The nmr relaxation rate results show unequivocally that there are at least two fractions of tissue water in both normal and edematous white matter which do not exchange on an nmr time scale (i.e. at times of the order of milliseconds to fraction of a second). In conjunction with the electron microscopic determination of the extracellular volumes of normal and edematous white matter, the relaxation results can be interpreted in terms of the following model. The two slowly-exchanging water components giving rise to the non-exponential relaxation correspond to cellular and extracellular water; edema changes the relaxation rate of the extracellular component much more than that of the cellular component (the extracellular component becoming more "liquid-like" in its relaxation). Such behavior is consistent with the properties of the extracellular water being due to rapid exchange between motionally restricted water adsorbed at the surface of myelin sheaths, and relatively unrestricted, bulk water. Edema presumably increases the relative amount of the "bulk" water between the axons.
- "Free" or "bound" state of water
- Biophysics of brain tissue
- Cerebral edema
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