Background: Determinants of successful recanalization likely differ for Merci thrombectomy (MT) and intra-arterial pharmacologic fibrinolysis interventions.While the amount of thrombotic material to be digested is an important consideration for chemical lysis, mechanical debulking may be more greatly influenced by other target lesion characteristics.Methods: In consecutive acute ischemic stroke patients treated with MT for middle cerebral artery M1 occlusions, we analyzed the influence on recanalization success and clinical outcome of target thrombus size (length) and shape (curvature and branching) on pretreatment T2* gradient echo magnetic resonance imaging (MRI).Results: Among 65 patients, pretreatment MRI showed susceptibility vessel signs (SVS) in 45 (69%).Thrombus length averaged 13.03 mm (range 5.56-34.91) and irregular shape (curvature or branching) was present in 17/45 (38%).Presence and length of SVS did not predict recanalization or good clinical outcome.Substantial recanalization (TICI 2b or 3) and good clinical outcome (mRS ≤ 2) were more frequent with regular than irregular SVS shape (57% vs 18%, P=0.013; 39% vs 6%, P=0.017).On multiple regression analysis, the only independent predictor of substantial recanalization was irregular SVS (OR, 0.16; 95% CI, 0.04 to 0.69; P=0.014); and leading predictors of good clinical outcome were baseline NIHSS (OR, 1.20; 95% CI, 1.03 to 1.40; P=0.019) and irregular SVS (OR, 9.36; 95% CI, 0.98 to 89.4; P=0.052).Conclusions: Extension thrombus into MCA division branches and curving shape of the MCA stem, but not thrombus length, decrease technical and clinical success of Merci thrombectomy in M1 occlusions.