Ford J C, Hackney D B, Lavi E, Phillips M, Patel U
Department of Radiology, University of Pennsylvania Medical Center, Philadelphia 19104, USA.
J Magn Reson Imaging. 1998 Jul-Aug;8(4):775-82. doi: 10.1002/jmri.1880080405.
We used a numerical simulation of water self-diffusion among permeable cylinders to predict the dependence of MR-based apparent diffusion coefficients in white matter on axonal separation, barrier permeability, and diffusion time (T). The transverse apparent diffusion coefficient (tADC), calculated with simulated diffusion-sensitizing gradients perpendicular to the axon fibers, remains a function of T down to diffusion times as short as .1 microsec for a range of diffusion barrier permeability. As the diffusion time lengthens, the response of tADC depends on axon diameter, with decreases in tADC occurring earliest, and most dramatically, for the smallest fiber diameter simulated (2 microm). For a given axonal separation, asymptotic values of ADC are determined by permeability alone and are the same for 2-microm and 11-microm fibers of equal membrane permeability. The effect of increased relative intracellular volume is manifested primarily in a decrease in tADC at short T. Increases in interaxonal spacing increase the tADC at asymptotically long diffusion times and reduce the dependence on permeability. However, at the widest plausible axonal separations, permeability remains an important determinant of tADC. These simulations may enhance interpretation of measured tADC in the context of the underlying physiologic and structural changes at the cellular level that accompany white-matter disease.
我们利用可渗透圆柱体间水的自扩散数值模拟，来预测白质中基于磁共振成像的表观扩散系数对轴突间距、屏障通透性和扩散时间（T）的依赖性。对于一系列扩散屏障通透性，使用垂直于轴突纤维的模拟扩散敏感梯度计算得到的横向表观扩散系数（tADC），在扩散时间短至0.1微秒时仍是T的函数。随着扩散时间延长，tADC的响应取决于轴突直径，对于模拟的最小纤维直径（2微米），tADC最早且最显著地下降。对于给定的轴突间距，ADC的渐近值仅由通透性决定，对于具有相同膜通透性的2微米和11微米纤维是相同的。相对细胞内体积增加的影响主要表现为在短T时tADC降低。轴突间距增加会在渐近长扩散时间时增加tADC，并降低对通透性的依赖性。然而，在最合理的轴突间距下，通透性仍是tADC的重要决定因素。这些模拟可能会增强在伴随白质疾病的细胞水平潜在生理和结构变化背景下对测量的tADC的解释。
