Delebarre Thaddée, Gras Vincent, Mauconduit Franck, Vignaud Alexandre, Boulant Nicolas, Ciobanu Luisa
NeuroSpin, UMR CEA/CNRS 9027, Paris-Saclay University, Gif-sur-Yvette, France.
Magn Reson Med. 2023 Jul;90(1):51-63. doi: 10.1002/mrm.29617. Epub 2023 Feb 13.
To optimize the homogeneity of the presaturation module in a Chemical Exchange Saturation Transfer (CEST) acquisition at 7 T using parallel transmission (pTx).
An optimized pTx-CEST presaturation scheme based on precomputed universal pulses was designed. The optimization was performed by minimizing the L2-norm between the effective and a given target while imposing energy constraints under virtual observation points (VOPs) supervision. The proposed method was evaluated through simulations and experimentally, both in vitro, on a realistic human head phantom, and in vivo, on healthy volunteers. The results were compared with circular polarization (CP) presaturation and other pTx approaches previously proposed. All experiments were conducted on a 7 T MRI scanner using a commercial 8Tx/32Rx head coil.
The simulations show that the proposed pTx strategy boosted with VOPs is superior to the CP mode and existent pTx approaches. While the best results are obtained with subject specific pulses, the gain provided by the use of VOPs renders the universal pulses superior to tailored pulses optimized under vendor provided Specific Absorption Rate (SAR) management. In the phantom, the glucose map was significantly more homogeneous than with CP (root mean square error [RMSE] 17% vs. 30%). The efficiency of the method for in vivo hydroxyl, glutamate and rNOE weighted CEST acquisitions was also demonstrated.
The use of a pTx presaturation scheme based on universal pulses optimized under VOP SAR management is significantly benefiting CEST imaging at high magnetic field.
在7T场强下使用并行发射（pTx）优化化学交换饱和转移（CEST）采集中预饱和模块的均匀性。
设计了一种基于预计算通用脉冲的优化pTx-CEST预饱和方案。通过在虚拟观测点（VOP）监督下施加能量约束，使有效信号与给定目标之间的L2范数最小化来进行优化。通过模拟以及在体外真实人体头部模型和体内健康志愿者身上进行实验，对所提出的方法进行评估。将结果与圆极化（CP）预饱和及先前提出的其他pTx方法进行比较。所有实验均在7T磁共振成像扫描仪上使用商用8Tx/32Rx头部线圈进行。
模拟结果表明，采用VOP增强的所提出的pTx策略优于CP模式和现有的pTx方法。虽然使用个体特异性脉冲可获得最佳结果，但使用VOP带来的增益使通用脉冲优于在供应商提供的比吸收率（SAR）管理下优化的定制脉冲。在模型中，葡萄糖图谱的均匀性明显优于CP（均方根误差[RMSE]为17%对30%）。还证明了该方法在体内羟基、谷氨酸和rNOE加权CEST采集中的有效性。
使用基于在VOP SAR管理下优化的通用脉冲的pTx预饱和方案，对高磁场下的CEST成像有显著益处。
