Diffusion generated T1 and T2 contrast
Diffusion generated T1 and T2 contrast
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DOI：
10.1016/j.jmr.2008.02.008
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发表时间：
2008-05-01
影响因子：
2.2
通讯作者：
Jakob, P.
中科院分区：
文献类型：
--
作者：
Kaufmann, I.;Seiberlich, N.;Jakob, P.
在水介质中的多孔有机样品（例如根或木材）的 MR 图像中，样品通常被明亮的环包围，T-1 图中的 T-1 值相应降低。当去除介质或添加造影剂时，环消失，表明信号并非源自样品的外层，而是源自介质本身。可以证明，只有当介质渗透样品时 T-1 减少时，才会观察到这种“亮环效应”。为了研究这种效应，使用计算机模型来模拟表现出不同弛豫常数的区域之间的磁化强度扩散。使用该模型，发现信号增加的根源是流入效应，因为扩散将松弛的磁化强度从样品的边界区域传输到周围介质中。在上述植物周围的“亮环”的情况下，样品内的短 T-1 值和介质内的长 T-1 值混合，产生两个值之间的“过渡区域”。与来自该过渡区域之外的介质的信号相比，可以在 T-1 加权图像中观察到信号增加。过渡区域的宽度与根据T-1值作为扩散时间计算出的扩散位移的数量级相同。除了在植物样品周围造成亮环之外，这种扩散效应还限制了弛豫时间图的分辨率。这种效应不仅限于T-1弛豫，也适用于T-2弛豫。然而，在高 B-0 场强（例如本研究中使用的场强 (11.7 T)）下，通常观察不到 T-2 效应，因为植物中的 T-2 时间相当短（大约 50 ms，而 T-1 时间高于 1 秒）。由于 T-2 弛豫过程中的扩散长度相对于成像实验的分辨率来说很短，因此没有看到 T-2 环效应。 (C) 2008 Elsevier Inc. 保留所有权利。
In MR images of porous organic samples (such as roots or wood) in water media, the sample is often surrounded by a bright ring, with a corresponding decreased T-1 value in T-1 maps. When the medium is removed, or contrast agents are added, the ring disappears, indicating that the signal does not originate in the outer layers of the sample, but from the medium itself. It can be shown that this "bright ring effect" is only observed when the medium experiences a reduction in T-1 when permeating the sample. In order to investigate this effect, a computer model was used to simulate the diffusion of magnetisation between regions that exhibit different relaxation constants. Using this model, the origin of the signal increase was found to be an inflow effect, as diffusion transports relaxed magnetisation from the boundary regions of the sample into the surrounding medium. In the case of the "bright ring" around the plants described above, a mixing of short T-1 values from within the sample and long T-1 values within the medium occurs, yielding a "transition region" between the two values. There, a signal increase can be observed at T-1 weighted images, compared to the signal from the medium beyond this transition region. The width of the transition region is on the order of magnitude of the diffusion displacement that is calculated from the T-1 value as diffusion time. In addition to causing the bright ring around the plant samples, this diffusion effect also limits the resolution of the relaxation time maps.This effect is not limited to T-1 relaxation but also applies to T-2 relaxation. However, at high B-0 field strengths such as those used in this study (11.7 T), a T-2 effect is not usually observed due to the considerably shorter T-2 times in plants (about 50 ms, compared to T-1 times of higher than 1s). Because the diffusion length during this T-2 relaxation is short with respect to the resolution of the imaging experiments, no T-2 ring effect is seen. (C) 2008 Elsevier Inc. All rights reserved.