基于低频表面超材料的超低场磁共振成像信噪比增强方法

doi:10.19595/j.cnki.1000-6753.tces.L10029

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摘要超低场磁共振成像（ULF-MRI）设备具有成本低、质量轻、体积小等优点,在特殊领域的应用需求越来越大,如移动磁共振成像救护车、床旁监护、偏远地区等。但由于其磁场强度低,导致信噪比也较低,图像分辨率差,因此提升信噪比对促进ULF-MRI技术的发展具有重要意义。该文基于低频表面超材料的特性,提出一种电感-电容谐振阵列,用来提升ULF-MRI的信噪比。首先,建立了等效电路模型来评估磁场增强效果;其次,利用部分元等效电路法计算射频线圈的等效电阻和信噪比,根据成像要求设计优化谐振线圈结构,并提出考虑谐振耦合的匹配方法和线圈阵列的解耦方法。最后,利用自主研发的54.6 mT的ULF-MRI系统（工作频率：2.32 MHz）进行成像实验,图像信噪比得到显著提高。

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关键词 ：磁共振成像, 射频线圈, 低频超材料, 信噪比

Abstract：Ultra-low field magnetic resonance imaging (ULF-MRI) offers the benefits of compact size and lightweight. However, the imaging signal-to-noise ratio (SNR) is lower due to limitations in the main magnetic field strength. Thus, a resonant coil array is used for localized signal enhancement in ULF-MRI at 54.6 mT (at a frequency of 2.32 MHz).
Given the significant frequency contrast with high-field MRI, the application and design approach for resonant coils in ULF-MRI is presented. The principle is explained using an equivalent circuit. The resonant coil is positioned on the surface of the sample, coaxially aligned with the radio frequency (RF) receiving coil for maximum enhancement. It's tuned to the resonant frequency with an additional capacitor. During RF reception, the coil amplifies the local RF magnetic field, enhancing the received magnetic resonance signals and improving the SNR of the image. A matching method for the receiving coil with the resonance coil is introduced. Due to the strong coupling between the resonant coil and the receiving coil, traditional methods of matching the receiving coil become ineffective. An appropriate deviation of the matching capacitor for the resonant coil is required to maximize the transfer factor from the receiving coil to the preamplifier. The optimization design of the resonant coil is then carried out using a combination of the partial element equivalent circuit (PEEC) method and unitary design. The PEEC method allows rapid calculation of the AC resistance, while the uniform design method facilitates the selection of optimal coil structures under multiple constraints and design objectives. Finally, an overlapped decoupling approach minimizes the coupling between individual coils within the resonant coil array, and the optimum center-to-center distance between the coils is given.
For the single resonant coil, simulation and imaging results show that the SNR enhancement decreases with distance. The overall SNR remains higher than in the absence of resonant coils. Clear trends in the imaging results for a 5.5 cm diameter water phantom demonstrate this effect, with a maximum SNR enhancement of up to 8.4 times. Resonant coil arrays are also used in imaging experiments with a large water phantom of 110 mm diameter, and a maximum SNR enhancement of 2.7 times is observed. Although the SNR enhancement decreases compared to single coils, the array provides a larger and more uniform enhancement region. Arm imaging experiments verify the performance of the array in human imaging by wrapping the resonant array around the subject's forearm. After the array is applied, the maximum SNR is improved by a factor of 4.9. The SNR improvement is slightly higher than for the large water phantom due to the smaller diameter of the forearm.
The effectiveness of localized enhancement using resonant coils has been confirmed. Compared to conventional surface coils, they offer lower cost, a wireless design for flexible placement, and eliminate the need for additional RF channels. Resonant coil arrays are suitable for enhancing the local SNR in different human body regions and can be freely configured into arrays based on imaging requirements. In future work, resonant coil arrays can be designed specifically for other body regions to achieve optimal enhancement effects.

Key words： Magnetic resonance imaging radio-frequency coil low-frequency metamaterial signal-to-noise ratio

收稿日期: 2023-01-10

PACS:

TM153

基金资助:国家自然科学基金项目（52077023）和深圳科创委承接国家重大科技项目（CJGJZD20200617102402006）及深圳市科技重大专项（KJZD20230923114110019）资助

通讯作者: 徐征男,1980年生,教授,博士生导师,研究方向为电磁场数值计算方法、磁共振成像装置研究等。E-mail: xuzheng@cqu.edu.cn

作者简介: 孔晓涵女,1994年生,博士研究生,研究方向为磁共振成像系统硬件设计与应用。E-mail: kongxiaohan@cqu.edu.cn

引用本文:

孔晓涵, 张雅娜, 吴嘉敏, 贺玉成, 徐征. 基于低频表面超材料的超低场磁共振成像信噪比增强方法[J]. 电工技术学报, 2024, 39(13): 3917-3927. Kong Xiaohan, Zhang Yana, Wu Jiamin, He Yucheng, Xu Zheng. Signal-to-Noise Ratio Enhancement Method for Ultra-Low Field Magnetic Resonance Imaging Based on Low-Frequency Surface Metamaterials. Transactions of China Electrotechnical Society, 2024, 39(13): 3917-3927.

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