USPIO-enhanced direct MR imaging of thrombus: Preclinical evaluation in rabbits

PURPOSE: To test the hypothesis that ultrasmall superparamagnetic iron oxid e (USPIO) particles may diffuse into nonendothelialized fresh thrombi and t hus allow for direct magnetic resonance (MR) imaging of a thrombus. MATERIALS AND METHODS: Stagnation thrombi of different thrombus ages (1, 3, 5, 7, and 9 days) were induced in the external jugular veins of 25 rabbits . Direct MR imaging of thrombi was performed by using a fat-saturated Tl-we ighted gradient-echo sequence (three-dimensional [3D] magnetization prepare d rapid acquisition gradient echo) before and 24 hours after intravenous ad ministration of USPIO (particle size, 25 nm; 200 mu mol per kilogram of bod y weight). Thrombus length on 3D reconstruction images was compared with th at depicted on a radiographic venogram and with histologic findings (joint reference standard). In addition, T2*-weighted gradient-echo images were ac quired and scored semiquantitatively. RESULTS: The hyperintensity of the thrombus segment depicted on T1-weighted images (thrombus length determined with 3D reconstruction images divided b y true thrombus length) increased significantly after administration of con trast medium at a thrombus age of 3 days (0.6 +/- 0.4 [SD] to 0.8 +/- 0.4; P = .02), 5 days (0.1 +/- 0.1 to 1.0 +/- 0.1; P < .001), and 7 days (0 to 0 .6 +/- 0.4; P = .02), but not at an age of 1 and 9 days. No significant cha nge in the thrombus signal intensity was observed on T2*-weighted images. CONCLUSION: The animal model showed that direct MR imaging of the thrombus improved 24 hours after USPIO administration with a T1-weighted sequence. N o improvement was seen with the T2*-weighted sequence.