Abstract only Background: TICI scores are routinely used to measure reperfusion on angiography after endovascular therapies for acute stroke. Reperfusion may also be quantified by Tmax parameter changes on serial perfusion MRI before and after treatment. Such definitions of reperfusion used in trials may vary and we therefore investigated the correlation between TICI and quantification of Tmax changes on serial MRI in proximal middle cerebral artery (MCA) stroke cases treated with endovascular therapy. Methods: Consecutive acute ischemic stroke patients treated with endovascular therapy for proximal or M1 MCA occlusions with serial perfusion MRI at baseline and 3-6 hours after treatment were analyzed. TICI scores were noted for reperfusion on angiography. Reperfusion on serial MRI was separately defined as interval volume of Tmax>6s lesion size, dichotomous change by > 70% reduction in Tmax>6s, and voxel-wise changes across all Tmax values. Results: 57 stroke patients (mean age 64±20 years, 68% female) with M1 MCA occlusions imaged with perfusion MRI both before and after treatment with endovascular therapy were studied. TICI angiographic outcomes included 8 TICI 0, 4 TICI 1, 22 TICI 2a, 22 TICI 2b and 1 TICI 3. Both the interval volume of Tmax>6s lesion size and voxel-wise changes across all Tmax values varied extensively. Dichotomous reduction in Tmax>6s lesion volume by 70% was noted in only 12.9% of cases, with reduction by 60% in 19.4%, and by 50% in 30.6%. TICI reperfusion did not correlate with either: interval volume of Tmax>6s lesion size, dichotomous change by > 70% reduction in Tmax>6s, or voxel-wise changes across all Tmax values. Even when only TICI 2a/2b/3 or TICI 2b/3 cases were analyzed, no correlation could be established between the TICI angiographic measure of reperfusion at post-procedure with the change in Tmax lesion from baseline to 3-6 hours after revascularization. Conclusions: Reperfusion measured by TICI at angiography and changes in Tmax on serial perfusion MRI provide distinct information, likely reflecting heterogeneity and different vascular phases (arterial vs. microcirculation). Determining the clinical impact of such reperfusion measures on recovery after stroke remains paramount.