Shao Xin, Zhang Zhe, Ma Xiaodong, Liu Fan, Guo Hua, Ugurbil Kamil, Wu Xiaoping
Center for Biomedical Imaging Research, School of Biomedical Engineering, Tsinghua University, Beijing, China.
Tiantan Neuroimaging Center of Excellence, Beijing Tiantan Hospital, Capital Medical University, Beijing, China.
Magn Reson Med. 2025 Mar;93(3):1238-1255. doi: 10.1002/mrm.30346. Epub 2024 Oct 31.
To propose a novel method for parallel-transmission (pTx) spatial-spectral pulse design and demonstrate its utility for robust uniform water-selective excitation (water excitation) across the entire brain.
Our design problem is formulated as a magnitude-least-squares minimization with joint RF and k-space optimization under explicit specific-absorption-rate constraints. For improved robustness against off-resonance effects, the spectral component of the excitation target is prescribed to have a water passband and a fat stopband. A two-step algorithm was devised to solve our design problem, with Step 1 aiming to solve a reduced problem to find a sensible start point for Step 2 to solve the original problem. The efficacy of our pulse design was evaluated in simulation, phantom, and human experiments using the commercial Nova head coil. Universal pulses were also designed based on a 10-subject training data set to demonstrate the utility of our method for plug-and-play pTx.
For k-points and spiral nonselective parameterizations, our design method outperformed the pTx interleaved binomial approach, reducing RMS error by up to about 35% for water excitation and about 97% for fat suppression (over a 200-Hz bandwidth) while decreasing local specific absorption rate by about 30%. Both our subject-specific and universal pulses improved water excitation, restoring signal loss in the cerebellum while suppressing fat signal even in regions of large susceptibility-induced off-resonances.
Demonstrated useful for 4D (3D spatial, one-dimensional spectral) pTx spatial-spectral pulse design, our proposed method provides an effective solution for robust volumetric uniform water excitation, holding a promise to many ultrahigh-field applications.
提出一种用于并行传输（pTx）空间谱脉冲设计的新方法，并证明其在全脑进行稳健的均匀水选择性激发（水激发）的效用。
我们将设计问题表述为在明确的比吸收率约束下，通过联合射频和k空间优化进行幅度最小二乘最小化。为了提高对失谐效应的鲁棒性，规定激发目标的频谱成分具有水通带和脂肪阻带。设计了一种两步算法来解决我们的设计问题，第一步旨在解决一个简化问题以找到一个合理的起始点，以便第二步解决原始问题。我们使用商用Nova头部线圈在模拟、体模和人体实验中评估了脉冲设计的效果。还基于一个包含10名受试者的训练数据集设计了通用脉冲，以证明我们的方法用于即插即用pTx的效用。
对于k点和螺旋非选择性参数化，我们的设计方法优于pTx交错二项式方法，在水激发方面将均方根误差降低了约35%，在脂肪抑制方面（在200赫兹带宽内）降低了约97%，同时将局部比吸收率降低了约30%。我们的受试者特异性脉冲和通用脉冲都改善了水激发，恢复了小脑的信号损失，同时即使在大的磁化率诱导失谐区域也能抑制脂肪信号。
我们提出的方法被证明对4D（3D空间，1D频谱）pTx空间谱脉冲设计有用，为稳健的体积均匀水激发提供了一种有效解决方案，有望应用于许多超高场应用。
