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

Background and Purpose-Diffusion-weighted imaging (DWI) and perfusion-weigh ted imaging (PWI) are relatively new MR techniques increasingly used in acu te stroke. During the first hours of stroke evolution, the regions with abn ormal perfusion are typically larger than the DWI lesions, and this mismatc h region has been suggested to be "tissue at risk." The aim of this study w as to evaluate the PWI/DWI mismatch region in acute stroke patients and fin d parameters indicative of both infarct progression and functional impairme nt. Methods-Twenty patients with nonlacunar ischemic stroke were imaged with DW I, PWI, and conventional MRT within 24 hours of symptom onset and after 1 w eek; in addition, the European Stroke Scale (ESS) score was recorded. With PWI, the volumes of regions with "time-to-peak" (TTP) delays of greater tha n 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 sco re. Results-In 80% of patients the acute DWI lesion was surrounded by regions w ith abnormal TTP delays (PWI>DWI lesion). A TTP delay of greater than or eq ual to 6 s in the mismatch region was found to be associated with lesion en largement 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 equ al 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 great er 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 w ith ESS (r = -0.88, P < 0.001) than other PWI (or DWI) volumes, which indic ated that a TTP delay of approximate to 4 s might be the threshold for func tional impairment of brain tissue. Conclusions-Only patients with severe perfusion deficits in the PWI/DWI mis match (TTP delays of greater than or equal to 6 s) are at high risk of lesi on 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.