Metamaterial-Inspired Radiofrequency (RF) Shield With Reduced Specific Absorption Rate (SAR) and Improved Transmit Efficiency for UHF MRI
Metamaterial-Inspired Radiofrequency (RF) Shield With Reduced Specific Absorption Rate (SAR) and Improved Transmit Efficiency for UHF MRI
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DOI：
10.1109/tbme.2020.3022884
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发表时间：
2021-04-01
影响因子：
4.6
通讯作者：
Crozier, Stuart
中科院分区：
文献类型：
--
作者：
Chen, Haiwei;Guo, Lei;Crozier, Stuart
为了防止射频(RF)线圈与磁共振成像(MRI)系统中的其他组件(如梯度线圈)之间的干扰，在射频线圈和梯度线圈之间放置射频屏蔽是必不可少的。然而，传统射频屏蔽上的感应电流对射频线圈的性能有负面影响。为了减少这些影响，超材料吸波材料(MA)是一类对入射电磁场具有接近单位吸收率的超材料，可用于设计一种新型的射频屏蔽。然而，由于超材料结构的庞大，在磁共振成像系统中采用超材料通常是有问题的。在这项工作中，使用电容器和金属互连来实现MA的小型化，使得单位MA单元可以工作在7T和9.4T磁共振的Larmor频率下，并且保持紧凑。这种MA-RF屏蔽用于提高射频表面线圈的发射效率，并降低感兴趣区域(ROI)的比吸收率(SAR)。仿真和实验表明，与传统的射频屏蔽结构相比，MA-RF屏蔽结构的发射效率提高了32%以上，峰值SAR值降低了22%。此外，还观察到当表面线圈与MA-RF屏蔽一起使用时，发射场穿透能力得到了改善。这项概念验证研究为超材料在超高场磁共振应用中的应用提供了一条新的实用途径。
To prevent the interferences between radiofrequency (RF) coils and other components in the magnetic resonance imaging (MRI) system such as gradient coils, it is essential to place an RF shield between the RF coils and gradient coils. However, the induced currents on conventional RF shields have negative influences on the RF coil performance. To reduce these influences, metamaterial absorbers (MA), a class of metamaterials exhibiting nearly unity absorption rate for the incident electromagnetic fields, can be employed for the design of a novel RF shield. However, the adoption of metamaterials in MRI systems is usually problematic because of the bulkiness of the metamaterial structure. In this work, capacitors and metallic interconnectors are used to miniaturize the MA so that the unit MA cell can operate at the Larmor frequencies of 7T and 9.4T MRI and stay compact. This MA-RF shield is used to improve the transmit efficiency of RF surface coils and reduce the specific absorption rate (SAR) in the region of interest (ROI). It is successfully demonstrated by simulations and experiments that, compared with conventional RF shield structure, the transmit efficiency can be enhanced by more than 32% and the peak SAR value can be reduced by 22% using the MA-RF shield. Moreover, it is observed that the transmit field penetration is improved when the surface coil is used with the MA-RF shield. This proof-of-concept study suggests a new practical way for the utilization of metamaterials in ultra-high field MRI applications.