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.

中文翻译：

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一种研究射频匀场最佳传输效率的形式。

传输效率指定了在感兴趣区域上产生的磁共振激发场的振幅，相对于沉积在样品中的射频 (RF) 功率。该指标在超高场磁共振成像（≥7 T）中非常重要，其中激发不均匀性和电场干扰效应可能会阻止实现所需的翻转角分布，同时满足功率安全限制。这项工作的目的是引入一种方法来计算射频匀场的发射效率 (OPTXE) 的理论上限，独立于任何特定的线圈设计。我们通过最大化每单位功率沉积的净传输场的平方来计算用于模拟头部的均匀球形样品和真实的异构头部模型的 OPTXE。相应的 RF 匀场权重用于将分析表面电流模式组合成理想的电流模式。当目标激发体素接近物体表面时，OPTXE 单调增长，并在较高场强下整体下降，在均匀球体和异质头部模型中呈现出相似的趋势。具有越来越多回路的阵列可以在物体的中心区域接近 OPTXE，但在靠近表面和更高的磁场强度下性能下降。当基于理想电流模式的形状排列时，7 T 二维激发区的 32 个回路的性能从 34% 增加到 93%。OPTXE 为评估线圈设计和 RF 匀场算法提供了绝对参考，而理想的电流模式可以作为超高场下新型线圈设计的指南。均匀球体模型可实现快速分析模拟，并在具有可比尺寸的真实异质头部模型中提供 OPTXE 分布的良好近似。