WATER SIGNAL ATTENUATION IN DIFFUSION-WEIGHTED H-1-NMR EXPERIMENTS DURING CEREBRAL-ISCHEMIA - INFLUENCE OF INTRACELLULAR RESTRICTIONS, EXTRACELLULAR TORTUOSITY, AND EXCHANGE

The ''concept of restricted intracellular water diffusion at permeable boundaries,'' which was recently used to model diffusion-weighted H-1 NMR experiments on glioma cells, was applied to measurements on the r at brain in vivo. Combined with the ''concept of extracellular tortuos ity,'' various physiological states of the brain were simulated. Hereb y, a variable intracellular volume fraction, intracellular exchange ti me, and extracellular tortuosity factor were considered for young, adu lt, and ischemic rat brains. The model simulated the cytotoxic shift o f extracellular water, changes in membrane permeability and tissue mor phology, and was able to explain the diffusion time dependence as well as the non-monoexponentiality of the diffusion attenuation curves. Pr eliminary diffusion time dependent experiments on the healthy rat brai n (H-1 NMR imaging) agreed well with the theoretical concept. Hereby, the intracellular water signal was separated from extracellular signal contributions by large diffusion weighting. It showed the characteris tic of restricted diffusion as well as a signal decay due to the excha nge of intracellular water across the plasma membrane. A map of the me an intracellular exchange time for water in living animal brain was de termined, and the upper limit in rat brain was evaluated to 15 ms, The presented methods can be applied to correlate local differences in a map of exchange times with tissue morphology and to detect pathologica l deviations of the exchange time, e,g,, during ischemia, (C) 1998 Els evier Science Inc.