Extracellular Apparent Diffusion in Rat Brain

The apparent diffusion coefficients (ADCs) of a series of mark-ers concentrated in the extracellular space of normal rat brain were measured to evaluate, by inference, the ADC of water in the extracellular space. The markers (mannitol, phenylphos-phonate, and polyethylene glycols) are defined as “compart-ment selective ” because tissue culture experiments demon-strate some leakage into the intracellular space, making them less “compartment specific ” than commonly believed. These primarily extracellular markers have ADCs similar to those of intracellular metabolites of comparable hydrodynamic radius, suggesting that water ADC values in the intra- and extracellular spaces are similar. If this is the case, then it is unlikely that a net shift of water from the extra- to the intracellular space contrib-utes significantly to the reduction in water ADC detected fol-lowing brain injury. Rather, this reduction is more likely du