Development of a light-weight, high-performance magnetic shielding system for MEG measurements

这项为期三年的研究的主要目的是将自行设计的高2.76m、外径0.98m、内径0.67m的开口圆柱形结构的磁屏蔽应用于一阶DC-SQUID梯度仪的脑磁图测量，并研究屏蔽系统的哪些部分需要改进才能进行脑磁图测量。我们已经成功地从23岁男性志愿者的大脑中测量了脑磁α节律，但我们发现屏蔽层内的磁噪声仍需进一步改进以测量诱发的脑磁图。其他重要的结果和未来的问题总结如下：-包着磁屏蔽的导电布射频屏蔽被发现成功地将DC-SQUID的磁通-电压(φ-v)特性恢复到没有射频干扰的水平。-在磁振荡过程中观察到低频磁波动噪声，这被发现与巴克豪森磁噪声有很强的关系。Metglas 2714A非晶态磁带的涨落噪声比Metglas 2705M非晶态磁带小得多。对圆柱形磁壳的开口进行了主动补偿，对螺旋壳和轴向壳进行了补偿。在2赫兹均匀外加磁场下，磁通门磁力计的性能提高了55倍，在10赫兹和50赫兹外加磁场时，磁通门磁强计的性能分别提高了20倍和5倍。-将偏置反转技术引入到正交化结构中，研制出了稳定的磁通门磁力计。-提出了一种产生均匀磁场的简单线圈系统，用于主动磁屏蔽--需要进一步清除磁噪声，这可以通过在屏蔽设计中考虑第二和第三结果来实现。

The main objectives of this three-year research are to apply a light-weight, high-performance magnetic shield of open-ended cylindrical structure (height=2.76 m, outer diameter=0.98 m and inner diameter=0.67 m), which was built by our own design using Co-based amorphous magnetic tapes and applying magnetic shaking technique, for the MEG measurements with first-order dc-SQUID gradiometers, and to study which parts of shielding system should be improved for the MEG measurement to be done. We have succeeded in measuring MEG alpha rhythm from a human brain of 23-year old male volunteer, however, we found magnetic noise inside the shield should be further improved for the measurement of evoked MEG. Other important results and future problems are summarized below :- Conductive cloth RF shield enclosing the magnetic shield was found to be successful to recover flux-voltage (φ-v) characteristics of dc-SQUID to the level of no RF interference.- Low frequency magnetic fluctuation noise is observed during magnetic shaking, which was found to be in strong relation to the Barkhousen magnetic noise. Metglas 2714A amorphous magnetic tape exhibits much smaller fluctuation noise than Metglas 2705M one.- Active compensation for openings of the cylindrical magnetic shell has been established for the helical shell and for the axial shell. 55 times improvement was obtained at 2 Hz of uniformly impressed magnetic field and 20 times at 10 Hz and 5 times at 50 Hz.- Stable fluxgate magnetometer was developed by incorporating bias reversal technique to the orthogonal structure.- A simple coil system for producing uniform magnetic field was proposed for active magnetic shields-Further magnetic noise cleaning should be needed, which can be realized by taking account of the second and the third-results in the design of the shield.