Ferris Jennifer K, Greeley Brian, Vavasour Irene M, Kraeutner Sarah N, Rinat Shie, Ramirez Joel, Black Sandra E, Boyd Lara A
Graduate Programs in Rehabilitation Sciences, University of British Columbia, Vancouver, Canada.
Department of Physical Therapy, University of British Columbia, Vancouver, Canada.
Brain Commun. 2022 Jun 6;4(3):fcac142. doi: 10.1093/braincomms/fcac142. eCollection 2022.
White matter hyperintensities negatively impact white matter structure and relate to cognitive decline in aging. Diffusion tensor imaging detects changes to white matter microstructure, both within the white matter hyperintensity and extending into surrounding (perilesional) normal-appearing white matter. However, diffusion tensor imaging markers are not specific to tissue components, complicating the interpretation of previous microstructural findings. Myelin water imaging is a novel imaging technique that provides specific markers of myelin content (myelin water fraction) and interstitial fluid (geometric mean T). Here we combined diffusion tensor imaging and myelin water imaging to examine tissue characteristics in white matter hyperintensities and perilesional white matter in 80 individuals (47 older adults and 33 individuals with chronic stroke). To measure perilesional normal-appearing white matter, white matter hyperintensity masks were dilated in 2 mm segments up to 10 mm in distance from the white matter hyperintensity. Fractional anisotropy, mean diffusivity, myelin water fraction, and geometric mean T were extracted from white matter hyperintensities and perilesional white matter. We observed a spatial gradient of higher mean diffusivity and geometric mean T, and lower fractional anisotropy, in the white matter hyperintensity and perilesional white matter. In the chronic stroke group, myelin water fraction was reduced in the white matter hyperintensity but did not show a spatial gradient in perilesional white matter. Across the entire sample, white matter metrics within the white matter hyperintensity related to whole-brain white matter hyperintensity volume; with increasing white matter hyperintensity volume there was increased mean diffusivity and geometric mean T, and decreased myelin water fraction in the white matter hyperintensity. Normal-appearing white matter adjacent to white matter hyperintensities exhibits characteristics of a transitional stage between healthy white matter and white matter hyperintensities. This effect was observed in markers sensitive to interstitial fluid, but not in myelin water fraction, the specific marker of myelin concentration. Within the white matter hyperintensity, interstitial fluid was higher and myelin concentration was lower in individuals with more severe cerebrovascular disease. Our data suggests white matter hyperintensities have penumbra-like effects in perilesional white matter that specifically reflect increased interstitial fluid, with no changes to myelin concentration. In contrast, within the white matter hyperintensity there are varying levels of demyelination, which vary based on the severity of cerebrovascular disease. Diffusion tensor imaging and myelin imaging may be useful clinical markers to predict white matter hyperintensity formation, and to stage neuronal damage within white matter hyperintensities.
白质高信号对白质结构产生负面影响，并与衰老过程中的认知衰退相关。扩散张量成像可检测白质高信号内部以及延伸至周围（病灶周围）看似正常的白质中的白质微观结构变化。然而，扩散张量成像标记并非组织成分特异性的，这使得以往微观结构研究结果的解读变得复杂。髓磷脂水成像（Myelin water imaging）是一种新型成像技术，可提供髓磷脂含量（髓磷脂水分数）和细胞间液（几何平均T）的特异性标记。在此，我们将扩散张量成像与髓磷脂水成像相结合，以研究80名个体（47名老年人和33名慢性卒中患者）白质高信号及病灶周围白质的组织特征。为测量病灶周围看似正常的白质，将白质高信号掩膜以2毫米的段进行扩展，直至距离白质高信号10毫米处。从白质高信号和病灶周围白质中提取分数各向异性、平均扩散率、髓磷脂水分数和几何平均T。我们观察到在白质高信号和病灶周围白质中，平均扩散率和几何平均T较高，分数各向异性较低的空间梯度。在慢性卒中组中，白质高信号中的髓磷脂水分数降低，但病灶周围白质中未显示出空间梯度。在整个样本中，白质高信号内的白质指标与全脑白质高信号体积相关；随着白质高信号体积增加，白质高信号中的平均扩散率和几何平均T升高，髓磷脂水分数降低。与白质高信号相邻的看似正常的白质表现出健康白质与白质高信号之间过渡阶段的特征。这种效应在对细胞间液敏感的标记中观察到，但在髓磷脂浓度的特异性标记髓磷脂水分数中未观察到。在白质高信号内，脑血管疾病更严重的个体细胞间液更高，髓磷脂浓度更低。我们的数据表明，白质高信号在病灶周围白质中具有类似半暗带的效应，具体表现为细胞间液增加，而髓磷脂浓度无变化。相比之下，在白质高信号内存在不同程度的脱髓鞘，其程度因脑血管疾病的严重程度而异。扩散张量成像和髓磷脂成像可能是预测白质高信号形成以及对白质高信号内神经元损伤进行分期的有用临床标记。
