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: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=715123
• 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) 采集的关键组件。并行发射 (pTx) 阵列提供单独的灵敏度配置文件，当与优化的梯度和 RF 波形配合使用时，可以加速激励 k 空间的遍历。该原理可用于加速多维选择性激励、执行 B1+ 或 RF 匀场以克服超高场 (UHF) 下的不均匀性，或 降低比吸收率 (SAR)。同样，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)，并将其扩展到整个视场，同时执行空间编码，用于加速成像。 随着主磁场强度的增加，负责在射频传输期间激发自旋以及从横向磁化后激发接收信号的电磁场从纯反应性近场相互作用转变为近场和远场的混合。因此，与更传统的射频环路元件相比，电偶极子天线在 UHF 上的实用性越来越高，并且将环路和偶极子阵列元件组合到单个射频线圈组件中，预计将在每单位 B1+ 和 SNR 的 SAR 效率方面表现出性能提升。 使用电偶极子元件和/或组合不同的元件的发展提出了技术挑战。电偶极子产生的辐射图及其几何形状受到传统去耦方法的适用性的限制。这在密集的偶极子接收阵列中得到了最好的证明，其中未缓解的耦合和增强的噪声相关性会降低测量的 SNR，无论对样本的灵敏度是否增加。我们开发了一种通用的去耦方法，允许使用梯形滤波器方法有效地去耦混合元件类型和可变耦合系数的复杂射频阵列。使用这种方法，我们将模拟、设计和构建多通道传输阵列，该阵列将环路和偶极子组合在一起，以捕获尽可能多的人体头部成像传输效率。一旦硬件经过优化以通过射频匀场产生最佳的 B1+ 均匀性，射频脉冲将用于消除残留的不均匀性。实时脉冲优化一直是一个挑战，因此目前使用折衷的方法。使用来自受试者和电磁仿真的 B1+ 和静态磁场图，我们将使用机器学习方法来帮助实时设计最佳解决方案，同时考虑射频均匀性和 SAR 的附加约束。 结合起来，这些方法将克服在临床和基础神经科学环境中采用 7 T 扫描仪多通道 pTx 的剩余障碍。