Center for Computational and Data-Intensive Science and Engineering (CDISE), Skolkovo Institute of Science and Technology, Moscow, Russian Federation.
Q Bio, Redwood City, California.
NMR Biomed. 2020 Nov;33(11):e4383. doi: 10.1002/nbm.4383. Epub 2020 Jul 28.
Transmit efficiency specifies the amplitude of the magnetic resonance excitation field produced over a region of interest with respect to the radiofrequency (RF) power deposited in the sample. This metric is highly important at ultra-high field magnetic resonance imaging (≥7 T), where excitation inhomogeneities and electric field interference effects could prevent achieving the desired flip angle distribution while satisfying the power safety limits. The aim of this work was to introduce an approach to calculate a theoretical upper bound on the transmit efficiency (OPTXE) for RF shimming, independent from any particular coil design. We computed the OPTXE for head-mimicking uniform spherical samples and a realistic heterogeneous head model by maximizing the square of the net transmit field per unit power deposition. The corresponding RF shimming weights were used to combine the analytical surface current modes into ideal current patterns. OPTXE grew monotonically as the target excitation voxel approached the surface of the object, and overall decreased at higher field strengths, presenting similar trends in both the uniform sphere and heterogeneous head model. Arrays with an increasing number of loops could closely approach OPTXE in the central region of the object, but performance decreased closer to the surface and at higher magnetic field strengths. The performance of 32 loops for a two-dimensional excitation region at 7 T increased from 34% to 93% when they were arranged based on the shape of the ideal current patterns. OPTXE provides an absolute reference to evaluate coil designs and RF shimming algorithms, whereas ideal current patterns could serve as guidelines for novel coil designs at ultra-high field. The uniform sphere model enables rapid analytic simulations and provides a good approximation of the OPTXE distribution in a realistic heterogeneous head model with comparable dimensions.
传输效率指定了相对于样品中沉积的射频 (RF) 功率，在感兴趣区域产生的磁共振激励场的幅度。在超高场磁共振成像（≥7T）中，该指标非常重要，因为激励不均匀和电场干扰效应可能会阻止在满足功率安全限制的同时实现所需的翻转角分布。这项工作的目的是引入一种计算 RF 调谐的理论上的传输效率上限（OPTXE）的方法，该方法与任何特定的线圈设计无关。我们通过最大化单位功率沉积的净传输场的平方来计算模拟头部的均匀球形样本和现实的异质头部模型的 OPTXE。相应的 RF 调谐权重用于将分析的表面电流模式组合成理想的电流模式。OPTXE 随着目标激励体素接近物体表面而单调增长，并且在更高的场强下总体减小，在均匀球体和异质头部模型中呈现出相似的趋势。具有更多环路的阵列可以在物体的中心区域紧密接近 OPTXE，但在靠近表面和更高的磁场强度时性能下降。在 7T 时，对于二维激励区域的 32 个环路，当根据理想电流模式的形状排列时，性能从 34%增加到 93%。OPTXE 为评估线圈设计和 RF 调谐算法提供了绝对参考，而理想电流模式可以作为超高温场新型线圈设计的指南。均匀球体模型能够实现快速的解析模拟，并提供与可比尺寸的现实异质头部模型中 OPTXE 分布的良好近似。
