Laboratory for Imaging Science and Technology, Department of Electrical and Computer Engineering, Seoul National University, Seoul, South Korea.
Department of Biomedical Engineering, Hankuk University of Foreign Studies, Yongin, South Korea.
Neuroimage. 2019 Mar;188:835-844. doi: 10.1016/j.neuroimage.2018.11.040. Epub 2018 Nov 23.
Gradient echo myelin water imaging (GRE-MWI) is an MRI technique to measure myelin concentration and involves the analysis of signal decay characteristics of the multi-echo gradient echo data. The method provides a myelin water fraction as a quantitative biomarker for myelin. In this work, a new sequence and post-processing methods were proposed to generate high quality GRE-MWI images at 3T and 7T. In order to capture the rapidly decaying myelin water signals, a bipolar readout GRE sequence was designed with "gradient pairing," compensating for the eddy current effects. The flip angle dependency from the multi-compartmental T effects was explored and avoided using a 2D multi-slice acquisition with a long TR. Additionally, the sequence was tested for the effects of inflow and magnetization transfer and demonstrated robustness to these error sources. Lastly, the temporal and spatial B inhomogeneity effects were mitigated by using the B navigator and field inhomogeneity corrections. Using the method, high-quality myelin water images were successfully generated for the in-vivo human brain at both field strengths. When the myelin water fraction at 3T and 7T were compared, they showed a good correlation (R≥ 0.88; p < 0.001) with a larger myelin water fraction at 7T. The proposed method also opens the possibility of high resolution (isotropic 1.5 mm resolution) myelin water mapping at 7T.
梯度回波髓鞘水成像（GRE-MWI）是一种测量髓鞘浓度的 MRI 技术，涉及多回波梯度回波数据信号衰减特征的分析。该方法提供了髓鞘水分数作为髓鞘的定量生物标志物。在这项工作中，提出了一种新的序列和后处理方法，以在 3T 和 7T 上生成高质量的 GRE-MWI 图像。为了捕获迅速衰减的髓鞘水信号，设计了具有“梯度配对”的双极读出 GRE 序列，以补偿涡流效应。通过使用具有长 TR 的 2D 多切片采集来探索和避免来自多隔室 T 效应的翻转角依赖性。此外，还测试了序列对流入和磁化转移的影响，并证明了对这些误差源的稳健性。最后，通过使用 B 导航仪和场不均匀性校正来减轻时间和空间 B 不均匀性的影响。使用该方法，在两种场强下均成功生成了体内人脑的高质量髓鞘水图像。当比较 3T 和 7T 的髓鞘水分数时，它们显示出很好的相关性（R≥0.88;p<0.001），7T 的髓鞘水分数较大。该方法还为 7T 上的高分辨率（各向同性 1.5mm 分辨率）髓鞘水映射开辟了可能性。
