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To test whether T1-weighted MRI can detect the differences in the rate of thrombolysis induced by recombinant tissue plasminogen activator (rt-PA) between platelet-rich regions and red blood cell (RBC)-rich regions of venous thrombi ex vivo.Each of 21 venous thrombi ex vivo (8 pulmonary emboli and 13 in situ thrombi) was dissected along the longitudinal axis. Half of it was analyzed for the presence of platelet, fibrin, and RBC components by immunohistochemistry and the other half was imaged serially by high-resolution T1-weighted three-dimensional MRI to assess the progression of thrombolysis. The MR images were analyzed for proportions of the remaining platelet-rich and RBC-rich regions.Laminated platelet-rich regions, corresponding to Zahn lines, were confirmed immunohistochemically and by MRI in 18/21 venous thrombi. In T1-weighted MR images (TE/TR = 10/105 ms) the mean signal intensity of platelet-rich regions was on average 2.3 higher than that of RBC-rich regions. The rate of thrombolysis in platelet-rich regions was on average 30% lower than in RBC-rich regions. After 120 min of thrombolysis the proportion of lysed platelet-rich regions was 0.27 ± 0.04 versus 0.40 ± 0.08 in RBC regions, which resulted in 1.4% decrease of lysed thrombus volume per 1% increase of platelet-rich content.Venous thrombi are most often composed of interspersed platelet-rich and RBC-rich regions. T1-weighted MRI is capable of noninvasive discrimination between those two components of venous thrombi ex vivo which have a different susceptibility to thrombolysis by rt-PA. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.