Oh Se-Hong, Bilello Michel, Schindler Matthew, Markowitz Clyde E, Detre John A, Lee Jongho
Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA.
Neuroimage. 2013 Dec;83:485-92. doi: 10.1016/j.neuroimage.2013.06.047. Epub 2013 Jun 22.
White matter of the brain has been demonstrated to have multiple relaxation components. Among them, the short transverse relaxation time component (T2<40 ms; T2⁎<25 ms at 3 T) has been suggested to originate from myelin water whereas long transverse relaxation time components have been associated with axonal and/or interstitial water. In myelin water imaging, T2 or T2⁎ signal decay is measured to estimate myelin water fraction based on T2 or T2⁎ differences among the water components. This method has been demonstrated to be sensitive to demyelination in the brain but suffers from low SNR and image artifacts originating from ill-conditioned multi-exponential fitting. In this study, a novel approach that selectively acquires short transverse relaxation time signal is proposed. The method utilizes a double inversion RF pair to suppress a range of long T1 signal. This suppression leaves short T2⁎ signal, which has been suggested to have short T1, as the primary source of the image. The experimental results confirm that after suppression of long T1 signals, the image is dominated by short T2⁎ in the range of myelin water, allowing us to directly visualize the short transverse relaxation time component in the brain. Compared to conventional myelin water imaging, this new method of direct visualization of short relaxation time component (ViSTa) provides high quality images. When applied to multiple sclerosis patients, chronic lesions show significantly reduced signal intensity in ViSTa images suggesting sensitivity to demyelination.
大脑白质已被证明具有多种弛豫成分。其中，短横向弛豫时间成分（在3T时T2<40ms；T2⁎<25ms）被认为源自髓鞘水，而长横向弛豫时间成分则与轴突和/或间质水有关。在髓鞘水成像中，通过测量T2或T2⁎信号衰减，基于水成分之间的T2或T2⁎差异来估计髓鞘水分数。该方法已被证明对大脑脱髓鞘敏感，但存在信噪比低和源于病态多指数拟合的图像伪影问题。在本研究中，提出了一种选择性获取短横向弛豫时间信号的新方法。该方法利用双反转射频对来抑制一系列长T1信号。这种抑制使得短T2⁎信号（被认为具有短T1）成为图像的主要来源。实验结果证实，在抑制长T1信号后，图像在髓鞘水范围内以短T2⁎为主，使我们能够直接可视化大脑中的短横向弛豫时间成分。与传统的髓鞘水成像相比，这种直接可视化短弛豫时间成分的新方法（ViSTa）提供了高质量的图像。当应用于多发性硬化症患者时，慢性病变在ViSTa图像中显示出明显降低的信号强度，表明对脱髓鞘敏感。
