Analysis of water diffusion in white matter using a hydration layer model

Abstract

and membrane hindrance effects. If the hydration water is associated primarily with membrane bound proteins then similar behavior can be expected for intra- and extracellular water. Our results are also consistent with the observed high FA values since diffusion is higher for =0 in the hydration layers and in the free water. An important assumption inherent to this model is that the hydration water and the non-hydration water are not in fast exchange since, if they were, a single exponential decay would be observed. If cellular water molecules are considered to move like they do in free water, then hydration and non-hydration water would be completely mixed within 50 ms. However, there is considerable evidence to suggest that cellular water behaves more like a gel than a liquid. Our results confirm the anisotropy of hydration water diffusion and are consistent with the gel model of cellular water. The results reported here for the GCC and the SCC are remarkably similar. The most significant difference is in the signal fractions for =90°. However, the smaller hydration water fraction for the GCC correlates with reduced membrane surface area, consistent with smaller diameter fibers, on average, in the GCC ffast ADCfast fslow ADCslow 0 SCC 0.80 ± 0.04 0.95 ± 0.01 1.81 ± 0.08 0.04 ± 0.01 0.07 ± 0.03 0 GCC 0.80 ± 0.03 0.95 ± 0.03 1.74 ± 0.10 0.05 ± 0.03 0.09 ± 0.06 15 SCC 0.79 ± 0.03 0.95 ± 0.02 1.69 ± 0.06 0.05 ± 0.02 0.12 ± 0.03 30 SCC 0.79 ± 0.01 0.92 ± 0.03 1.50 ± 0.06 0.07 ± 0.02 0.13 ± 0.05 45 SCC 0.80 ± 0.02 0.87 ± 0.02 1.10 ± 0.08 0.12 ± 0.03 0.12 ± 0.04 60 SCC 0.79 ± 0.03 0.74 ± 0.04 0.82 ± 0.05 0.25 ± 0.03 0.11 ± 0.01 75 SCC 0.79 ± 0.02 0.52 ± 0.08 0.70 ± 0.06 0.46 ± 0.07 0.08 ± 0.01 90 SCC 0.80 ± 0.02 0.42 ± 0.06 0.66 ± 0.13 0.56 ± 0.06 0.06 ± 0.02 90 GCC 0.79 ± 0.03 0.60 ± 0.05 0.91 ± 0.21 0.35 ± 0.07 0.06 ± 0.02