of a pair of MI elements with 1 cm length each: a sensing element and a reference element with a baseline of 3 cm. The developed MI gradiometer has good linearity and a high sensitivity of $1.2 \times 105$ V/T with no amplification by semiconductor device. The noise level of the gradiometer is approximately 2 pT/Hz1/2. The MI sensor has system noise higher than SQUIDs, but it has considerable advantages that can measure closely to the scalp. Because of proximity measurement, the magnetic field from brain measured by MI sensor is larger than the case of SQUIDs. The precise MI gradiometer was tested as MEG detectors. The spontaneous brain activity (alpha rhythm) measurements were carried out on a female subject. As expected, the alpha rhythm signals simultaneously measured by electroencephalogram and MI gradiometer were significantly attenuated when the subject opens eyes and intensified with eyes closed. The subject’s $\alpha $ wave MEG signal has a main frequency component of 10–11 Hz and the maximum amplitude is about 25 pT (effective value). Significant susceptibility of MI sensor to spontaneous brain activity was confirmed by signal-to-noise ratio of 8/1. In addition, the measurement of evoked magnetic field N100 was carried out in the unshielded environment. The measurement results about N100 in this paper are almost consistent with the previous study by SQUIDs.