Differences between Gray Matter and White Matter Water Diffusion in Stroke: Diffusion-Tensor MR Imaging in 12 Patients

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Abstract
PURPOSE: To investigate differences in water diffusion between white matter and gray matter in acute to early subacute stroke with diffusion-tensor magnetic resonance (MR) imaging. MATERIALS AND METHODS: Twelve patients with unilateral middle cerebral arterial infarcts were examined with diffusion tensor–encoded echo-planar MR imaging 17 hours to 5 days after stroke onset. Isotropic diffusion coefficient (D̄) and diffusion anisotropy (Aσ) images were computed. D̄ values were measured in ischemic and contralateral gray matter and white matter by using Aσ images to differentiate white matter from gray matter. D̄ images were compared with unidirectional and directionally averaged diffusion-weighted images. RESULTS: In all patients, D̄ images showed two distinct levels of diffusion reduction in the infarct; more severe reduction occurred exclusively in white matter. D̄ values were significantly less in infarcted white matter than in infarcted gray matter, whereas D̄ values in the contralateral white matter and gray matter were not significantly different. Relative to the contralateral side, D̄ values in the infarct were reduced by 46% in white matter and by 31% in gray matter (P < .001). Diffusion-weighted imaging caused underestimation of the magnitude and, in some cases, the spatial extent of the white matter diffusion abnormality. CONCLUSION: Isotropic diffusion is more reduced in white matter than in gray matter in acute to early subacute middle cerebral arterial stroke. Diffusion-tensor imaging may be more sensitive than diffusion-weighted imaging to white matter ischemia.