Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels
Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels
复制标题
DOI：
10.1002/mrm.21513
复制
发表时间：
2008-04-01
影响因子：
3.3
通讯作者：
Adalsteinsson, E.
中科院分区：
文献类型：
--
作者：
Setsompop, K.;Wald, L. L.;Adalsteinsson, E.
利用并行发射系统加速的空间定制射频（RF）激励提供了创建成形体积激励或缓解具有临床相关脉冲长度的不均匀B-1激励曲线的机会。虽然这样的激励通常被设计为对目标幅度和相位轮廓的最小二乘优化近似，但是只要激励相位与体素尺寸相比缓慢变化，则对目标相位轮廓的遵守通常并不重要。在这项工作中，我们展示了一种方法的幅度最小二乘优化的目标磁化曲线多通道并行激励，以提高幅度分布和减少RF功率的成本不太均匀的相位分布。该方法使设计者能够权衡允许的空间相位变化，以改善幅度分布和降低RF功率。我们验证的方法与模拟研究，并证明其性能在四倍加速的二维螺旋激励，以及均匀的平面内切片选择性并行激励使用8通道发射阵列上的7 T人体MRI扫描仪。实验结果与模拟结果吻合良好，模拟结果显示幅度分布显著改善，所需RF功率降低，同时仍保持可忽略的体素内相位变化。
Spatially tailored radio frequency (RF) excitations accelerated with parallel transmit systems provide the opportunity to create shaped volume excitations or mitigate inhomogeneous B-1 excitation profiles with clinically relevant pulse lengths. While such excitations are often designed as a least-squares optimized approximation to a target magnitude and phase profile, adherence to the target phase profile is usually not important as long as the excitation phase is slowly varying compared with the voxel dimension. In this work, we demonstrate a method for a magnitude least squares optimization of the target magnetization profile for multichannel parallel excitation to improve the magnitude profile and reduce the RF power at the cost of a less uniform phase profile. The method enables the designer to trade off the allowed spatial phase variation for the improvement in magnitude profile and reduction in RF power. We validate the method with simulation studies and demonstrate its performance in fourfold accelerated two-dimensional spiral excitations, as well as for uniform in-plane slice selective parallel excitations using an eight-channel transmit array on a 7T human MRI scanner. The experimental results are in good agreement with the simulations, which show significant improvement in the magnitude profile and reductions in the required RF power while still maintaining negligible intravoxel phase variation.