Diffusion-and perfusion-weighted MRI -: The DWI/PWI mismatch region in acute stroke

Background and Purpose-Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) are relatively new MR techniques increasingly used in acute stroke. During the first hours of stroke evolution, the regions with abnormal perfusion are typically larger than the DWI lesions, and this mismatch region has been suggested to be "tissue at risk." The aim of this study was to evaluate the PWI/DWI mismatch region in acute stroke patients and find parameters indicative of both infarct progression and functional impairment. Methods-Twenty patients with nonlacunar ischemic stroke were imaged with DWI, PWI, and conventional MRT within 24 hours of symptom onset and after 1 week; in addition, the European Stroke Scale (ESS) score was recorded. With PWI, the volumes of regions with "time-to-peak" (TTP) delays of greater than or equal to 2, 4, 6, 8, and 10 seconds were measured; these volumes were compared with the acute DWI lesion volumes, final infarct size, and ESS score. Results-In 80% of patients the acute DWI lesion was surrounded by regions with abnormal TTP delays (PWI>DWI lesion). A TTP delay of greater than or equal to 6 s in the mismatch region was found to be associated with lesion enlargement between the initial and follow-up MRI scans. Lesions increased in 9 of 12 patients (75%) in whom the area with TTP delay greater than or equal to 6 a was larger than the DWI lesion, but they increased in only I of 8 (12.5%) of the remaining patients, in whom the area with a TTP delay greater than or equal to 6 s was smaller than the DWI lesion. The volume of the regions with TTP delays of greater than or equal to 4 s correlated better with ESS (r = -0.88, P < 0.001) than other PWI (or DWI) volumes, which indicated that a TTP delay of approximate to 4 s might be the threshold for functional impairment of brain tissue. Conclusions-Only patients with severe perfusion deficits in the PWI/DWI mismatch (TTP delays of greater than or equal to 6 s) are at high risk of lesion enlargement. Functionally, more moderate perfusion deficits (TTP delays greater than or equal to 4 and <6 s) appear to also contribute to the acute clinical deficit.