Rotatable Small Permanent Magnet Array for Ultra-Low Field Nuclear Magnetic Resonance Instrumentation: A Concept Study

Object
We studied the feasibility of generating the variable magnetic fields required for ultra-low field nuclear magnetic resonance relaxometry with dynamically adjustable permanent magnets. Our motivation was to substitute traditional electromagnets by distributed permanent magnets, increasing system portability.
Materials and Methods
The finite element method (COMSOL®) was employed for the numerical study of a small permanent magnet array to calculate achievable magnetic field strength, homogeneity, switching time and magnetic forces. A manually operated prototype was simulated and constructed to validate the numerical approach and to verify the generated magnetic field.
Results
A concentric small permanent magnet array can be used to generate strong sample pre-polarisation and variable measurement fields for ultra-low field relaxometry via simple prescribed magnet rotations. Using the array, it is possible to achieve a pre-polarisation field strength above 100 mT and variable measurement fields ranging from 20–50 μT with 200 ppm absolute field homogeneity within a field-of-view of 5 x 5 x 5 cubic centimetres.
Conclusions
A dynamic small permanent magnet array can generate multiple highly homogeneous magnetic fields required in ultra-low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) instruments. This design can significantly reduce the volume and energy requirements of traditional systems based on electromagnets, improving portability considerably.

研究目标（Object）
本研究探讨了采用动态可调永磁体，生成超低场核磁共振弛豫谱（ultra-low field nuclear magnetic resonance relaxometry）所需可变磁场的可行性。本研究的初衷在于以分布式永磁体替代传统电磁体，从而提升系统的便携性。

材料与方法（Materials and Methods）
本研究采用有限元法（finite element method，COMSOL®）对小型永磁体阵列开展数值研究，计算其可实现的磁场强度、均匀性、切换时间与磁力。我们针对一款手动操作的原型机进行了仿真建模并实际搭建，以此验证数值分析方法与生成的磁场特性。

研究结果（Results）
通过简单的预设磁体旋转操作，同心小型永磁体阵列可用于为超低场弛豫谱生成较强的样本预极化磁场与可变测量磁场。采用该阵列可实现100 mT以上的预极化场强，以及20~50 μT范围的可变测量磁场；在5×5×5立方厘米的视场（field-of-view）内，磁场绝对均匀性可达200 ppm。

研究结论（Conclusions）
动态小型永磁体阵列可生成超低场核磁共振（ultra-low field nuclear magnetic resonance, NMR）与磁共振成像（magnetic resonance imaging, MRI）仪器所需的多种高均匀性磁场。该设计可显著降低传统电磁体系统的体积与能耗需求，大幅提升系统便携性。