Temporal evolution of water diffusion parameters is different in grey and white matter in human ischaemic stroke.

OBJECTIVES: Our purpose was to investigate whether differences exist in the values and temporal evolution of mean diffusivity (<D>) and fractional anisotropy (FA) of grey and white matter after human ischaemic stroke.
METHODS: Thirty two patients with lesions affecting both grey and white matter underwent serial diffusion tensor magnetic resonance imaging (DT-MRI) within 24 hours, and at 4-7 days, 10-14 days, 1 month, and 3 months after stroke. Multiple small circular regions of interest (ROI) were placed in the grey and white matter within the lesion and in the contralateral hemisphere. Values of <D>[grey], <D>[white], FA[grey] and FA[white] were measured in these ROI at each time point and the ratios of ischaemic to normal contralateral values (<D>R and FAR) calculated.
RESULTS: <D> and FA showed different patterns of evolution after stroke. After an initial decline, the rate of increase of <D>[grey] was faster than <D>[white] from 4-7 to 10-14 days. FA[white] decreased more rapidly than FA[grey] during the first week, thereafter for both tissue types the FA decreased gradually. However, FA[white] was still higher than FA[grey] at three months indicating that some organised axonal structure remained. This effect was more marked in some patients than in others. <D>R[grey] was significantly higher than <D>R[white] within 24 hours and at 10-14 days (p<0.05), and FAR[white] was significantly more reduced than FAR[grey] at all time points (p<0.001).
CONCLUSIONS: The values and temporal evolution of <D> and FA are different for grey and white matter after human ischaemic stroke. The observation that there is patient-to-patient variability in the degree of white matter structure remaining within the infarct at three months may have implications for predicting patient outcome.