低场核磁共振电磁场屏蔽问题的研究

【作者】 陈海永；

【导师】 张一鸣；

【作者基本信息】 中国科学院研究生院（电工研究所） ， 电工理论与新技术， 2006， 硕士

【摘要】 由于核磁共振回波信号很微弱,核磁共振仪器的设计和制造要求就特别高。近年来,对NMR频谱仪总体性能的改善所作的努力很大一部分都是对NMR探头的改善,以减少信号中的噪声。一种可行的技术就是对探头中的线圈进行屏蔽。首先,本文对低场核磁共振探头进行了分析,初步确定屏蔽体的基本形状为上面带开口的圆柱体,用来屏蔽外部空间的干扰噪声,同时也使螺线管线圈产生的交变电磁场对静磁场均匀性的干扰和影响减少到最低程度。其次,本文计算了当屏蔽体尺寸变化时,屏蔽体的屏蔽效能,得出了屏蔽效能随屏蔽体尺寸变化的曲线。这些尺寸变化包括侧壁半径和厚度、下底深度和厚度、上底高度和厚度以及上底的开口半径。屏蔽体在完成屏蔽职能的同时,会对螺线管线圈产生不利影响。计算螺线管内部的磁场、线圈的电感L和品质因数Q,得出随屏蔽体尺寸变化的曲线。最后,通过球壳屏蔽体屏蔽效能的解析公式,用Matlab编程计算,验证了两种方法所得结果有相同的变化规律。对相关问题进行总结,并给出一些指导性的建议。更多还原

【Abstract】 Because of the very weak echo signal generated by NMR, the manufacture anddesign of NMR spectrometer are very demanding. A large proportion of the totalNMR spectrometer improvement effort in recent years has been spent in improvingthe probe part of the NMR spectrometer instrument to reduce the noise in the signal.One practical technique is to place conductive shields around the solenoidal coil of theprobe.Firstly, the probe part of the NMR spectrometer is analyzed, and a cylindricalshield with an opening is determined to be the basic form. The shield is used to shieldthe instrument from outer space noise, and minimize the interference and influence ofthe alternating magnetic fields generated by the solenoidal coil upon the homogeneityof the static magnetic fields.Secondly, while the dimensions of the shield varying, the shielding effectivenessof the shield and some parameters of the coil are computed, and some related curvesare gained. These dimensions include the radius, the depth, the height, the openingradius, as well as the thickness in the three dimensions. The shield does harm to thesolenoidal coil as it practices its shielding function. The coil parameters include theinductance, quality factor, as well as the magnetic flux density.Finally, the shielding effectiveness of a spherical shell is calculated with Matlabaccording to the analytic formulae. The results of two types of shields are in goodagreement. Some instructive suggestion of designing the shield is given.更多还原