Fetal-Neonatal Neuroimaging & Developmental Science Center, Boston Children's Hospital, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA.
Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER Maastricht, the Netherlands.
Magn Reson Imaging. 2022 Nov;93:87-96. doi: 10.1016/j.mri.2022.08.006. Epub 2022 Aug 5.
We develop and test a parallel transmit (pTx) pulse design framework to mitigate transmit field inhomogeneity with control of local specific absorption rate (SAR) in 2D rapid acquisition with relaxation enhancement (RARE) imaging at 7T.
We design large flip angle RF pulses with explicit local SAR constraints by numerical simulation of the Bloch equations. Parallel computation and analytical expressions for the Jacobian and the Hessian matrices are employed to reduce pulse design time. The refocusing-excitation "spokes" pulse pairs are designed to satisfy the Carr-Purcell-Meiboom-Gill (CPMG) condition using a combined magnitude least squares-least squares approach.
In a simulated dataset, the proposed approach reduced peak local SAR by up to 56% for the same level of refocusing uniformity error and reduced refocusing uniformity error by up to 59% (from 32% to 7%) for the same level of peak local SAR compared to the circularly polarized birdcage mode of the pTx array. Using explicit local SAR constraints also reduced peak local SAR by up to 46% compared to an RF peak power constrained design. The excitation and refocusing uniformity error were reduced from 20%-33% to 4%-6% in single slice phantom experiments. Phantom experiments demonstrated good agreement between the simulated excitation and refocusing uniformity profiles and experimental image shading.
PTx-designed excitation and refocusing CPMG pulse pairs can mitigate transmit field inhomogeneity in the 2D RARE sequence. Moreover, local SAR can be decreased significantly using pTx, potentially leading to better slice coverage, enabling larger flip angles or faster imaging.
我们开发并测试了一种并行发射（pTx）脉冲设计框架，以减轻 7T 下二维快速采集弛豫增强（RARE）成像中局部特定吸收率（SAR）控制的发射场不均匀性。
我们通过数值模拟 Bloch 方程来设计具有明确局部 SAR 约束的大翻转角 RF 脉冲。并行计算和雅可比矩阵和 Hessian 矩阵的解析表达式用于减少脉冲设计时间。采用幅度最小二乘-最小二乘组合方法设计聚焦-激励“辐条”脉冲对，以满足 Carr-Purcell-Meiboom-Gill（CPMG）条件。
在模拟数据集上，与 pTx 阵列的圆形极化鸟笼模式相比，该方法将相同水平的重聚焦均匀性误差下的峰值局部 SAR 降低了 56%，并将相同水平的峰值局部 SAR 降低了 59%（从 32%降至 7%）。使用明确的局部 SAR 约束也将峰值局部 SAR 降低了 46%，与 RF 峰值功率约束设计相比。在单切片体模实验中，激励和重聚焦均匀性误差从 20%-33%降低到 4%-6%。体模实验证明了模拟激发和重聚焦均匀性轮廓与实验图像阴影之间的良好一致性。
pTx 设计的激励和重聚焦 CPMG 脉冲对可减轻二维 RARE 序列中的发射场不均匀性。此外，pTx 可显著降低局部 SAR，从而可能实现更好的切片覆盖范围、更大的翻转角或更快的成像速度。
