Diffusion in compartmental systems. II. Diffusion‐weighted measurements of rat brain tissue in vivo and postmortem at very large b‐values

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Abstract
Diffusion-weighted single-voxel 1H spectroscopic measurements were performed on rat brain tissue in vivo and postmortem. Diffusion weighting was achieved by varying the diffusion time from 23 ms to 1.18 sec via the mixing time in a stimulated echo sequence. A series of constant gradient (cg-) experiments of eight effective gradient strengths q̃2(q̃2 = γ2δ2g2) from 24.2 × 103 to 490.2 × 103 mm−2 was performed, resulting in a maximum attenuation factor of b = 580,000 s/mm2. A fit of three exponential terms was found to be appropriate to represent the attenuation signal over the whole b-range. The behavior of the slowest decaying component can be fully understood in terms of a long time limit of a modified Kärger formalism for a two-compartment system. This allowed estimation of the transmembrane water exchange rate: the intracellular exchange time was determined to be 622 ± 29 ms and 578 ± 20 ms in vivo and postmortem, respectively. Magn Reson Med 50:510–514, 2003.