Restricted diffusion and exchange of intracellular water: theoretical modelling and diffusion time dependence of 1H NMR measurements on perfused glial cells.

Intracellular diffusion properties of water in F98 glioma cells immobilized in basement membrane gel threads, are investigated with a pulsed-field-gradient spin-echo NMR technique at diffusion times from 6 to 2000 ms and at different temperatures. In extended model calculations the concept of 'restricted intracellular diffusion at permeable boundaries' is described by a combined Tanner-Kärger formula. Signal components in a series of ct experiments (constant diffusion time) are separated due to different diffusion properties (Gaussian and restricted diffusion), and physiological as well as morphological cell parameters are extracted from the experimental data. The intracellular apparent diffusion coefficients strongly depend on the diffusion time and are up to two orders of magnitude smaller than the self diffusion constant of water. Propagation lengths are found to be in the range of 4-7 microns. Hereby intracellular signals of compartments with a characteristic diameter could be selected by an appropriate gradient strength. With cg experiments (constant gradient) a mean intracellular residence time for water is determined to be about 50 ms, and the intrinsic intracellular diffusion constant is estimated to 1 x 10(-3)mm2/s. Studying the water diffusion in glial cells provides basic understanding of the intracellular situation in brain tissue and may elucidate possible influences on the changes in the diffusion contrast during ischemic conditions.