The occlusion of the middle cerebral artery was used as an experimental acute stroke model in 30 cats. The diffusion of water was followed by diffusion-sensitized MRI between 1 and 15 h after induction of stroke. It is demonstrated that images representing the trace of the diffusion tensor provide a much more accurate delineation of affected area than images representing the diffusion in one direction only. The reason is that the strong contrast caused by the anisotropy and orientation of myelin fibers is completely removed in the trace of the diffusion tensor. The trace images show a small contrast between white and gray matter. The diffusion coefficient of white matter is decreased in acute stroke to approximately the same extent as gray matter. It is further shown that the average lifetime of water in extra and intracellular space is shorter than 20 ms both for healthy and ischemic tissue indicating that myelin fibers are permeable to water. The anisotropy contrast did not change before or after induction of stroke, nor after sacrifice. Together, these observations are consistent with the view that the changes in water diffusion during acute stroke are directly related to cytotoxic oedema, i.e., to the change in relative volume of intra- and extracellular spaces. Changes in membrane permeability do not appear to contribute significantly to the changes in diffusion.

The occlusion of the middle cerebral artery in cat brain was used as an experimental stroke model to investigate the physical basis of the recently reported lowered diffusion constant of water in acute infarcted brain tissue (Moseley et al., Magn. Reson. Med. 14, 330, 1990). The original findings were confirmed in this study of 12 animals investigated with the diffusion-sensitized stimulated echo sequence. The following additional results were obtained: First, the onset of significant lowering of the diffusion constant in the stroke area varied significantly (up to 2.5 h depending on the animal). Second, the affected area is much more clearly outlined in diffusion-weighted images than in T2-weighted images, even in the period between 3 to 12 h following occlusion. Third, for diffusion times between 50 and 2000 ms. the diffusion constant of water is independent of diffusion time in healthy tissue, as well as in the stroke area. Fourth, the diffusion anisotropy is similar in healthy and in stroke area and remains similar regardless of the diffusion time used.