IEEE Trans Med Imaging. 2016 Aug;35(8):1837-44. doi: 10.1109/TMI.2016.2531120. Epub 2016 Feb 18.
Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions means that the uniformity of the excitation field is much poorer than at lower field strengths. In addition to techniques such as transmit arrays, one simple but effective method to counteract this effect is to use high permittivity "pads". Very high permittivities enable thinner, flexible pads to be used, but the limiting factor is wavelength effects within the pads themselves, which can lead to image artifacts. So far, all studies have used simple continuous rectangular/circular pad geometries. In this work we investigate how the wavelength effects can be partially mitigated utilizing shaped pad with holes. Several arrangements have been simulated, including low order pre-fractal geometries, which maintain the overall coverage of the pad, but can provide better image homogeneity in the region of interest or higher sensitivity depending on the setup. Experimental data in the form of in vivo human images at 7T were acquired to validate the simulation results.
超高场 MRI 由于其具有更高的信噪比、空间分辨率和基于磁化率的对比，因此成为临床研究和基础科学领域的研究热点。然而，由于组织中的电磁波长与相关的身体尺寸相当，因此激励场的均匀性比在较低场强下要差得多。除了发射阵列等技术外，一种简单但有效的方法是使用高介电常数“衬垫”来对抗这种效应。非常高的介电常数可以使用更薄、更灵活的衬垫，但限制因素是衬垫本身内的波长效应，这可能导致图像伪影。到目前为止，所有的研究都使用了简单的连续矩形/圆形衬垫几何形状。在这项工作中，我们研究了如何利用带有孔的成形衬垫来部分缓解波长效应。已经模拟了几种布置，包括低阶预分形几何形状，这些形状保持了衬垫的整体覆盖范围，但可以根据设置在感兴趣区域提供更好的图像均匀性或更高的灵敏度。以 7T 下人体图像的形式获取了实验数据来验证模拟结果。
