QUANTITATION OF DIFFUSION EFFECTS IN MR IMAGING OF BRAIN

这项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 扩散加权成像(DWI)技术的发展，对于 更好地了解影响扩散的因素 纸巾。我们要解决的具体问题是：(L)量化 水变化的大小和时间进程的测定 在病理发生期间发生的扩散系数，例如 中风和癫痫，使用我们开发的动物模型。扩散- 将获得时间分辨率为数量级的加权成像 在中风和癫痫的急性期；(2) 对不同机制的量化评估 改变表观水扩散系数(ADC)，包括 病变过程中水扩散受限的变化， 胞液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/ 脱氧血红蛋白转化率。我们将使用专门设计的实验 在简单的幻象、灌流细胞、新鲜切除的组织和动物中 针对这些问题建立模型；(3)建立水扩散模型 在对组织等非均质系统有清晰认识的基础上 以及对影响的每个个体机制的定量评估 水扩散。我们自己对ADC降低的初步观察 在癫痫发作期间强调了定量验证 已经提出的关于缺血时ADC值降低的假设 由其他研究人员提供。因为缺血和癫痫是两个相当重要的 不同的生物条件(血流、氧合和能量 地位等)，两种模型的密切比较和定量研究 对个别机制的了解应有助于增进对 ADC在这两个方面都发生了变化。我们将使用核磁共振波谱和成像方法 基于松弛和扩散测量来量化水的传输 在扩散屏障之间以及穿过细胞膜或毛细管壁 有限渗透率。我们将使用数值分析和计算机 模拟以量化不同形状的势垒之间的扩散， 大小和不同的边界条件。我们将使用核磁共振Q空间 由Callaghan(1991)提出的用于研究微结构和 超出常规核磁共振分辨率的动力学。Q空间成像是 基于脉冲梯度自旋回波(PGSE)方法，该方法由 Stejeskal和Tanner(1965)，它可以用来表征水 位移分布，如果分析得当，它们反映了 舱室尺寸的自相关函数以及相关系数 不同扩散的隔室的数量和大小。这些新方法 在提供有关扩散的新见解方面可能非常强大 不同的隔室系统，如组织，但到目前为止它们的使用 在很大程度上仅限于无生命样本。我们将表演 在我们的2T和7T扫描仪上进行实验，这两台扫描仪都配备了HIGH 强度，屏蔽磁场梯度。的另一个重要意义 这项工作是为了评估Q空间成像的价值 生物样品的技术。

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.