Diffusion- and perfusion-weighted MR imaging during the hyperacute phase of stroke.

The sensitivity of diffusion-weighted MR imaging to detect a lesion within 6 hours of stroke onset was approximately 90%. The false negative results were usually small lesions (less than or equal to 1 ml), were seen early, and were usually located in the brain stem. The specificity of this technique was nearly 100% when it was used correctly. The volume and the value of the apparent diffusion coefficient of the detected lesions provided prognostic information. After injection of a contrast agent (perfusion imaging), a time series of volumes were obtained using a T2* sensitive gradient echo EPI sequence. Hemodynamic perturbations of the cerebral parenchyma could be detected as well as the type of perturbation in the lesion. A map representing the mean transit time for each voxel was used to define the maximum volume of the perturbation. A hemodynamic penumbra was defined to be when this volume was larger than the volume detected on the diffusion images. The quantitative measure of cerebral blood flow could predict the irreversibility of the lesions when the value was below 18ml/min/100g, and the extension of the ischemia in the penumbra zone when the value was below a threshold of 30ml/min/100g.