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超低场磁共振双平面有源匀场线圈设计
首发时间：2017-12-27
摘要：超低场磁共振是目前便携式医学成像领域研究的热门方向，超低场磁共振设备由于其主磁场量值很低（低于50mt），对静态磁场均匀度提出了更高的要求，主磁场均匀度直接影响磁共振的信噪比。本文使用目标场法设计出有源匀场线圈，利用匀场线圈产生的微小磁场抵消主磁场中的不均匀分量，从而提高主磁场的均匀度。匀场对象是磁场强度为50mt，目标区域为200mm直径的球形空间，首先使用正交球谐函数对永磁体磁场进行展开，分析其不均匀分量，其次使用目标场法建立匀场线圈电流密度分布与主磁场球谐展开项之间的对应数学关系，再用矩阵求逆解出相应阶次的电流密度分布，最终利用流函数等值线离散法获得匀场线圈的绕线分布。利用此方法本文设计出多组有源匀场线圈，对主磁场扰动最大的z2-(x2 y2)/2匀场线圈进行实验验证，结果显示线圈实际产生的磁场与理论磁场相符，在线圈中通以1.4a电流时，磁场均匀度由174.4258ppm降至72.3911ppm。
关键词： 电工理论与新技术;超低场磁共振;目标场法;双平面匀场线圈
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the design of biplanar active shim coils for ultra low-field nmr
abstract：the ultra low-field nuclear magnetic resonance (nmr) is a popular research topic in the field of portable medical imaging. due to the low value of main magnetic field in the ultra low-field nuclear magnetic resonance device（lower than 50mt）, a higher request to homogeneity of the magnet field has been put forward. the homogeneity of the magnet field impacts signal noise ratio (snr) of nuclear magnetic resonance directly. an optimized target-field method is used to design bi-planar active shim coils, and the small magnetic field generated by the active shim coil can offset the inhomogeneous component of main magnetic field so that the homogeneity can be improved obviously. shimming object is a spherical space with a diameter of 200mm where the magnetic field strength is 50mt. first, we analyse all inhomogeneous components of the permanent magnetic field which are expanded by the orthogonal spherical harmonic function. second, mathematical expressions that relate the surface current distribution of shim coils to spherical harmonic terms in the magnetic field expansion are established by the optimized target-field method. and then a simple matrix inversion approach is used to obtain current distribution of shim coils. last, an isoline discrete method with stream function was utilized to obtain the winding distribution on the coil plane. a range of active shim coils are designed by using this method, the verification experiment of z2-(x2 y2)/2 shim coil，which is the most inhomogeneous component of the permanent magnetic field, shows that the actual magnetic field produced by the coil agrees with the theoretical field, which can basically meet the design requirements.magnetic field uniformity becomes 72.3911ppm from 174.4258ppm when the current of z2-(x2 y2)/2 shim coil is 1.4 a.
keywords： theory and new technology of electrical engineering ultra low-field nmr target-field method bi-planar shim coils
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