Uddin Md Nasir, Figley Teresa D, Figley Chase R
Department of Radiology, University of Manitoba, Winnipeg, MB, Canada; Division of Diagnostic Imaging, Health Sciences Centre, Winnipeg, MB, Canada; Neuroscience Research Program, Kleysen Institute for Advanced Medicine, Winnipeg, MB, Canada.
Department of Radiology, University of Manitoba, Winnipeg, MB, Canada; Division of Diagnostic Imaging, Health Sciences Centre, Winnipeg, MB, Canada; Neuroscience Research Program, Kleysen Institute for Advanced Medicine, Winnipeg, MB, Canada; Department of Physiology and Pathophysiology, University of Manitoba, Winnipeg, MB, Canada; Biomedical Engineering Graduate Program, University of Manitoba, Winnipeg, MB, Canada; Department of Psychological and Brain Sciences, Johns Hopkins University, Baltimore, MD, USA.
Magn Reson Imaging. 2018 Sep;51:35-43. doi: 10.1016/j.mri.2018.04.012. Epub 2018 Apr 20.
Tissue contrast can be enhanced by dividing T-weighted (Tw) images by T-weighted (Tw) images to map the so-called Tw/Tw ratio, which has become an increasingly popular technique for quantifying brain tissue changes associated with neurodevelopment, aging, and a variety of neurodegenerative diseases. However, although it is self-evident that Tw/Tw ratios increase with the amount of T-weighting in the Tw image - which is determined by the echo time (TE), all else being equal - longer TEs also reduce the signal-to-noise ratio (SNR) of the Tw images, and it is not clear how these SNR characteristics affect the reliability of Tw/Tw measurements. Therefore, the current study systematically investigated how different amounts of T-weighting affected Tw/Tw measurements in order to determine whether there is an optimal amount of T-weighting. Tw images were acquired from 10 neurologically healthy adults using a 3D turbo field echo (TFE) sequence, and a series of T-weighted images were extracted from a multi-echo 3D combined gradient- and spin-echo (GRASE) sequence. Analyses of 12 anatomically defined brain regions revealed that both the mean and standard deviation of the Tw/Tw measurements increased exponentially with TE of the Tw images, and that Tw images with TE ≈ 120-160 ms yielded the most consistent/reproducible contrast between white matter ROIs and the whole-brain Tw/Tw signal. Furthermore, comparisons between Tw/Tw measurements and multi-component T-relaxation myelin water fractions (MWFs) in the same brain regions revealed that Tw images with TE ≥ 160 ms drastically reduced the degree of correlation between Tw/Tw measurements and MWF. Overall, these findings suggest that: 1) there is a substantial trade-off between increased Tw/Tw contrast (based on longer TEs for the Tw images) and the reliability of quantitative Tw/Tw signals; and 2) the optimal TE for Tw GRASE scans is between 120 ms and 160 ms for calculating Tw/Tw ratios.
通过将T加权（Tw）图像除以T加权（Tw）图像来映射所谓的Tw/Tw比率，可以增强组织对比度，这已成为一种越来越流行的技术，用于量化与神经发育、衰老和各种神经退行性疾病相关的脑组织变化。然而，尽管很明显Tw/Tw比率会随着Tw图像中的T加权量增加而增加——这由回波时间（TE）决定，在其他条件相同的情况下——更长的TE也会降低Tw图像的信噪比（SNR），并且尚不清楚这些SNR特性如何影响Tw/Tw测量的可靠性。因此，当前研究系统地研究了不同量的T加权如何影响Tw/Tw测量，以确定是否存在最佳的T加权量。使用3D涡轮场回波（TFE）序列从10名神经系统健康的成年人中获取Tw图像，并从多回波3D组合梯度和自旋回波（GRASE）序列中提取一系列T加权图像。对12个解剖学定义的脑区的分析表明，Tw/Tw测量的平均值和标准差均随Tw图像的TE呈指数增加，并且TE≈120 - 160 ms的Tw图像在白质感兴趣区（ROI）和全脑Tw/Tw信号之间产生了最一致/可重复的对比度。此外，同一脑区Tw/Tw测量值与多组分T弛豫髓磷脂水分数（MWF）之间的比较表明，TE≥160 ms的Tw图像大大降低了Tw/Tw测量值与MWF之间的相关程度。总体而言，这些发现表明：1）Tw/Tw对比度增加（基于Tw图像更长的TE）与定量Tw/Tw信号的可靠性之间存在重大权衡；2）对于计算Tw/Tw比率，Tw GRASE扫描的最佳TE在120 ms至160 ms之间。
