Liang Ting-Ou, Koh Yan Hao, Qiu Tie, Li Erping, Yu Wenwei, Huang Shao Ying
Singapore University of Technology and Design 8 Somapah Road, 487372, Singapore.
Zhejiang University Hangzhou, Zhejiang Province, China.
J Magn Reson. 2022 Dec;345:107309. doi: 10.1016/j.jmr.2022.107309. Epub 2022 Oct 28.
Lightweight and compact permanent magnet arrays (PMAs) are suitable for portable dedicated magnetic resonance imaging (MRI). It is worth exploring different PMA design possibilities and optimization methods with an adequate balance between weight, size, and performance, in addition to Halbach arrays and C-shaped/H-shaped magnets which are widely used. In this paper, the design and optimization of a sparse high-performance inward-outward ring-pair PMA consisting of magnet cuboids is presented for portable imaging of the brain. The design is lightweight (151kg) and compact (inner bore diameter: 270mm, outer diameter: 616mm, length: 480mm, 5-Gauss range: 1840×1840×2340mm). The optimization framework is based on the genetic algorithm with a consideration of both field properties and simulated image quality. The resulting PMA design has an average field strength of 101.5 mT and a field pattern with a built-in linear readout gradient. Subtracting the best fit to the linear gradient target resulted in a residual deviation from the target field of 0.76mT and an average linear regression coefficient of 0.85 to the linear gradient. The required radiofrequency bandwidth is 6.9% within a field of view (FoV) with a diameter of 200mm and a length of 125mm. It has a magnetic field generation efficiency of 0.67mT/kg, which is high among the sparse PMAs that were designed for an FoV with a diameter of 200mm. The field can be used to supply gradients in one direction working with gradient coils in the other two directions, or can be rotated to encode signals for imaging with axial slice selection. The encoding capability of the designed PMA was examined through the simulated reconstructed images. The force experienced by each magnet in the design was calculated, and the feasibility of a physical implementation was confirmed. The design can offer an increased field strength, and thus, an increased signal-to-noise ratio. It has a longitudinal field direction that allows the application of technologies developed for solenoidal magnets. This proposed design can be a promising alternative to supplying the main and gradient fields in combination for dedicated portable MRI. Lastly, the design is resulted from a fast genetic algorithm-based optimization in which fast magnetic field calculation was applied and high design flexibility was feasible. Within optimization iterations, image quality metrics were used for the encoding field of a magnet configuration to guide the design of the magnet array.
轻量化且紧凑的永磁阵列（PMA）适用于便携式专用磁共振成像（MRI）。除了广泛使用的哈尔巴赫阵列以及C形/H形磁体之外，探索不同的PMA设计可能性以及在重量、尺寸和性能之间实现适当平衡的优化方法是很有价值的。本文提出了一种由磁体长方体组成的稀疏高性能内外环对PMA的设计与优化，用于脑部的便携式成像。该设计重量轻（151千克）且紧凑（内径：270毫米，外径：616毫米，长度：480毫米，5高斯范围：1840×1840×2340毫米）。优化框架基于遗传算法，同时考虑了场特性和模拟图像质量。所得的PMA设计平均场强为101.5毫特斯拉，场型具有内置的线性读出梯度。减去与线性梯度目标的最佳拟合后，与目标场的残余偏差为0.76毫特斯拉，与线性梯度的平均线性回归系数为0.85。在直径为200毫米、长度为125毫米的视野（FoV）内，所需的射频带宽为6.9%。其磁场产生效率为0.67毫特斯拉/千克，在为直径200毫米的FoV设计的稀疏PMA中处于较高水平。该场可用于在一个方向上提供梯度，与另外两个方向上的梯度线圈配合使用，或者可以旋转以编码信号用于轴向切片选择成像。通过模拟重建图像检验了所设计PMA的编码能力。计算了设计中每个磁体所受的力，并确认了物理实现的可行性。该设计能够提供更高的场强，从而提高信噪比。其纵向场方向允许应用为螺线管磁体开发的技术。对于专用便携式MRI而言，这种提议的设计有望成为提供主磁场和梯度场组合的一种有前途的替代方案。最后，该设计源于基于快速遗传算法的优化，其中应用了快速磁场计算且实现了高设计灵活性。在优化迭代过程中，将图像质量指标用于磁体配置的编码场，以指导磁体阵列的设计。
