Liu Jinhao, Wang Yaohui, Wang Miutian, Wang Wenchen, Yang Gang, Wang Weimin, Wang Qiuliang, Liu Feng
School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, 710049, China.
School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing, 100190, China; Division of Superconducting Magnet Science and Technology, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
J Magn Reson. 2024 Nov;368:107787. doi: 10.1016/j.jmr.2024.107787. Epub 2024 Oct 9.
Ultrahigh field systems (≥ 7 T) can increase the signal-to-noise ratio of magnetic resonance imaging (MRI), improving imaging performance compared to systems with lower fields. However, these enhancements heavily rely on a high B magnetic field homogeneity level, which can be achieved through superconducting shimming. This paper presents a novel two-stage superconducting shimming method designed to achieve precise shimming for a 7 T MRI superconducting magnet. In the initial stage, detailed measurements and fittings were conducted to determine the current polarity and the axial or circumferential positions of the shim fields. Subsequently, an optimization strategy was implemented to determine the optimal shim currents with a flexible target field. The second stage involves an iterative process to fine-tune the current of a specific shim coil, identified as having the most significant impact on field homogeneity. The overall fitness of 99.5% underscores the precision in determining the current polarity and position of the shim fields. Significantly, the calibrated shim system substantially improves the peak-to-peak and Root Mean Square Error (RMSE) field homogeneities from 107.42 ppm and 37.00 ppm to 11.12 ppm and 3.26 ppm, respectively, representing improvements of 89.65% and 91.19%. Furthermore, the simulation results of the fine-tuning stage demonstrate additional enhancements in peak-to-peak field homogeneity, to 9.9 ppm by reducing the current of the Z2 shim coil by 51.3 mA. Additionally, the shimmed magnetic field exhibited high time stability, with a maximum variation of only 27 µT observed within 48 h. Thus, the proposed two-stage superconducting shimming framework effectively addresses the challenge of imperfect B magnetic fields, enhancing peak-to-peak and RMSE field homogeneity. The stepwise optimized approach also mitigates deviations caused by shim-to-shim coupling, demonstrating its efficacy in achieving precise shimming in ultrahigh-field MRI systems.
超高场系统（≥7T）可以提高磁共振成像（MRI）的信噪比，与低场系统相比，成像性能得到改善。然而，这些增强很大程度上依赖于高B磁场均匀性水平，这可以通过超导匀场来实现。本文提出了一种新颖的两阶段超导匀场方法，旨在为7T MRI超导磁体实现精确匀场。在初始阶段，进行了详细的测量和拟合，以确定匀场线圈的电流极性以及轴向或周向位置。随后，实施了一种优化策略，以确定具有灵活目标场的最佳匀场电流。第二阶段涉及一个迭代过程，以微调对场均匀性影响最大的特定匀场线圈的电流。99.5%的整体拟合度突出了确定匀场线圈电流极性和位置的精度。值得注意的是，校准后的匀场系统将峰峰值和均方根误差（RMSE）场均匀性分别从107.42 ppm和37.00 ppm大幅提高到11.12 ppm和3.26 ppm，分别提高了89.65%和91.19%。此外，微调阶段的模拟结果表明，通过将Z2匀场线圈的电流降低51.3 mA，峰峰值场均匀性进一步提高到9.9 ppm。此外，匀场后的磁场表现出很高的时间稳定性，在48小时内观察到的最大变化仅为27 µT。因此，所提出的两阶段超导匀场框架有效地解决了B磁场不完善的挑战，提高了峰峰值和RMSE场均匀性。逐步优化方法还减轻了匀场线圈间耦合引起的偏差，证明了其在超高场MRI系统中实现精确匀场的有效性。
