MacKay Alex, Laule Cornelia, Vavasour Irene, Bjarnason Thorarin, Kolind Shannon, Mädler Burkhard
Department of Radiology, University of British Columbia, Vancouver, BC, Canada.
Magn Reson Imaging. 2006 May;24(4):515-25. doi: 10.1016/j.mri.2005.12.037. Epub 2006 Mar 20.
T2 weighting is particularly sensitive, but notoriously unspecific, to a wide range of brain pathologies. However, careful measurement and analysis of the T2 decay curve from brain tissue promise to provide much improved pathological specificity. In vivo T2 measurement requires accurate 180 pulses and appropriate manipulation of stimulated echoes; the most common approach is to acquire multiple echoes from a single slice. The T2 distribution, a plot of component amplitude as a function of T2, can be estimated using an algorithm capable of fitting a multi-exponential T2 decay with no a priori assumptions about the number of exponential components. T2 distributions from normal brain show peaks from myelin water, intra/extracellular water and cerebral spinal fluid; they can be used to provide estimates of total water content (total area under the T2 distribution) and myelin water fraction (MWF, fractional area under the myelin water peak), a measure believed to be related to myelin content. Experiments on bovine brain suggest that magnetization exchange between water pools plays a minor role in the T2 distribution. Different white matter structures have different MWFs. In normal white matter (NWM), MWF is not correlated with the magnetization transfer ratio (MTR) or the diffusion tensor fractional anisotropy (FA); hence it provides unique information about brain microstructure. Normal-appearing white matter (NAWM) in multiple sclerosis (MS) brain possesses a higher water content and lower MWF than controls, consistent with histopathological findings. Multiple sclerosis lesions demonstrate great heterogeneity in MWF, presumably due to varying myelin contents of these focal regions of pathology. Subjects with schizophrenia were found to have significantly reduced MWF in the minor forceps and genu of the corpus callosum when compared to controls, suggesting that reduced frontal lobe myelination plays a role in schizophrenia. In normal controls, frontal lobe myelination was positively correlated with both age and education; this result was not observed in subjects with schizophrenia. A strong correlation between MWF and the optical density from the luxol fast blue histological stain for myelin was observed in formalin-fixed brain, supporting the use of the MWF as an in vivo myelin marker.
T2加权对多种脑病变特别敏感，但特异性很差。然而，仔细测量和分析脑组织的T2衰减曲线有望显著提高病理特异性。体内T2测量需要精确的180°脉冲和对受激回波的适当操控；最常用的方法是从单个层面采集多个回波。T2分布是成分幅度作为T2的函数的曲线图，可以使用一种能够拟合多指数T2衰减的算法来估计，且无需对指数成分的数量做先验假设。正常脑的T2分布显示出髓鞘水、细胞内/外水和脑脊液的峰值；它们可用于估计总含水量（T2分布下的总面积）和髓鞘水分数（MWF，髓鞘水峰值下的分数面积），这一指标被认为与髓鞘含量有关。对牛脑的实验表明，水池间的磁化交换在T2分布中起的作用较小。不同的白质结构具有不同的MWF。在正常白质（NWM）中，MWF与磁化传递率（MTR）或扩散张量分数各向异性（FA）无关；因此它提供了有关脑微观结构的独特信息。多发性硬化症（MS）脑的正常外观白质（NAWM）与对照组相比，含水量更高，MWF更低，这与组织病理学结果一致。多发性硬化症病变在MWF上表现出很大的异质性，可能是由于这些局部病变区域的髓鞘含量不同。与对照组相比，发现精神分裂症患者胼胝体小钳和膝部的MWF显著降低，这表明额叶髓鞘形成减少在精神分裂症中起作用。在正常对照组中，额叶髓鞘形成与年龄和教育程度均呈正相关；在精神分裂症患者中未观察到这一结果。在福尔马林固定的脑中观察到MWF与髓鞘的卢氏固蓝组织学染色的光密度之间有很强的相关性，支持将MWF用作体内髓鞘标记物。
