Abstract
In order to measure weak magnetic fields such as those detected by magnetoencephalography (MEG), a magnetically shielded room (MSR) with multiple layers of μ-metal is usually used to reduce environmental magnetic field noise. However, such a heavily shielded room is not suitable for a magnetocardiogram (MCG) measurement system from the viewpoint of requirements of low cost, light weight, and small space.As an alternative to an MSR, we have developed a hybrid cancellation method which combines an electronic gradiometer and a normal conduction coil cancellation. In this paper, the compensation setups are described and experimental results are presented and discussed. The normal coil cancellation consisted of two coils (100 turns each) to detect uniform and gradient magnetic fields. The two detection coils were wound around a cubic wooden frame (1m×1m×1m) at the top (upper detection coil) and bottom (lower detection coil). The noises detected with the detection coils were added and subtracted to compensate for uniform and gradient noise, respectively. The following results were obtained for the suppression of environmental noises: 20 dB at 0.1-100 Hz and 30 dB at 50 Hz.
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