Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA.
Magn Reson Med. 2012 Jan;67(1):110-7. doi: 10.1002/mrm.22990. Epub 2011 May 31.
Visualizing myelin in human brain may help the study of diseases such as multiple sclerosis. Previous studies based on T(1) and T(2) relaxation contrast have suggested the presence of a distinct water pool that may report directly on local myelin content. Recent work indicates that T(2) contrast may offer particular advantages over T(1) and T(2) contrast, especially at high field. However, the complex mechanism underlying T(2) relaxation may render interpretation difficult. To address this issue, T(2) relaxation behavior in human brain was studied at 3 and 7 T. Multiple gradient echoes covering most of the decay curve were analyzed for deviations from mono-exponential behavior. The data confirm the previous finding of a distinct rapidly relaxing signal component (T(2) ∼ 6 ms), tentatively attributed to myelin water. However, in extension to previous findings, this rapidly relaxing component displayed a substantial resonance frequency shift, reaching 36 Hz in the corpus callosum at 7 T. The component's fractional amplitude and frequency shift appeared to depend on both field strength and fiber orientation, consistent with a mechanism originating from magnetic susceptibility effects. The findings suggest that T(2) contrast at high field may be uniquely sensitive to tissue myelin content and that proper interpretation will require modeling of susceptibility-induced resonance frequency shifts.
在人脑组织中可视化髓鞘可能有助于多发性硬化症等疾病的研究。先前基于 T(1) 和 T(2) 弛豫对比的研究表明存在一个独特的水池，该水池可能直接反映局部髓鞘含量。最近的研究表明，T(2)对比可能比 T(1)和 T(2)对比具有特别的优势，尤其是在高场强下。然而，T(2)弛豫背后的复杂机制可能使得其解释变得困难。为了解决这个问题，在 3 和 7 T 下研究了人脑的 T(2)弛豫行为。分析了覆盖大部分衰减曲线的多个梯度回波，以研究其是否偏离单指数行为。这些数据证实了先前发现的一个明显的快速弛豫信号分量（T(2)∼6 ms），该信号分量可能归因于髓鞘水。然而，与先前的发现相比，这个快速弛豫分量显示出显著的共振频率偏移，在 7 T 时胼胝体中的频率达到 36 Hz。该分量的分数幅度和频率偏移似乎既依赖于场强又依赖于纤维方向，这与起源于磁导率效应的机制一致。这些发现表明，高场下的 T(2)对比可能对组织髓鞘含量具有独特的敏感性，并且需要对诱导的共振频率偏移进行建模才能进行正确的解释。
