Ming Hsieh Department of Electrical and Computer Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, California, USA.
Alfred E. Mann Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, California, USA.
Magn Reson Med. 2025 Feb;93(2):718-729. doi: 10.1002/mrm.30311. Epub 2024 Oct 4.
To develop a small-tip multidimensional RF pulse design procedure that incorporates linear time-invariant gradient imperfections and concomitant field effects. This could be particularly important for contemporary low-field MRI systems with high-performance gradients.
We developed an extension of the small-tip excitation k-space formalism, where concomitant fields were approximated as a Bloch-Siegert shift in the rotating frame. This was evaluated using realistic simulations of 2D selective excitation at various field strengths (0.2T, 0.55T, 1.5T, 3T, and 7T) with single and parallel transmit. Simulated excitation profiles from the original and extended k-space formalisms were compared. Experimental validations were performed at 0.55T with a single-channel transmit.
The extended formalism provides improved 2D excitation profiles in all scenarios simulated, compared against the original formalism. The proposed method corrects the concomitant field effects on 2D selective excitations for B > 0.2T when the magnitude of the B is far larger than that of nonrotating concomitant fields. Simulation and phantom experiments at 0.55T match well for both original and proposed methods, with the proposed method providing sharper and more accurate excitation profiles at off-isocenter distances up to 15 cm. The impact of the proposed method is greatest in scenarios where concomitant fields are substantial, such as low field strengths and off-isocenter.
Concomitant fields can be modeled as a Bloch-Siegert shift in the rotating frame during multidimensional RF pulse design, resulting in improved excitation profiles with sharp edges. This is important to consider for off-isocenter excitations and imaging at low field strengths with strong gradients.
开发一种小型尖端多维射频脉冲设计程序，该程序将线性时不变梯度误差和伴随场效应结合在一起。这对于具有高性能梯度的现代低场 MRI 系统可能尤为重要。
我们开发了小型尖端激励 k 空间形式的扩展，其中伴随场被近似为旋转框架中的 Bloch-Siegert 偏移。这是通过在各种场强（0.2T、0.55T、1.5T、3T 和 7T）下使用单通道和并行发射对二维选择性激励进行的现实模拟进行评估的。比较了原始和扩展 k 空间形式的模拟激励轮廓。在 0.55T 下进行了单通道发射的实验验证。
与原始形式相比，扩展形式在所有模拟场景中提供了改进的二维激励轮廓。对于 B>0.2T 的情况，当 B 的大小远大于非旋转伴随场的大小时，该方法可以纠正二维选择性激励的伴随场效应。0.55T 的模拟和仿体实验对于原始和建议方法都非常匹配，在离等中心距离高达 15cm 时，建议的方法提供了更尖锐和更准确的激励轮廓。在伴随场较大的情况下，例如在低场强和离等中心位置，该方法的影响最大。
在多维 RF 脉冲设计过程中，可以将伴随场建模为旋转框架中的 Bloch-Siegert 偏移，从而产生具有锐利边缘的改进激励轮廓。对于低场强和具有强梯度的离等中心激励和成像，这一点非常重要。
