Physics Department, Sapienza University of Rome, P.le Aldo Moro, 5, 00185 Rome, Italy.
J Magn Reson. 2012 Mar;216:28-36. doi: 10.1016/j.jmr.2011.12.023. Epub 2012 Jan 9.
In this paper, we investigate the image contrast that characterizes anomalous and non-gaussian diffusion images obtained using the stretched exponential model. This model is based on the introduction of the γ stretched parameter, which quantifies deviation from the mono-exponential decay of diffusion signal as a function of the b-value. To date, the biophysical substrate underpinning the contrast observed in γ maps, in other words, the biophysical interpretation of the γ parameter (or the fractional order derivative in space, β parameter) is still not fully understood, although it has already been applied to investigate both animal models and human brain. Due to the ability of γ maps to reflect additional microstructural information which cannot be obtained using diffusion procedures based on gaussian diffusion, some authors propose this parameter as a measure of diffusion heterogeneity or water compartmentalization in biological tissues. Based on our recent work we suggest here that the coupling between internal and diffusion gradients provide pseudo-superdiffusion effects which are quantified by the stretching exponential parameter γ. This means that the image contrast of Mγ maps reflects local magnetic susceptibility differences (Δχ(m)), thus highlighting better than T(2)(∗) contrast the interface between compartments characterized by Δχ(m). Thanks to this characteristic, Mγ imaging may represent an interesting tool to develop contrast-enhanced MRI for molecular imaging. The spectroscopic and imaging experiments (performed in controlled micro-beads dispersion) that are reported here, strongly suggest internal gradients, and as a consequence Δχ(m), to be an important factor in fully understanding the source of contrast in anomalous diffusion methods that are based on a stretched exponential model analysis of diffusion data obtained at varying gradient strengths g.
在本文中，我们研究了使用扩展指数模型获得的异常和非高斯扩散图像的图像对比度。该模型基于引入γ扩展参数，该参数量化了扩散信号随 b 值变化的单指数衰减的偏离。迄今为止，γ图中观察到的对比度的生物物理基础，换句话说，γ参数（或空间中的分数阶导数，β参数）的生物物理解释尚不完全清楚，尽管它已经被应用于研究动物模型和人类大脑。由于γ图能够反映使用基于高斯扩散的扩散过程无法获得的附加微观结构信息，一些作者将该参数作为生物组织中扩散异质性或水隔室化的度量。基于我们最近的工作，我们在这里提出，内部梯度和扩散梯度之间的耦合提供了伪超扩散效应，该效应由扩展指数参数γ来量化。这意味着 Mγ 图的图像对比度反映了局部磁化率差异（Δχ(m)），因此比 T(2)(∗)对比度更好地突出了由 Δχ(m) 表征的隔室之间的界面。由于这一特性，Mγ 成像可能代表一种用于发展分子成像的对比增强 MRI 的有趣工具。本文报道的光谱和成像实验（在受控的微珠分散体中进行）强烈表明，内部梯度，因此是 Δχ(m)，是充分理解基于扩展指数模型分析在不同梯度强度 g 下获得的扩散数据的异常扩散方法对比度来源的重要因素。
