Wong Dickson, Schranz Amy L, Bartha Robert
Department of Medical Biophysics, The University of Western Ontario, London, ON, Canada.
Centre for Functional and Metabolic Mapping, Robarts Research Institute, The University of Western Ontario, London, ON, Canada.
NMR Biomed. 2018 Nov;31(11):e4002. doi: 10.1002/nbm.4002. Epub 2018 Aug 24.
A short echo time (T ) is commonly used for brain glutamate measurement by H MRS to minimize drawbacks of long T such as signal modulation due to J evolution and T relaxation. However, J coupling causes the spectral patterns of glutamate to change with T , and the shortest achievable T may not produce the optimal glutamate measurement. The purpose of this study was to determine the optimal T for glutamate measurement at 7 T using semi-LASER (localization by adiabatic selective refocusing). Time-domain simulations were performed to model the T dependence of glutamate signal energy, a measure of glutamate signal strength, and were verified against measurements made in the human sensorimotor cortex (five subjects, 2 × 2 × 2 cm voxel, 16 averages) on a 7 T MRI scanner. Simulations showed a local maximum of glutamate signal energy at T = 107 ms. In vivo, T = 105 ms produced a low Cramér-Rao lower bound of 6.5 ± 2.0% across subjects, indicating high-quality fits of the prior knowledge model to in vivo data. T = 105 ms also produced the greatest glutamate signal energy with the smallest inter-subject glutamate-to-creatine ratio (Glu/Cr) coefficient of variation (CV), 4.6%. Using these CVs, we performed sample size calculations to estimate the number of participants per group required to detect a 10% change in Glu/Cr between two groups with 95% confidence. 13 were required at T = 45 ms, the shortest achievable echo time on our 7 T MRI scanner, while only 5 were required at T = 105 ms, indicating greater statistical power. These results indicate that T = 105 ms is optimum for in vivo glutamate measurement at 7 T with semi-LASER. Using long T decreases power deposition by allowing lower maximum RF pulse amplitudes in conjunction with longer RF pulses. Importantly, long T minimizes macromolecule contributions, eliminating the requirement for acquisition of separate macromolecule spectra or macromolecule fitting techniques, which add additional scan time or bias the estimated glutamate fit.
短回波时间（T）通常用于通过氢磁共振波谱（H MRS）测量脑内谷氨酸，以尽量减少长T的缺点，如由于J演化和T弛豫导致的信号调制。然而，J耦合会使谷氨酸的光谱模式随T发生变化，并且可实现的最短T可能无法产生最佳的谷氨酸测量结果。本研究的目的是使用半激光（绝热选择性重聚焦定位）确定7T时谷氨酸测量的最佳T。进行了时域模拟，以模拟谷氨酸信号能量（一种衡量谷氨酸信号强度的指标）对T的依赖性，并根据在7T磁共振成像（MRI）扫描仪上对人类感觉运动皮层（5名受试者，2×2×2 cm体素，16次平均）进行的测量进行了验证。模拟显示在T = 107 ms时谷氨酸信号能量出现局部最大值。在体内，T = 105 ms在所有受试者中产生了6.5±2.0%的低克拉美-罗下限，表明先验知识模型与体内数据具有高质量的拟合。T = 105 ms还产生了最大的谷氨酸信号能量，受试者间谷氨酸与肌酸比值（Glu/Cr）的变异系数（CV）最小，为4.6%。使用这些CV，我们进行了样本量计算，以估计每组检测两组之间Glu/Cr 10%变化所需的参与者数量，置信度为95%。在我们的7T MRI扫描仪上可实现的最短回波时间T = 45 ms时需要13名参与者，而在T = 105 ms时仅需要5名参与者，这表明统计效力更高。这些结果表明，对于7T时使用半激光进行体内谷氨酸测量，T = 105 ms是最佳的。使用长T通过允许较低的最大射频脉冲幅度并结合较长的射频脉冲来降低功率沉积。重要的是，长T可将大分子贡献降至最低，无需采集单独的大分子光谱或大分子拟合技术，这些技术会增加额外的扫描时间或使估计的谷氨酸拟合产生偏差。
