PURPOSE: To evaluate the temporal evolution of the ischemic area and trace ratio, and to define ischemic penumbra within the hyperacute experimental focal cerebral ischemia model. MATERIALS AND METHODS: A focal cerebral ischemia model of middle cerebral artery occlusion (MCAO) was constructed in twelve Korean cats weighing 2 -3 Kg. T2-weighted images (T2WI) and diffusion-weighted images(DWI) were obtained using a 1.5T MR imager. Trace images were reconstructed after post-image processing with IDL 5.0. The trace ratio (ipsilateral trace value/contralateral trace value) was calculated in the ischemic core and periphery, and MR images were obtained at 1, 3, 6, and 24 hrs after MCAO. The twelve cats were divided into three groups, and 4, 5, and 3 cats were sacrified after obtaining MR images at 3, 6, and 24 hrs after MCAO, respectively. After 2% triphenyl tetrazolium chloride (TTC) solution and formalin preparation, the infarction area of the brain slice and T2WI and DWI trace images of the same slice were compared. The trace ratio was calculated at the peripheral portion of the ischemic core, which was the presumed ischemic penumbra in images obtained 1hr after MCAO. Changes in trace ratio in the ischemic core and infarction territory were also evaluated according to time. RESULTS: The trace ratio in the peripheral portion of the ischemic core was 0.71 +/- 0.03. The region where the trace ratio was 0.83 +/- 0.06 in images obtained 1 hour after MCAO was presumed to be ischemic penumbra; the region progressed to infarction in images taken during the next time period. In all cases the abnormal area of trace images was larger than that seen on DWI. The trace ratio was lower, by 6.2 %, 3 -6hrs after MCAO than at any other time. In cat number 3, the trace radio decreased rapidly and progressively, by 21%, during the first six hours. For 3 -6hrs after MCAO, the area of infarction showed progressive enlargement. CONCLUSION: Within six hours of MCAO, ischemic penumbra is progressively incorporated into the ischemic core. In the experimental focal cerebral ischemia model, an area of trace ratio larger than 0.71 and less than 1may be defined as ischemic penumbra.