Radio Frequency Coil Design for Ultra High Field Magnetic Resonance Imaging

超高场磁共振成像射频线圈设计

• 批准号: RGPIN-2019-04743
• 负责人: Menon, Ravi
• 金额: $ 6.92万
• 依托单位: University of Western Ontario
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=760731
• 关键词: Radio; Frequency; Coil; Design; Ultra

Multichannel radio frequency (RF) coil arrays, composed of multiple resonant elements, are a critical component in magnetic resonance imaging (MRI) acquisition. Parallel transmit (pTx) arrays provide individual sensitivity profiles that when used in concert with optimized gradient and RF waveforms can accelerate the traversal of excitation kspace. This principle can be used to accelerate multidimensional selective excitation, perform B1+ or RF shimming to overcome inhomogeneity at ultrahigh field (UHF), or reduce specific absorption rate (SAR). Similarly, parallel receive (pRx) arrays exploit the locally high signal-to-noise ratio (SNR) of surface coils to the MRI signal (B1-) and extend it across a full field of view while simultaneously performing spatial encoding, utilized in accelerated imaging.    The electromagnetic fields responsible for exciting spins during RF transmission, as well as receiving signal from the transverse magnetization post-excitation, transition from purely reactive nearfield interaction towards a mixture of both near and far fields as the main magnetic field strength increases. Due to this, electric dipole antennas are finding increasing utility at UHF when compared to more conventional RF loop elements and combinations of loop and dipole array elements into a single RF coil assembly are expected to show performance gains in SAR efficiency per unit B1+ and SNR.    The evolution towards using electric dipole elements and/or combining dissimilar elements present a technical challenge. The radiation patterns produced from the electric dipole, in conjunction with its geometry, are limited by the applicability of conventional decoupling methods. This is best demonstrated in densely populated dipole receive arrays where unmitigated coupling and enhanced noise correlations degrade measured SNR, regardless of an increased sensitivity to the sample. We have developed a generalized decoupling approach that allows complex RF arrays of mixed element types and variable coupling coefficients to be efficiently decoupled with a ladder filter approach. Using this approach, we will simulate, design and build multichannel transmit  arrays that combine loops and dipoles in a manner that captures as much of the transmit efficiency as possible for human head imaging. Once the hardware is optimized to generate the best B1+ homogeneity via RF shimming, RF pulses will be used to remove the residual inhomogeneity. Real-time pulse optimization has been a challenge, so compromised approaches are currently used. Using B1+ and static magnetic field maps from subjects and from electromagnetic simulations, we will use machine learning approaches to help design the optimal solutions in real-time that take into account RF homogeneity with the added constraints of SAR.    In combination, these approaches will surmount the remaining barriers to the adoption of multichannel pTx for 7 T scanners in clinical and basic neuroscience settings.

多通道射频 (RF) 线圈阵列由多个谐振元件组成，是磁共振成像 (MRI) 采集中的关键组件。并行发射 (pTx) 阵列提供单独的灵敏度分布，当与优化的梯度和射频波形配合使用时，可以提供单独的灵敏度分布。加速激励k空间的遍历 该原理可用于加速多维选择性激励，进行B1+或RF匀场来克服。超高场 (UHF) 的不均匀性，或降低比吸收率 (SAR) 类似地，并行接收 (pRx) 阵列利用表面线圈对 MRI 信号 (B1-) 的局部高信噪比 (SNR) 和将其扩展到整个视场，同时执行空间编码，用于加速成像以及在射频传输期间激发旋转的电磁场。从横向磁化后激励接收信号，随着主磁场强度的增加，从纯反应性近场相互作用转变为近场和远场混合。因此，与其他天线相比，电偶极子天线在 UHF 下的实用性越来越高。传统的射频环路元件以及将环路和偶极子阵列元件组合到单个射频线圈组件中预计将显示出每单位 B1+ SAR 效率和信噪比的性能提升​向使用电动的发展。偶极子元件和/或组合不同的元件提出了技术挑战，由电偶极子结的几何形状产生的辐射图案受到便利去耦方法的适用性的限制，其中E接收阵列采用未缓解的耦合和增强的噪声相关性测量的SNR去耦方法。允许混合元件类型的复杂射频阵列LD 多通道传输的可变耦合系数，以尽可能多地捕获人体头部传输效率的方式组合环路和偶极子，一旦我通过射频匀场实时脉冲优化优化了硬件，即可实现最佳 B1+ 均匀性。一直是一个挑战，因此目前使用来自主题和电磁仿真的 B1+ 和静态磁场图，我们希望学习一些方法来帮助实时设计解决方案，同时考虑到射频均匀性和附加的约束。综合起来，这些方法将消除在基础神经科学设置中采用多通道 Ptx 的剩余障碍。