Low-field (B0 < 0.2 T) magnetic resonance imaging (MRI) is emerging as a low cost, point-of-care alternative to provide access to diagnostic imaging technology even in resource scarce environments. MRI magnets can be constructed based on permanent neodymium-iron-boron (NdFeB) magnets in discretized arrangements, leading to substantially lower mass and costs. A challenge with these designs is, however, a good B0 field homogeneity, which is needed to produce high quality images free of distortions. In this work, we describe an iterative approach to build a low-field MR magnet based on a B0-shimming methodology using genetic algorithms. The methodology is tested by constructing a small bore (inner bore diameter = 130 mm) desktop MR magnet (< 15 kg) at a field strength of B0 = 0.1 T and a target volume of 4cm in diameter. The configuration consists of a base magnet and shim inserts, which can be placed iteratively without modifying the base magnet assembly and without changing the inner dimensions of the bore or the outer dimensions of the MR magnet. Applying the shims, B0 field inhomogeneity could be reduced by a factor 8 from 5448 ppm to 682 ppm in the target central slice of the magnet. Further improvements of these results can be achieved in a 2nd or 3rd iteration, using more sensitive magnetic field probes (e.g. nuclear magnetic resonance based magnetic field measurements). The presented methodology is scalable to bigger magnet designs. The MR magnet can be reproduced with off-the-shelf components and a 3D printer and no special tools are needed for construction. All design files and code to reproduce the results will be made available as open source hardware.

中文翻译：

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用于经济实惠且紧凑的低场磁共振成像磁体的 B0 匀场方法

低场 (B0 < 0.2 T) 磁共振成像 (MRI) 正在成为一种低成本、即时的替代方案，即使在资源稀缺的环境中也能提供诊断成像技术。MRI 磁体可以基于离散排列的永久钕铁硼 (NdFeB) 磁体构建，从而显着降低质量和成本。然而，这些设计的一个挑战是良好的 B0 场均匀性，这是产生无失真的高质量图像所必需的。在这项工作中，我们描述了一种使用遗传算法基于 B0 匀场方法构建低场 MR 磁体的迭代方法。该方法通过构建一个小孔（内孔直径 = 130 毫米）桌面 MR 磁体（< 15 千克）在 B0 = 0.1 T 的场强和直径为 4 厘米的目标体积下进行测试。该配置由基础磁铁和垫片插件组成，可以反复放置，无需修改基础磁铁组件，也无需改变孔的内部尺寸或 MR 磁铁的外部尺寸。应用垫片后，B0 场的不均匀性可以在磁铁的目标中心切片中从 5448 ppm 减少到 682 ppm 的 8 倍。这些结果的进一步改进可以在第 2 次或第 3 次迭代中实现，使用更灵敏的磁场探头（例如基于核磁共振的磁场测量）。所提出的方法可扩展到更大的磁体设计。MR 磁体可以使用现成的组件和 3D 打印机进行复制，并且不需要特殊工具进行构建。所有用于重现结果的设计文件和代码都将作为开源硬件提供。