Time course of trace of diffusion tensor [Trace(D)] and histology in brain edema.

We examined the correlation between changes in the trace of diffusion tensor [Trace(D)], regional water content and tissue ultrastructure relating to cellular (cytotoxic) and vasogenic brain edema. Cellular edema was induced by left middle cerebral artery occlusion in cats (Kuroiwa T et al., 1998). Vasogenic edema was induced in the white matter of cats by a cold lesion (Kuroiwa T et al., 1999). In cellular edema, the water content increase correlated linearly with the Trace(D) decrease in both the gray and white matter. However, both the slopes and intercepts of the correlation lines were significantly different. Hydropic astrocytic swelling was seen in both structures, and in the white matter, oligodendrocytic and myelinated axonal swelling were observed. In vasogenic edema, the increase in Trace(D) showed a significant linear correlation with the increase in tissue water content. Histologically, nerve fibers were dissociated and the extracellular space was markedly enlarged with protein-rich fluid. These result showed that the different slopes and intercepts of the water content--Trace(D) correlation lines for different subtype of brain edema, which reflect different ultrastructural localization of water, should be taken into account when evaluating brain edema using Trace(D) mapping.