背景:超高场(UHF)磁共振神经成像的主要优点是与低场强成像相比,信噪比(SNR)增加。然而,与UHFMRI相关的波长效应导致射频(RF)不均匀性,影响许多商业线圈的整个大脑覆盖。解决发射场不均匀性问题的方法包括并行发射系统(PTx)的设计,射频脉冲设计,并应用无源RF匀场,例如高介电材料。然而,这些方法有一些缺点,如不稳定的材料参数的电介质垫,高成本,以及PTx系统的复杂性。超表面是人造结构,具有独特的平台,可以控制电磁(EM)波的传播,他们是非常有前途的工程EM设备。先前在几项研究中已经探索了元阵列增强MRI的实施。
目的:本研究的目的是评估新的meta-array技术在7T时增强脑MRI的效果。设计了一种基于由宽边耦合裂环谐振器和高介电常数材料阵列组成的混合结构的元阵列,以在7特斯拉(7T)MRI扫描仪的拉莫尔频率下工作。当放在头部和颈部后面时,该结构提高了小脑区域的信噪比,脑干和颞叶的下部。
方法:进行了数值电磁模拟,以优化元阵列设计参数并确定RF电路配置。在体模中对所得的发射效率和信号灵敏度的改善进行了实验分析,随后健康志愿者使用配备标准单通道发射的7T全身MRI扫描仪,32通道接收头线圈。通过使用两个基本序列:梯度召回回波(GRE)和涡轮自旋回波(TSE),在有和没有元阵列的情况下进行采集来评估功效。
结果:实验体模分析证实了目标区域的传输效率提高了两倍,信号灵敏度提高了1.4倍。在体内GRE和TSE图像与meta-array的地方显示增强的可视化在大脑的下部区域,尤其是小脑,脑干,和颈脊髓。
结论:在常用的MRI线圈中添加元阵列可以增强SNR,以扩展线圈的解剖覆盖范围并改善MRI线圈的整体性能。可以利用SNR的这种增强来在相同的时隙上获得更高分辨率的图像,或者可以用相同的分辨率实现更快的采集。使用这种技术可以改善现有的商业线圈在7T下的性能,用于全脑和其他应用。
BACKGROUND: The main advantage of ultra-high field (UHF) magnetic resonance neuroimaging is theincreased signal-to-noise ratio (SNR) compared with lower field strength imaging. However, the wavelength effect associated with UHF MRI results in radiofrequency (RF) inhomogeneity, compromising whole brain coverage for many commercial coils. Approaches to resolving this issue of transmit field inhomogeneity include the design of parallel transmit systems (PTx), RF pulse design, and applying passive RF shimming such as high dielectric materials. However, these methods have some drawbacks such as unstable material parameters of dielectric pads, high-cost, and complexity of PTx systems. Metasurfaces are artificial structures with a unique platform that can control the propagation of the electromagnetic (EM) waves, and they are very promising for engineering EM device. Implementation of meta-arrays enhancing MRI has been explored previously in several studies.
OBJECTIVE: The aim of this study was to assess the effect of new meta-array technology on enhancing the brain MRI at 7T. A meta-array based on a hybrid structure consisting of an array of broadside-coupled split-ring resonators and high-permittivity materials was designed to work at the Larmor frequency of a 7 Tesla (7T) MRI scanner. When placed behind the head and neck, this construct improves the SNR in the region of the cerebellum,brainstem and the inferior aspect of the temporal lobes.
METHODS: Numerical electromagnetic simulations were performed to optimize the meta-array design parameters and determine the RF circuit configuration. The resultant transmit-efficiency and signal sensitivity improvements were experimentally analyzed in phantoms followed by healthy volunteers using a 7T whole-body MRI scanner equipped with a standard one-channel transmit, 32-channel receive head coil. Efficacy was evaluated through acquisition with and without the meta-array using two basic sequences: gradient-recalled-echo (GRE) and turbo-spin-echo (TSE).
RESULTS: Experimental phantom analysis confirmed two-fold improvement in the transmit efficiency and 1.4-fold improvement in the signal sensitivity in the target region. In vivo GRE and TSE images with the meta-array in place showed enhanced visualization in inferior regions of the brain, especially of the cerebellum, brainstem, and cervical spinal cord.
CONCLUSIONS: Addition of the meta-array to commonly used MRI coils can enhance SNR to extend the anatomical coverage of the coil and improve overall MRI coil performance. This enhancement in SNR can be leveraged to obtain a higher resolution image over the same time slot or faster acquisition can be achieved with same resolution. Using this technique could improve the performance of existing commercial coils at 7T for whole brain and other applications.