Department of Biomedical Engineering, The University of Melbourne, Parkville, Victoria, Australia.
Melbourne Brain Centre Imaging Unit, The University of Melbourne, Parkville, Victoria, Australia.
Magn Reson Med. 2024 Feb;91(2):570-582. doi: 10.1002/mrm.29884. Epub 2023 Oct 17.
The aim of the study was to explore a novel methodology for designing universal pulses (UPs) that balances the benefits of a calibration-free approach with subject-specific online pulse design.
The proposed method involves segmenting the population into subpopulations with variability in anatomical shapes and positions reduced to 75%, 50%, and 25% of their original values while keeping the mean values unchanged. An additional 25% extreme case with a large volume of interest and shifted position was included. For each group, a 5kT-points universal inversion pulse was designed and assessed by the normalized root mean square error (NRMSE) on the target longitudinal magnetization profile. The performance was compared to the conventional one-size-fits-all approach. A total of 132 electromagnetic simulations were executed to generate representative anatomies and specific absorption rate (SAR) distributions in a three-dimensional parameter space comprised of head breadth, head length, and Y-shift. The 99.9 percentile on the peak local SAR distribution was utilized to establish an intersubject variability safety margin.
UPs designed for subpopulations with decreased head shape and position variability reduced the anatomical safety margin by up to 20%. Furthermore, when a head was significantly different to the average case, the proposed approach improved the inversion homogeneity by up to 24%, compared to the conventional one-size-fits-all approach.
Subpopulation UPs present an opportunity to improve the homogeneity and reduce anatomical SAR safety margins at 7T without additional acquisition time for calibration.
本研究旨在探索一种新的通用脉冲（UP）设计方法，该方法将无校准方法的优势与针对特定个体的在线脉冲设计相结合。
该方法将人群分为解剖形状和位置变异降低到原始值的 75%、50%和 25%的亚群，同时保持平均值不变。还包括另外 25%的感兴趣体积大且位置偏移的极端情况。对于每个组，设计一个 5kT 点的通用反转脉冲，并通过目标纵向磁化率曲线的归一化均方根误差（NRMSE）进行评估。将性能与传统的一刀切方法进行比较。总共进行了 132 次电磁模拟，以在由头部宽度、头部长度和 Y 偏移组成的三维参数空间中生成具有代表性的解剖结构和特定吸收率（SAR）分布。利用峰局部 SAR 分布的 99.9%百分位数来确定个体间变异性的安全裕度。
针对头部形状和位置变异较小的亚群设计的 UP 降低了最大可达 20%的解剖安全裕度。此外，当头部与平均情况明显不同时，与传统的一刀切方法相比，该方法可将反转均匀性提高多达 24%。
亚群 UP 提供了一种机会，可以在不增加校准采集时间的情况下，提高 7T 下的均匀性并降低解剖 SAR 安全裕度。
