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Fig. 1B —MRI depiction of a diffusion–perfusion mismatch in 70-year-old man with right hemiparesis. Axial MR mean transit time (MTT) image obtained at same time as A shows region of prolonged MTT (depicted in orange) in left middle cerebral artery territory that is much larger than diffusion-weighted abnormality seen in A. Area of mismatch, that is, region showing abnormal MTT, but no abnormal diffusion, is usually considered as representing ischemic penumbra or tissue at risk.