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 w ere usually small lesions (less than or equal to 1 ml), were seen early, an d 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 th e apparent diffusion coefficient of the detected lesions provided prognosti c information. After injection of a contrast agent (perfusion imaging), a t ime series of volumes were obtained using a T2* sensitive gradient echo EPI sequence. Hemodynamic perturbations of the cerebral parenchyma could be de tected as well as the type of perturbation in the lesion. A map representin g the mean transit time for each voxel was used to define the maximum volum e of the perturbation. A hemodynamic penumbra was defined to be when this v olume was larger than the volume detected on the diffusion images. The quan titative measure of cerebral blood flow could predict the irreversibility o f 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 3 0ml/min/100g.