Joint Design of Excitation k-Space Trajectory and RF Pulse for Small-Tip 3D Tailored Excitation in MRI.
Joint Design of Excitation k-Space Trajectory and RF Pulse for Small-Tip 3D Tailored Excitation in MRI.
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DOI：
10.1109/tmi.2015.2478880
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发表时间：
2016-02
影响因子：
10.6
通讯作者：
Nielsen JF
中科院分区：
文献类型：
--
作者：
Hao S;Fessler JA;Noll DC;Nielsen JF
提出了一种三维小尖端定制激励中k空间轨迹和射频脉冲联合设计的新方法。针对MRI中的期望3D目标激励模式设计时变RF和梯度波形提出了具有相对大的问题大小的非线性、非凸、约束优化问题，其难以直接求解。因此，现有的联合脉冲设计方法通常限于预定义的轨迹类型，例如EPI或螺旋堆叠，其本质上满足梯度最大值和转换速率约束并显著减小问题大小（维度），但导致给定脉冲持续时间的次优激励精度。在这里，我们使用一个二阶B样条基，可以拟合到任意的k空间轨迹，并允许梯度约束有效地实现。我们表明，这使得联合优化问题可以用非常一般的k空间轨迹来解决。从任意的初始轨迹出发，先用B样条基逼近轨迹，然后优化相应的系数。我们评估我们的方法在模拟使用四种不同的k空间初始化：堆栈的螺旋，SPINS，KT点，和一个新的方法的基础上KT点。在所有情况下，我们的方法导致激励精度的大幅改善，对于一个给定的脉冲持续时间。我们还验证了我们的方法内体积激发使用幻影实验。计算速度足够快，适合在线应用。
We propose a new method for the joint design of k-space trajectory and RF pulse in 3D small-tip tailored excitation. Designing time-varying RF and gradient waveforms for a desired 3D target excitation pattern in MRI poses a non-linear, non-convex, constrained optimization problem with relatively large problem size that is difficult to solve directly. Existing joint pulse design approaches are therefore typically restricted to predefined trajectory types such as EPI or stack-of-spirals that intrinsically satisfy the gradient maximum and slew rate constraints and reduce the problem size (dimensionality) dramatically, but lead to suboptimal excitation accuracy for a given pulse duration. Here we use a 2nd-order B-spline basis that can be fitted to an arbitrary k-space trajectory, and allows the gradient constraints to be implemented efficiently. We show that this allows the joint optimization problem to be solved with quite general k-space trajectories. Starting from an arbitrary initial trajectory, we first approximate the trajectory using B-spline basis, and then optimize the corresponding coefficients. We evaluate our method in simulation using four different k-space initializations: stack-of-spirals, SPINS, KT-points, and a new method based on KT-points. In all cases, our approach leads to substantial improvement in excitation accuracy for a given pulse duration. We also validated our method for inner-volume excitation using phantom experiments. The computation is fast enough for online applications.