Norris D G
Max-Planck-Institute of Cognitive Neuroscience, Leipzig, Germany.
NMR Biomed. 2001 Apr;14(2):77-93. doi: 10.1002/nbm.682.
This review examines the way in which microscopic tissue parameters can affect MR experiments which are sensitive to diffusion. The interaction between the intra- and extravascular as well as that between the intra- and extracellular spaces is examined. Susceptibility gradients due to the presence of deoxyhemoglobin can cause diffusion-induced signal losses which are significant in functional magnetic resonance experiments, particularly at higher main magnetic field strengths. This is also true of the fast response that manifests itself as an early negative signal change in functional magnetic resonance experiments. The fields surrounding paramagnetic vessels are described and the way in which diffusion in these fields contributes to functional signal changes is examined. Flow in the capillary bed can be a confounding factor in experiments which aim to examine the diffusion characteristics of extravascular water. It is potentially also a method for assessing capillary perfusion. The intravoxel incoherent motion experiment is described in terms of how significantly this effect can influence diffusion attenuation curves from water. The major models for describing water diffusion in tissue are presented, as are the main experimental results that have contributed to an understanding of the mechanisms of diffusion contrast. The widely accepted view that changes in the diffusion characteristics are caused by a shift of water to the intracellular space and a concomitant change in extracellular tortuosity is examined critically. More recent experiments that indicate that a reduction in the intracellular diffusion may occur simultaneously with the cell swelling are described and their compatibility with existing models discussed.
本综述探讨了微观组织参数如何影响对扩散敏感的磁共振实验。研究了血管内与血管外以及细胞内与细胞外空间之间的相互作用。脱氧血红蛋白的存在所导致的磁化率梯度会引起扩散诱导的信号损失，这在功能磁共振实验中很显著，尤其是在较高的主磁场强度下。功能磁共振实验中表现为早期负信号变化的快速响应也是如此。描述了顺磁性血管周围的场，并研究了这些场中的扩散对功能信号变化的贡献方式。在旨在研究血管外水扩散特性的实验中，毛细血管床中的血流可能是一个混杂因素。它也可能是一种评估毛细血管灌注的方法。体素内不相干运动实验是根据这种效应能在多大程度上影响水的扩散衰减曲线来描述的。介绍了描述组织中水扩散的主要模型，以及有助于理解扩散对比机制的主要实验结果。对广泛接受的观点进行了批判性审视，该观点认为扩散特性的变化是由水向细胞内空间的转移以及细胞外曲折度的相应变化引起的。描述了表明细胞内扩散减少可能与细胞肿胀同时发生的最新实验，并讨论了它们与现有模型的兼容性。
