超低场磁共振膝关节正交接收线圈设计

doi:10.19595/j.cnki.1000-6753.tces.230089

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摘要

射频接收线圈是磁共振成像（MRI）系统中的关键部件,用于检测变化的磁场,其性能直接影响了图像的质量。超低场MRI射频接收线圈工作在较低频率,其阻抗特性不同于传统临床MRI射频接收线圈。理论上正交接收线圈比单通道线圈的信噪比（SNR）可以提高41%。而与阵列线圈相比,正交接收线圈可以用更低的成本与设计复杂度达到很好的成像效果。因此该文研究了一种用于超低场MRI（50.4 mT）系统的膝关节正交接收线圈,它由鞍形线圈和亥姆霍兹线圈组成。首先,本文确定了线圈的支撑结构及尺寸,并通过计算单匝接收线圈的效率以及磁场的不均匀性来确定鞍形以及亥姆霍兹线圈的最优尺寸。然后,该文实测了线圈交流电阻,来评估线圈不同匝数的SNR,发现匝数增多会使得交流电阻显著增加,从而使SNR随着线圈匝数增加先增加后减少。最后,本文得到了两线圈的最佳匝数,并制作了正交接收线圈。从CuSO₄·5H₂O体膜以及体内膝关节成像来评估正交接收线圈的性能。在体膜和体内膝关节成像中,正交接收线圈与单通道线圈相比,图像SNR最大程度上分别提升了约33.5%和 30.9%。

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关键词 ：磁共振成像, 超低场, 膝关节成像, 正交接收线圈

Abstract：

The radio frequency (RF) receive coil is a key component in a magnetic resonance imaging (MRI) system, and its performance directly affects the quality of the MR images. Currently, the RF coil performance is mostly measured by RF magnetic field inhomogeneity and signal-to-noise ratio (SNR). In SNR calculations, the main contribution to noise is made by the coil resistance, including conductor losses and sample losses caused by RF currents. At low frequencies, the latter can be neglected. The ultra-low-field (ULF) MRI RF receive coil has a low working frequency and, therefore, has different impedance characteristics than the conventional clinical MRI RF receive coil, leading to different optimal RF coil configurations. This paper optimized a knee RF receive coil for an ultra-low field MRI system and investigated the relationship between RF coil construction and performance.
Specifically, this paper investigated a knee quadrature receive coil. Theoretically, a quadrature receive coil can improve the SNR by 41% over a single-channel coil. Compared to an array coil, a quadrature receive coil can achieve good imaging results at a lower cost and with less design complexity. The quadrature receive coil in this paper consists of a saddle coil and a Helmholtz coil. Firstly, the support structure and dimensions of the coil were determined, and the optimum size of the saddle and Helmholtz coils was determined by calculating the efficiency of the single-turn receive coil and the RF magnetic field inhomogeneity. Then, the paper further investigated the performance of the optimally sized RF coil in terms of the number of turns. Using the measured coil AC resistance, it is found that the number of turns is a significant feature affecting the AC resistance of the RF coil for ultra-low field MRI, and it is found that an increase in the number of turns leads to a significant increase in the AC resistance, resulting in an increase and then a decrease in SNR as the number of turns increases.
Finally, the optimal number of turns for both coils was obtained, and a quadrature receive coil was fabricated. The performance of the quadrature receive coil was evaluated using CuSO₄·5H₂O phantom and in vivo knee imaging. In phantom imaging, the quadrature receive coil shows a maximum image SNR improvement of approximately 33.5% compared to the single channel coil. In in vivo knee imaging, T₁- and T₂-weighted images were obtained using GRE3D and FSE sequences, respectively, from which the structural composition of the knee can be clearly observed. The quadrature receive coil shows maximum image SNR improvements of approximately 30.9% and 27.9%, respectively, over the single-channel coil. The results show that knee imaging in an ultra-low-field MRI system is feasible and that the quadrature receive coil performance is superior to that of a single-channel receive coil. This paper analyses and optimizes the coil from the perspective of the number of turns of the RF receive coil, revealing that the number of turns of the coil is an important factor affecting the SNR of the ULF-MRI RF receive coil, providing new evidence for the design and optimization of RF coils.

Key words： Magnetic resonance imaging ultra-low field knee imaging quadrature receive coil

收稿日期: 2023-01-18

PACS:	TM153+.1
	R445.2

基金资助:

国家自然科学基金（52077023）、重庆市自然科学基金（cstc2020jcyj-msxmX0340）、重庆市教委自然科学项目（KJQN202100533）和深圳市科技创新委员会（CJGJZD20200617102402006）资助项目

通讯作者: 徐征, 男,1980年生,教授,博士生导师,研究方向为生物电磁成像技术。E-mail：xuzheng@cqu.edu.cn

作者简介: 万裁 , 男,1996年生,博士研究生,研究方向生物电磁技术。E-mail：20181113069t@cqu.edu.cn

引用本文:

万裁, 何为, 沈晟, 徐征. 超低场磁共振膝关节正交接收线圈设计[J]. 电工技术学报, 0, (): 9014-14. Wan Cai, He Wei, Shen Sheng, Xu Zheng. Design of a Quadrature Receive Coil for Ultra-low-field Knee Magnetic Resonance Imaging. Transactions of China Electrotechnical Society, 0, (): 9014-14.

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