Wilkinson Molly, Lim Ashley R, Cohen Andrew H, Galaburda Albert M, Takahashi Emi
Department of Behavioral Neuroscience, Northeastern UniversityBoston, MA, United States.
Division of Newborn Medicine, Department of Medicine, Boston Children's Hospital, Harvard Medical SchoolBoston, MA, United States.
Front Neurosci. 2017 Jul 14;11:389. doi: 10.3389/fnins.2017.00389. eCollection 2017.
Fractional anisotropy (FA) threshold is commonly used to perform diffusion MRI tractography. However, FA threshold may be one aspect of tractography that needs additional scrutiny in accurately assessing pathways in immature, developing brains, as well as in adult brains. Using high-angular resolution diffusion MRI (HARDI) tractography without an FA threshold, we identified the arcuate fasciculus (AF) of 83 healthy subjects ranging in age from 40 gestational weeks (GW) (newborns) to 28-year-old adults. The AF was identified in both hemispheres in all subjects with high inter-rater reliability. The detected AF included regions with very low FA values. The entire AF was segmented into anterior, posterior, and long tracts. Growth and laterality patterns were investigated using tract count (number of detected streamlines), total volume of imaging voxels (touched by the detected streamlines), mean length, mean FA, and mean apparent diffusion coefficient (ADC). Comparison of subjects under 3 years old, to those that were older, revealed the three AF tracts that took different developmental courses. As expected, the anterior and long tracts showed lower ADC values in subjects over 3 years old, while the posterior tract showed higher ADC in that same age range. The posterior tract did not show age-related effect in terms of FA, tract count, length, and volume. These results suggest that the posterior AF tract shows a matured state, indexed by most of the used measurements in early postnatal developmental ages, and ADC is a measurement that can detect further maturation of the posterior tract. Interestingly, in all tracts, hemispheric asymmetries were found in raw (left<right), and in whole brain (WB)-normalized (left>right) tract count, as well as in raw volume (left<right). In raw, and in WB-normalized length, as well as in WB-normalized volume, rightward asymmetry (left<right) was found only in the anterior tract; other tracts were not significantly affected by hemisphere. Although many previous studies have observed a leftward asymmetry in the AF, rightward asymmetry has also been reported in other studies, and together with the present report, the results in the literature are likely to reflect differences in the methods used.
分数各向异性（FA）阈值常用于进行扩散磁共振成像纤维束成像。然而，在准确评估未成熟发育大脑以及成人大脑中的神经通路时，FA阈值可能是纤维束成像中需要额外审视的一个方面。我们使用无FA阈值的高角分辨率扩散磁共振成像（HARDI）纤维束成像技术，识别了83名年龄范围从40孕周（GW）（新生儿）到28岁成年人的健康受试者的弓状束（AF）。在所有受试者的两个半球中均识别出了AF，且评分者间可靠性较高。检测到的AF包括FA值非常低的区域。整个AF被分为前束、后束和长束。使用纤维束计数（检测到的流线数量）、成像体素总体积（被检测到的流线触及）、平均长度、平均FA和平均表观扩散系数（ADC）来研究生长和偏侧性模式。将3岁以下的受试者与年龄较大的受试者进行比较，发现三条AF束呈现出不同的发育过程。正如预期的那样，3岁以上受试者的前束和长束显示出较低的ADC值，而后束在同一年龄范围内显示出较高的ADC值。后束在FA、纤维束计数、长度和体积方面未显示出与年龄相关的影响。这些结果表明，后AF束呈现出成熟状态，在出生后早期发育阶段，大多数所使用的测量指标都可作为其指标，并且ADC是一种能够检测后束进一步成熟的测量方法。有趣的是，在所有束中，原始数据（左<右）、全脑（WB）标准化纤维束计数（左>右）以及原始体积（左<右）中均发现了半球不对称性。在原始数据和WB标准化长度以及WB标准化体积中，仅在前束中发现了向右的不对称性（左<右）；其他束未受到半球的显著影响。尽管之前许多研究都观察到AF存在向左的不对称性，但其他研究也报道了向右的不对称性，结合本报告来看，文献中的结果可能反映了所使用方法的差异。
