Department of Electronics and Electromagnetism, Facultad de Física, University of Seville, Avenida Reina Mercedes s/n, 41012, Sevilla, Spain.
Sci Rep. 2023 Feb 21;13(1):2998. doi: 10.1038/s41598-023-30185-y.
This work investigates the use of a metasurface made up of a two-dimensional array of capacitively loaded metallic rings to enhance the signal-to-noise ratio of magnetic resonance imaging surface coils and to tailor the magnetic near-field radio frequency pattern of the coils. It is found that the signal-to-noise ratio is increased if the coupling between the capacitively loaded metallic rings in the array is increased. The input resistance and the radiofrequency magnetic field of the metasurface loaded coil are numerically analyzed by means of an efficient algorithm termed the discrete model to determine the signal-to-noise ratio. Standing surface waves or magnetoinductive waves supported by the metasurface introduce resonances in the frequency dependence of the input resistance. The signal-to-noise ratio is found to be optimal at the frequency corresponding to a local minimum existing between these resonances.The discrete model is used in an optimization procedure to fit the structural parameters of a metasurface to enhance the signal-to-noise ratio at the frequency corresponding to this local minimum in the input resistance. It is found that the signal-to-noise ratio can be greatly improved if the mutual coupling between the capacitively loaded metallic rings of the array is made stronger by bringing them closer or by using rings of squared shape instead of circular. These conclusions derived from the numerical results provided by the discrete model are double-checked by means of numerical simulations provided by the commercial electromagnetic solver Simulia CST and by experimental results. Numerical results provided by CST are also shown to demonstrate that the surface impedance of the array of elements can be adjusted to provide a more homogeneous magnetic near-field radio frequency pattern that ultimately leads to a more uniform magnetic resonance image at a desired slice. This is achieved by preventing the reflection of propagating magnetoinductive waves at the edges of the array by matching the elements arranged at the edges of the array with capacitors of suitable value.
这项工作研究了使用由二维电容加载金属环阵列组成的超表面来提高磁共振成像表面线圈的信噪比，并调整线圈的近磁场射频模式。研究发现，如果增加阵列中电容加载金属环之间的耦合，可以提高信噪比。通过一种称为离散模型的高效算法对加载超表面的线圈的输入电阻和射频磁场进行数值分析，以确定信噪比。超表面支持的驻波或磁电感波在输入电阻的频率依赖性中引入共振。发现信噪比在对应于这些共振之间存在局部最小值的频率处最佳。离散模型用于优化过程，以调整超表面的结构参数，以在对应于输入电阻局部最小值的频率处提高信噪比。研究发现，如果通过使金属环更靠近或使用方形而不是圆形的金属环来增强阵列中电容加载金属环之间的互耦合，则可以大大提高信噪比。这些结论是从离散模型提供的数值结果中得出的，并通过商业电磁求解器 Simulia CST 提供的数值模拟和实验结果进行了双重检查。还展示了 CST 提供的数值结果，以证明可以调整元件阵列的表面阻抗，以提供更均匀的近磁场射频模式，最终在期望的切片上获得更均匀的磁共振图像。通过在阵列边缘处的元件与适当值的电容器匹配，可以防止传播的磁电感波在阵列边缘处反射，从而实现这一点。
