Allard Mathieu, Henkelman R Mark
Mouse Imaging Centre, Hospital for Sick Children, Toronto, Ont., Canada M5G 1X8.
J Magn Reson. 2006 Oct;182(2):200-7. doi: 10.1016/j.jmr.2006.06.029. Epub 2006 Jul 21.
Swiss rolls are one instance of metamaterials, and can be described as an effective medium with a complex, anisotropic magnetic permeability. It has been shown that bundles of Swiss rolls can guide the magnetic flux in magnetic resonance measurements and increase the coupling between the nuclear spins and the receiver coil. Here, we investigate, with a numerical model, whether Swiss rolls can boost the detected signal in a NMR experiment, where the rolls could provide a low-reluctance return path for the magnetic flux when shaped in a yoke encircling the sample. The system consisting of the nuclear spin, the rolls and the receiver coil is analyzed with the method of finite differences in time domain (FDTD). The results show that small gains in the received signal are possible, but only if the losses (resistive and dielectric) in the rolls are reduced by over one order of magnitude from their present value in state-of-the-art materials. This situation arises because of the energy dissipation in the rolls and the mode splitting caused by the coupling between the rolls and the resonant coil.
瑞士卷是超材料的一个实例，可被描述为具有复杂各向异性磁导率的有效介质。研究表明，在磁共振测量中，瑞士卷束能够引导磁通量，并增强核自旋与接收线圈之间的耦合。在此，我们通过数值模型研究瑞士卷在核磁共振实验中是否能够增强检测信号，在该实验中，当瑞士卷围绕样品形成轭状时，可为磁通量提供低磁阻返回路径。采用时域有限差分法（FDTD）对由核自旋、瑞士卷和接收线圈组成的系统进行分析。结果表明，接收信号有可能实现小幅增益，但前提是瑞士卷中的损耗（电阻性和介电性）要比现有先进材料中的当前值降低一个数量级以上。出现这种情况是由于瑞士卷中的能量耗散以及瑞士卷与谐振线圈之间的耦合所导致的模式分裂。
