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Background and Purpose: Better understanding about thrombus composition seems necessary, as treatment of acute ischemic stroke (AIS) is focus on clot chemical dissolution and mechanical extraction. We propose to evaluate whether magnetic resonance imaging (MRI) can differentiate white from red clots and estimate red blood cells percentage (RBC%) using clots with predetermined components and an index based on MRI signal intensity (SI).Material and Methods: 5 clots (A=100% fibrin, B=80% RBC, C=50% RBC, D=20% RBC, E=unknown) were fixed in gelatin-manganese solution and studied by: high-resolution 3D T1-weighted (T1MPR), T2-weighted turbo spin echo (T2TSE), T2-weighted gradient echo (T2GE), susceptibility weighted (SWI), fluid-attenuated inversion recovery (FLAIR) and diffusion weighted imaging (DWI) with apparent diffusion coefficient (ADC). SI index was calculated with clot SI and gelatin SI. Statistical analysis compared RBC-clots to fibrin-clot SI index and the correlation of RBC% and SI index in each MRI sequence.Results: Each red clot was different from clot A except clot D in FLAIR. Correlation between clots SI index and RBC concentration were found in T1MPR (r=-0.84), SWI (r=-0.79), T2GE (r=-0.72) and FLAIR (r=0.80). Linear regression resolution provided an indirect RBC estimation for clot E: 47.3 % in T1MPR, SWI 41.5%, T2GE 45.1% and FLAIR 50.9%. Histological analysis confirmed clot E composition.Conclusion: This in vitro study suggests that MRI can differentiate white from red clots except clots with low RBC% in FLAIR and also provide approximate RBC%.