Sir Peter Mansfield Imaging Centre, School of Physics and Astronomy, University of Nottingham, Nottingham, UK.
Russell H. Morgan Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Magn Reson Med. 2021 Aug;86(2):611-624. doi: 10.1002/mrm.28749. Epub 2021 Mar 21.
Achieving a desired RF transmit field ( ) in small regions of interest is critical for single-voxel MRS at ultrahigh field. Radio-frequency (RF) shimming, using parallel transmission, requires mapping and optimization, which limits its ease of use. This work aimed to generate calibration-free RF shims for predefined target regions of interest, which can be applied to any participant, to produce a desired absolute magnitude (| |).
The RF shims were found offline by joint optimization on a database comprising maps from 11 subjects, considering regions of interest in occipital cortex, hippocampus and posterior cingulate, as well as whole brain. The | | achieved was compared with a tailored shimming approach, and MR spectra were acquired using tailored and calibration-free shims in 4 participants. Global and local 10g specific-absorption-rate deposition were estimated using Duke and Ella dielectric models.
There was no difference in the mean | | produced using calibration-free versus tailored RF shimming in the occipital cortex (p = .15), hippocampus (p = .5), or posterior cingulate (p = .98), although differences were observed in the RMS error | |. Spectra acquired using calibration-free shims had similar SNR and low residual water signal. Under identical power settings, specific-absorption-rate deposition was lower compared with operating in quadrature mode. For example, the total head specific absorption rate was around 35% less for the occipital cortex.
This work demonstrates that static RF shims, optimized offline for small regions, avoid the need for mapping and optimization for each region of interest and participant. Furthermore, power settings may be increased when using calibration-free shims, to better take advantage of RF shimming.
在超高场单体素 MRS 中，实现小感兴趣区域所需的射频发射场（）是至关重要的。使用并行传输的射频调谐需要映射和优化，这限制了其易用性。本工作旨在为预定义的感兴趣区域生成无校准的 RF 调谐器，可应用于任何参与者，以产生所需的绝对幅度（|）。
通过对包含 11 个受试者的数据库进行联合优化，离线找到 RF 调谐器，考虑到枕叶皮质、海马体和后扣带回以及整个大脑的感兴趣区域。与定制调谐方法相比，比较了 | | 的实现情况，并在 4 名参与者中使用定制和无校准调谐器采集了 MR 谱。使用 Duke 和 Ella 介电模型估计了全局和局部 10g 特定吸收率沉积。
在枕叶皮质（p=0.15）、海马体（p=0.5）或后扣带回（p=0.98）中，使用无校准与定制 RF 调谐器产生的平均| | 没有差异，尽管在 RMS 误差| | 方面存在差异。使用无校准调谐器采集的光谱具有相似的 SNR 和低残留水信号。在相同的功率设置下，与正交模式相比，特定吸收率沉积较低。例如，枕叶皮质的总头部特定吸收率降低了约 35%。
本工作表明，针对小区域离线优化的静态 RF 调谐器避免了针对每个感兴趣区域和参与者进行映射和优化的需要。此外，当使用无校准调谐器时，可以增加功率设置，以更好地利用 RF 调谐器。
