基于Halbach阵列磁体的桌面式磁共振主从式匀场线圈优化研究

Halbach array magnets have great potential in desktop magnetic resonance device, but field inhomogeneity is the main problem of this type magnet. Active shimming is the main method to solve this problem. At present, the design of multiple shimming coils based on the orthogonal decomposition of Spherical Harmonics (SH) theory has the problems of cross-interference, high adjustment difficulty and the calibration process is time-consuming. In addition, the application in Halbach magnet may occupy too much effective magnet inner diameter, such that it is not suitable for small size magnetic resonance device shimming. Here a master-slave shimming coil design is proposed. The inhomogeneous magnetic field of Halbach magnet is decomposed into original non-uniform field and dynamic non-uniformity according to the source. The boundary element method is applied directly to design the shimming coils to compensate the original non-uniform field. Then the matrix coils theory is applied for dynamic shimming, including coil structure, spatial distribution optimization and multi-parameter optimization algorithm design. This theory targets at solving the problem of magnetic field crossover in the original SH shimming theory, to form a high-precision, high-efficiency, compact shimming coil design method that can be applied to Halbach magnets, which could promote the application of Halbach magnets in desktop magnetic resonance equipment. In a broad sense, this work may provide a new idea and solution for the active shimming design of normal magnetic resonance equipment.

Halbach阵列磁体在桌面式磁共振中具有重大应用潜力，但磁场均匀度差是其存在的主要问题，主动匀场技术是解决上述问题的主要手段。目前基于球谐函数正交分解的多组主动匀场线圈设计存在磁场交叉干扰，调节难度大、耗时等问题，且在Halbach磁体应用中占用大量有效磁体口径，不适宜于小型化磁共振的匀场。基于此本课题提出一种主从式匀场线圈设计，将Halbach磁体的非均匀磁场根据来源分解为原始非均匀场和动态非均匀场，研究边界元法直接针对复杂分布的原始非均匀场设计主匀场线圈，以及阵列线圈理论进行动态非均匀场补偿，包括线圈结构、空间分布优化和多参数优化算法设计。解决原有设计理论中磁场交叉影响的问题，形成一种能够应用于Halbach磁体的高精度，高效率，紧凑型的匀场线圈设计方法，促进Halbach磁体在桌面式磁共振设备中的应用，同时广义上为常规磁共振设备主动匀场设计提供了一种新的思路和解决方案。

Halbach磁体在桌面式磁共振设备中具有重大应用潜力，但均匀度差是制约其应用的主要问题，需要利用主动匀场手段提高磁场均匀度。基于球谐函数分解的主动匀场线圈设计理论，需要多组线圈组合补偿场非均匀度，在Halbach磁体高非均匀度匀场中占用体积大，且线圈产生的磁场存在交叉，导致主动匀场过程耗时且难度高。.有鉴于此，本课题研究优化阵列线圈进行动态匀场补偿，包括线圈结构、分布优化和多参数优化算法研究。针对Halbach磁体载流平面结构，本项目分析建立了基于谐波函数分解和傅里叶级数展开的阵列线圈模型，直接建立了线圈产生磁场的谐波正交基系数与线圈形状之间的对应关系，使得针对多目标匀场的阵列线圈快速优化设计成为可能。根据建立的模型，对1st, 2nd, 3rd阶分量的多个正交基进行设计，结果证明相比于常规的规则形状阵列线圈，磁场产生精度和电流效率都有很大的提升。最后，对1st阶阵列线圈进行了实物制作和测试，结果与理论结果误差在10%以内，证明了理论模型的正确性。.本项目成果解决原有设计理论磁场交叉干扰的问题，形成一种能够应用于Halbach磁体的高精度，小体积，高效率的匀场线圈设计理论，促进Halbach磁体在小型化便携式磁共振设备中的应用，同时广义上为常规磁共振设备主动匀场线圈设计提供了一种新的思路和解决方案。