Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
F. M. Kirby Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.
J Magn Reson Imaging. 2018 Mar;47(3):673-681. doi: 10.1002/jmri.25813. Epub 2017 Jul 22.
To investigate frequency-offset effects in edited magnetic resonance spectroscopy (MRS) experiments arising from B eddy currents.
Macromolecule-suppressed (MM-suppressed) γ-aminobutyric acid (GABA)-edited experiments were performed at 3T. Saturation-offset series of MEGA-PRESS experiments were performed in phantoms, in order to investigate different aspects of the relationship between the effective editing frequencies and eddy currents associated with gradient pulses in the sequence. Difference integrals were quantified for each series, and the offset dependence of the integrals was analyzed to quantify the difference in frequency (Δf) between the actual vs. nominal expected saturation frequency.
Saturation-offset N-acetyl-aspartate-phantom experiments show that Δf varied with voxel orientation, ranging from 10.4 Hz (unrotated) to 6.4 Hz (45° rotation about the caudal-cranial axis) and 0.4 Hz (45° rotation about left-right axis), indicating that gradient-related B eddy currents vary with crusher-gradient orientation. Fixing the crusher-gradient coordinate-frame substantially reduced the orientation dependence of Δf (to ∼2 Hz). Water-suppression crusher gradients also introduced a frequency offset, with Δf = 0.6 Hz ("excitation" water suppression), compared to 10.2 Hz (no water suppression). In vivo spectra showed a negative edited "GABA" signal, suggesting Δf on the order of 10 Hz; with fixed crusher-gradient coordinate-frame, the expected positive edited "GABA" signal was observed.
Eddy currents associated with pulsed field gradients may have a considerable impact on highly frequency-selective spectral-editing experiments, such as MM-suppressed GABA editing at 3T. Careful selection of crusher gradient orientation may ameliorate these effects.
2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:673-681.
研究编辑磁共振波谱（MRS）实验中因涡流引起的频率偏移效应。
在 3T 进行了大分子抑制（MM 抑制）γ-氨基丁酸（GABA）编辑实验。在体模中进行了 MEGA-PRESS 饱和偏移系列实验，以研究序列中梯度脉冲相关的有效编辑频率和涡流之间关系的不同方面。对每个系列进行了差分积分量化，并分析积分的偏移依赖性，以量化实际与预期饱和频率之间的频率差异（Δf）。
N-乙酰天冬氨酸幻影的饱和偏移实验表明，Δf 随体素取向而变化，范围从 10.4 Hz（未旋转）到 6.4 Hz（沿头尾轴旋转 45°）和 0.4 Hz（沿左右轴旋转 45°），表明与梯度相关的涡流随碎波器梯度的方向而变化。固定碎波器梯度坐标框架显著降低了Δf 的取向依赖性（至约 2 Hz）。水抑制碎波器梯度也引入了频率偏移，与无 0.6 Hz 水抑制相比，Δf 为 10.2 Hz（无水抑制）。体内光谱显示出负的编辑“GABA”信号，表明Δf 约为 10 Hz；使用固定的碎波器梯度坐标框架，观察到了预期的正的编辑“GABA”信号。
与脉冲磁场梯度相关的涡流可能对高度频率选择性光谱编辑实验（如 3T 时的 MM 抑制 GABA 编辑）产生相当大的影响。仔细选择碎波器梯度方向可能会减轻这些影响。
2 技术功效：阶段 1 J. Magn. Reson. Imaging 2018;47:673-681.
