Imaging corticospinal degeneration in neonatal rats with unilateral cerebral infarction.

Recent human studies indicate that magnetic resonance (MR) imaging, particularly diffusion weighted imaging, detects abnormalities within the descending cortico-spinal tract following stroke. Whether these changes are directly related to processes of axonal degeneration and how MR changes (e.g. apparent diffusion coefficient of water (ADC) and T(2)) vary in their diagnostic utility over time is not known. The present study demonstrates that a commonly used rat model of neonatal transient unilateral hypoxia-ischemia provides similar diffusion weighted and ADC changes in the cerebral peduncle as those observed in human neonates clinically. Imaging the descending cortico-spinal tract in this model at defined acute (1-3 days) and chronic (1 and 4 weeks) time points demonstrates increased T(2) and progressive changes in ADC within the descending cortico-spinal tract in the first days to weeks following hypoxia-ischemia with a normalization by 1 week and further increases in ispilateral cerebral cortex by 4 weeks. These imaging changes are associated with reduced axonal neurofilament staining both at the subacute and more chronic time points. This demonstrates directly the utility of ADC and T(2) MRI to detect acute changes in axons associated with early Wallerian degeneration.