Astrocytic swelling in the ipsilateral substantia nigra after occlusion ofthe middle cerebral artery in rats

BACKGROUND AND PURPOSE: Focal cerebral ischemia results in neuronal changes in remote areas that have tiber connections with the ischemic area. We rep orted previously that a high-signal-intensity lesion was observed in the su bstantia nigra after striatal infarction on T2-weighted MR images in both c linical and experimental cases. However, the origin of these changes in sig nal intensity remains unclear, The aim of this study was to investigate the nigral changes by examining the correlation between the apparent diffusion coefficient (ADC) and the tissue structure. METHODS: Sprague-Dawley rats were subjected to middle cerebral artery occlu sion, Four days after the occlusion, when T2-weighted images revealed the p resence of an area of high signal intensity in the ipsilateral substantia n igra, diffusion-weighted imaging was performed using a 4.7-T superconductiv e MR unit, and the ADCs were calculated and imaged. Histopathologic examina tion by both light and electron microscopy was performed on day 4 after sur gery. RESULTS: Diffusion-weighted images showed an area of high signal intensity in the ipsilateral substantia nigra, and the ADC map revealed uniform reduc tion of the ADC in this area. Swelling of astrocytic end-feet was observed, especially in the pars reticulata, CONCLUSION: These findings suggest that MR changes in the ipsilateral subst antia nigra after striatal injury consist mainly of swelling in the astrocy tic end-feet.