Yuan Jing, Zhao Feng, Chan Queenie, Wang Yi-Xiang J
Department of Imaging and Interventional Radiology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
Acta Radiol. 2012 Jul;53(6):675-81. doi: 10.1258/ar.2012.120108. Epub 2012 Jul 3.
Spin-lattice relaxation in the rotating frame, or T(1ρ) relaxation, is normally described by a mono-exponential decay model. However, compartmentation of tissues and microscopic molecular exchange may lead to bi-exponential or multi-exponential T(1ρ) relaxation behavior in certain tissues under the application of spin lock pulse field strength.
To investigate the presence of bi-exponential T(1ρ) relaxations in in-vivo rat head tissues of brain and muscle.
Five Sprague-Dawley rats underwent T(1ρ) imaging at 3T. A B(1)-insensitive rotary echo spin lock pulse was used for T(1ρ) preparation with a spin lock frequency of 500Hz. Twenty-five T(1ρ)-weighted images with spin lock times ranging from 1 to 60 ms were acquired using a 3D spoiled gradient echo sequence. Image intensities over different spin lock times were fitted using mono-exponential as well as bi-exponential models both on region-of-interest basis and pixel-by-pixel basis. F-test with a significance level P value of 0.01 was used to evaluate whether bi-exponential model gave a better fitting than mono-exponential model.
In rat brains, only mono-exponential but no apparent bi-exponential T(1ρ) relaxation (70-71 ms) was found. In contrast, bi-exponential T(1ρ) relaxation was observed in muscles of all five rats (P < 10(-4)). A longer and a shorter T(1ρ) relaxation component were extracted to be ~37- ~41 ms (a fraction of ~80- ~88%) and ~9- ~11 ms (12-20%), compared to the normal single T(1ρ) relaxation of ~30- ~33 ms.
Bi-exponential relaxation components were detected in rat muscles. The long and the short T(1ρ) relaxation component are thought to correspond to the restricted intracellular water population and the hydrogen exchange between amine and hydroxyl groups, respectively.
旋转坐标系中的自旋-晶格弛豫，即T(1ρ)弛豫，通常用单指数衰减模型来描述。然而，组织的分隔和微观分子交换可能会在自旋锁定脉冲场强作用下，导致某些组织中出现双指数或多指数T(1ρ)弛豫行为。
研究大鼠脑和肌肉等活体头部组织中双指数T(1ρ)弛豫的存在情况。
对5只Sprague-Dawley大鼠在3T场强下进行T(1ρ)成像。使用对B(1)不敏感的旋转回波自旋锁定脉冲进行T(1ρ)准备，自旋锁定频率为500Hz。采用三维扰相梯度回波序列采集25幅自旋锁定时间范围为1至60ms的T(1ρ)加权图像。在感兴趣区域和逐像素基础上，分别使用单指数和双指数模型对不同自旋锁定时间下的图像强度进行拟合。使用显著性水平P值为0.01的F检验来评估双指数模型是否比单指数模型拟合得更好。
在大鼠脑中，仅发现单指数T(1ρ)弛豫（~70 - 71ms），未发现明显的双指数T(1ρ)弛豫。相反，在所有5只大鼠的肌肉中均观察到双指数T(1ρ)弛豫（P < 10(-4)）。与正常的约30 - 33ms的单一T(1ρ)弛豫相比，提取出较长和较短的T(1ρ)弛豫成分分别约为37 - 41ms（占比约80 - 88%）和约9 - 11ms（占比12 - 20%）。
在大鼠肌肉中检测到双指数弛豫成分。较长和较短的T(1ρ)弛豫成分分别被认为对应于受限的细胞内水群体以及胺基和羟基之间的氢交换。
