The design of a light and compact magnet for MRI

MRI是最好的非侵入性诊断成像方式，可以提供高质量的全身图像。然而，其高昂的成本和幽闭的磁铁几何结构阻碍了其更快的增长和更广泛的应用。因此，我们建议通过基于所谓的Halbach结构设计一种适用于MRI的新磁体来解决这些缺点。*Halbach磁体呈圆柱形，由放置在圆柱体表面的小磁片组成。虽然Halbach的设计已有30多年的历史，但只有在长径比非常高的情况下，磁场的数学描述才能简化为二维(2D)，才能发展出完整的磁体产生磁场的解析理论。小长径比(&lt；1)的有限长磁体(“长”磁体)的三维数学模型是最近(2014)才提出的，但基于该理论的磁体还没有建立起来。求解半解析场方程、使用有限元或边界方法计算场的计算机程序已经开发出来，但也不是没有缺点。此外，到目前为止，磁铁的重量或大小还没有得到优化。此外，没有完美的制造过程，但没有一个理论模型包括制造误差，例如圆柱体表面上的磁件分段不一致或其不完美的位置。例如，众所周知，2D分段会产生分段数量倍数量级的谐波。为了解决这一常见问题，用球谐函数分析了球面上的磁场。在此基础上，计算了小补偿磁体的放置位置，以消除球面谐波，改善磁体的均匀性。还可以通过校正分段的位置来改善场均匀度。然而，这些方法都不是完美的。在我们的方法中，我们建议开发一种在圆柱体表面上匀场的方法，而不是传统上使用的球体。我们希望这种方法将为“短”Halbach磁体提供有效的垫片。*综上所述，我们建议开发一种新的设计，适用于标准和“无梯度”MRI，由申请人共同开发的轻型永久磁体。由于“无梯度”磁共振成像不需要磁场和相关放大器的梯度，因此为许多新的成像应用生产廉价、轻便和安静的磁共振成像系统是可行的。

MRI is the best non-invasive diagnostic imaging modality providing high quality images of the entire body. However, its high costs and claustrophobic magnet geometry prevent a faster growth and broader applications. Therefore we propose to address these weaknesses by designing a new magnet suitable for MRI based on the so-called Halbach structure.****Halbach magnets have a cylindrical shape and comprise small magnetic pieces placed on a surface of a cylinder. Although the Halbach design has been known for over 30 years, a full analytical theory of the magnetic field produced by the magnet has been developed only for a very high length/diameter ratio, when the mathematical description of the field can be reduced to two dimensions (2D). A 3D mathematical model of the finite-length magnet ("long" magnet) with small length/diameter ratio (<1) has been proposed very recently (2014) but a magnet based on this theory has not yet been constructed. Computer programs solving semi-analytical field equations, using the finite elements or the boundary methods to calculate the field have been developed yet not without drawbacks. Furthermore magnet weight or its size have not been optimized so far. In addition, there is no perfect manufacturing process, yet none of the theoretical models include manufacturing errors, such as inconsistent segmentation of the magnetic pieces over the cylinder's surface or their imperfect position.****It is known that, for example, 2D segmentation causes creation of harmonics of the order of a multiple of the number of the segments. To deal with this common problem, the magnetic field over a sphere is analyzed using spherical harmonics. From this analysis, the placement of small compensating magnets is calculated to cancel the spherical harmonics and to improve magnet homogeneity. The field homogeneity may be also improved by correcting positions of the segments. However none of these methods is perfect. In our approach we propose to develop a method of shimming on a surface of a cylinder instead of a sphere, as traditionally used. We expect this approach will provide efficient shimming for "short" Halbach magnets.****In summary, we propose to develop a new design of a light, permanent magnet suitable for standard and "gradient free" MRI, co-developed by the applicant. As "gradient free" MRI does not require gradients of the magnetic field and associated amplifiers, it is feasible to produce inexpensive, light and silent MRI system for many new imaging applications.******