Biomedical Engineering Program, University of Arizona, Tucson, Arizona 85721, USA.
Magn Reson Med. 2009 Dec;62(6):1414-22. doi: 10.1002/mrm.22155.
The apparent diffusion coefficient (ADC), as measured by diffusion-weighted MRI, has proven useful in the diagnosis and evaluation of ischemic stroke. The ADC of tissue water is reduced by 30-50% following ischemia and provides excellent contrast between normal and affected tissue. Despite its clinical utility, there is no consensus on the biophysical mechanism underlying the reduction in ADC. In this work, a numerical simulation of water diffusion is used to predict the effects of cellular tissue properties on experimentally measured ADC. The model indicates that the biophysical mechanisms responsible for changes in ADC postischemia depend upon the time over which diffusion is measured. At short diffusion times, the ADC is dependent upon the intrinsic intracellular diffusivity, while at longer, clinically relevant diffusion times, the ADC is highly dependent upon the cell volume fraction. The model also predicts that at clinically relevant diffusion times, the 30-50% drop in ADC after ischemia can be accounted for by cell swelling alone when intracellular T(2) is allowed to be shorter than extracellular T(2).
表观扩散系数（ADC），通过扩散加权磁共振成像（DWI）测量，已被证明在缺血性脑卒中的诊断和评估中有一定的作用。组织水的 ADC 在缺血后降低 30-50%，从而在正常组织和病变组织之间提供了极好的对比度。尽管具有临床实用性，但对于 ADC 降低的基础生物物理机制尚未达成共识。在这项工作中，使用水扩散的数值模拟来预测细胞组织特性对实验测量 ADC 的影响。该模型表明，缺血后 ADC 变化的生物物理机制取决于扩散测量的时间。在较短的扩散时间内，ADC 取决于细胞内固有扩散率，而在较长的、临床上相关的扩散时间内，ADC 高度依赖于细胞体积分数。该模型还预测，在临床上相关的扩散时间内，当允许细胞内 T2 短于细胞外 T2 时，仅通过细胞肿胀即可解释缺血后 ADC 降低 30-50%。
