Correlations between the apparent diffusion coefficient, water content, and ultrastructure after induction of vasogenic brain edema in cats

Object. The authors examined the correlation between changes in the apparen t diffusion coefficient, regional water content, and tissue ultrastructure after vasogenic brain edema. Methods. Vasogenic edema was induced in the white matter of six cats by cor tical cold lesioning. The trace of diffusion tensor (Trace[D]) obtained fro m magnetic resonance imaging to measure the orientationally averaged water diffusibility was compared with the corresponding tissue water content dete rmined by gravimetric studies and with ultrastructural water localization. Edema fluid had spread to the subcortical and deep white matter by 4.5 hour s postlesioning. The increase in Trace(D) showed a significant linear corre lation with the increase in tissue water content, both in the subcortical a nd deep white matter as follows: y = 45.5x - 2367 (r = 0.94) and y = 37.0x - 1769 (r = 0.93), respectively, where x is the water content (gram water/g ram tissue) and y the Trace(D) (X 10(-6) mm(2)/sec end). On histological ex amination, nerve fibers were found to be dissociated in the white matter an d the extracellular space was markedly enlarged with protein-rich fluid. No noticeable hydropic swelling of the cellular components was observed. Conclusions. A linear correlation was observed between increases in Trace(D ) and increases in extracellular water volume in in vivo vasogenic brain ed ema. A similar correlation between the subcortical and deep white matter sh owing different arrangements of nerve fibers (parallel compared with interm ingled, respectively) indicated that measurement of Trace(D) is a suitable parameter for the evaluation of vasogenic brain edema.