QUANTITATION OF DIFFUSION EFFECTS IN MR IMAGING OF BRAIN

本研究旨在更好地理解扩散所提供的信息 组织中的加权磁共振成像（MRI）。作为广泛使用的 随着弥散加权成像（DWI）技术的发展， 更深入地了解影响扩散的因素， 组织中我们将讨论的具体问题是：（l）数量 确定水的变化幅度和时间进程 扩散系数发生在病理事件， 中风和癫痫发作，使用我们开发的动物模型。扩散- 将获得时间分辨率为 秒，以及在中风和癫痫发作的急性阶段;（2） 对不同机制的定量评价 表观水扩散系数（ADC）的改变，包括 病理变化过程中水扩散受限的变化， 胞质流动运动和局部磁场的变化 氧合血红蛋白引起的磁化率差异引起的梯度/ 脱氧血红蛋白转化我们将使用专门设计的实验 在简单的幻影，灌注细胞，新鲜切除的组织，和动物 模型来解决这些问题;（3）开发一个水扩散模型 在异质系统如组织中， 以及对每一种影响 水扩散我们对ADC值降低的初步观察 在扣押期间，强调需要定量验证 已经提出的关于局部缺血中ADC降低的假设 其他研究人员。因为缺血和癫痫是两种 不同的生物条件（血流、氧合和能量 现状等），两种模型的密切比较和定量研究 应有助于更好地了解 两者中的ADC变化。我们将使用核磁共振光谱和成像方法 基于弛豫和扩散测量来量化水传输 在扩散屏障之间和穿过细胞膜或毛细血管壁， 有限磁导率我们将使用数值分析和计算机 模拟以量化不同形状的屏障之间的扩散， 尺寸和不同的边界条件。我们将使用NMR q空间 Callaghan（1991）提出的研究微观结构和 这是一种超越传统MRI分辨率的动态成像。q空间成像是 基于脉冲梯度自旋回波（PGSE）方法， Stejeskal和坦纳（1965），它可以用来表征水 位移曲线，如果分析得当， 隔室尺寸的自相关函数以及相对 不同扩散隔室的数量和尺寸。这些新方法 在提供关于传播的新见解方面， 异质分隔系统，如组织，但迄今为止， 主要局限于无生命的样本我们将执行 实验对我们的2 T和7 T扫描仪，这两个都配备了高 强度，屏蔽磁场梯度。的进一步意义 这项工作的目的是评估q空间成像的价值 生物样品技术。

This study aims to better understand the information provided by diffusion weighted magnetic resonance imaging (MRI) in tissues. As extensive uses of diffusion-weighted imaging (DWI) techniques evolve, it is essential to develop a greater understanding of the factors that affect diffusion in tissues. The specific issues we will address are: (l) Quantitative determination of the magnitude and time course of changes in water diffusion coefficient that happen during pathological occurrences such as stroke and seizure, using animal models we have developed. Diffusion- weighted imaging will be obtained with time resolution on order of seconds, and during the acute stages of stroke and seizure; (2) Quantitative evaluation of different mechanisms responsible for the alteration of apparent water diffusion coefficient (ADC), including changes in restriction of water diffusion during pathological changes, cytosolic streaming motion, and variations of local magnetic field gradient due to susceptibility difference caused by oxyhemoglobin/ deoxyhemoglobin conversion. We will use specifically designed experiments in simple phantoms, perfused cells, freshly excised tissues, and animal models to address each of these; (3) Develop a model for water diffusion in heterogeneous systems such as tissues based on a clear understanding and quantitative evaluation of each individual mechanism that affects water diffusion. Our own preliminary observation of reduction in ADC during seizure has highlighted the need to quantitatively validate the hypotheses concerning ADC reduction in ischemia that have been suggested by other researchers. Since ischemia and seizure represent two quite different biological conditions (blood flow, oxygenation, and energy status, etc), close comparison of the two models and quantitative studies of individual mechanisms should facilitate improved understanding of the ADC changes in both. We will use NMR spectroscopic and imaging methods based on relaxation and diffusion measurements to quantify water transport among diffusion barriers and across cell membranes or capillary walls of finite permeability. We will use numerical analysis and computer simulations to quantify diffusion among barriers of different shapes, sizes, and different boundary conditions. We will use the NMR q-space concepts developed by Callaghan (1991) to study microstructure and dynamics beyond the resolution of conventional MRI. The q-space imaging is based on the pulsed gradient spin-echo (PGSE) method first developed by Stejeskal and Tanner (1965), and it can be used to characterize water displacement profiles which reflect, if analyzed appropriately, the autocorrelation function of compartment dimensions as well as the relative number and sizes of differently diffusing compartments. These new methods are potentially very powerful at providing new insights into diffusion in heterogeneous compartmented systems such as tissue, but to date their use has been restricted largely to inanimate samples. We will perform experiments on our 2T and 7T scanners both of which are equipped with high strength, shielded magnetic field gradients. A further significance of this work is that it would evaluate the value of the q-space imaging technique for biological samples.