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: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=692558
• 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.**

由多个谐振元件组成的多通道射频（RF）线圈阵列是磁共振成像（MRI）采集中的关键部件。并行发射（pTx）阵列提供单独的灵敏度曲线，当与优化的梯度和RF波形一起使用时，可以加速激励kspace的遍历。该原理可用于加速多维选择性激发，执行B1+或RF匀场以克服超高频（UHF）处的不均匀性，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），并将其扩展到整个视场，同时执行空间编码，用于加速成像。 负责在RF传输期间激励自旋以及从横向磁化后激励接收信号的电磁场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两者的混合。因此，与更传统的RF环路元件相比，电偶极天线在UHF下的实用性越来越高，并且环路和偶极阵列元件组合成单个RF线圈组件预计将在每单位B1+和SNR的SAR效率方面表现出性能增益。****** 朝向使用电偶极元件和/或组合不同元件的演变提出了技术挑战。从电偶极子产生的辐射图案，结合其几何形状，受到传统去耦方法的适用性的限制。这在密集偶极接收阵列中得到了最好的证明，其中未减轻的耦合和增强的噪声相关性降低了测量的SNR，而不管对样品的灵敏度是否增加。我们已经开发出一种广义的解耦方法，允许复杂的RF阵列的混合元素类型和可变的耦合系数，以有效地解耦与梯形滤波器的方法。使用这种方法，我们将模拟，设计和建立多通道发射阵列，结合联合收割机环路和偶极子的方式，捕捉尽可能多的传输效率，为人类头部成像。一旦优化硬件以通过RF匀场产生最佳B1+均匀性，将使用RF脉冲来消除残余不均匀性。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模拟的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，该解决方案考虑了SAR的附加约束条件下的RF均匀性。 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扫描仪的多通道pTx的剩余障碍。