Differential evolution of tissue mean transit time, relative blood volume and flow deficits in acute ischemic stroke
Background and Aim: The presence of a perfusion-weighted imaging (PWI) volume deficit greater than diffusion-weighted imaging (DWI) volume predicts the presence of salvageable tissue. The aim of this prospective study was to determine the evolution of various parameters of perfusion within the ischemic volume using serial MR imaging during the first 48 hours after stroke. Method: Five patients with middle cerebral artery (MCA) territory ischemic stroke were imaged (echoplanar DWI, PWI, MR angiography) within 6–10 hours of stroke onset, then at 15–18 hours, 22–24 hours and 48 hours. No patient received thrombolytic or neuro-protective agents. Results: Four of the 5 patients had a MCA occlusion on the initial MR angiogram. Recanalization of the MCA was observed by the second scan in one patient and by the fourth scan in the remaining 3 patients. Significant DWI and tissue mean transit time (tMTT) volume mismatch (>75%) was observed in 4 patients and moderate mismatch in 1 patient on the initial scan. A significant mismatch between DWI and tMTT volume (>70%) was observed in 2 patients by the second and third scan. By the fourth scan no patient had a significant DWI and tMTT volume mismatch. Significant mismatch (>50%) between DWI and relative blood volume/ relative blood flow (rCBV/rCBF) volume was observed in 3 of the 5 patients on the initial scan. Matched DWI and rCBV was observed by the third scan in all patients. Matched DWI and rCBF was observed by the second scan in all patients. A decrease in the size of the tMTT volume prior to the recanalization of the MCA was likely due to opening of collateral channels but the most dramatic decrease in the size of tMTT volume occurred following recanalization of the MCA. Conclusion: The serial changes in tMTT, rCBV, rCBF progress at different rates following acute ischemic stroke. These results have implications for future studies basing thrombolytic therapy on DWI/PWI. We caution that treatment based only on a tMTT/DWI mismatch may overestimate the size of the salvageable tissue.