Venous thromboembolism is a significant source of morbidity and mortality worldwide. Catheter-directed thrombolytics is the primary treatment used to relieve critical obstructions, though its efficacy varies based on the thrombus composition. Non-responsive portions of the specimen often remain in situ, which prohibits mechanistic investigation of lytic resistance or the development of diagnostic indicators for treatment outcomes. In this study, thrombus samples extracted from venous thromboembolism patients were analyzed ex vivo to determine their histological structure, lytic susceptibility, and imaging characteristics. A wide range of thrombus morphologies were observed, with a dependence on age and etymology of the specimen. The lytic inhibitor PAI-1 was present in samples, and may contribute to the lytic resistance of venous thrombi. Finally, a correlation was observed between the lytic response of the sample and its magnetic microstructure assessed with a quantitative MRI sequence. These findings highlight the myriad of changes in venous thrombi that may promote lytic resistance, and imaging metrics that correlate with treatment outcomes.