Characterization of intra-aneurysmal thrombus by correlation of MRA and histology in a canine model: Implications of thrombus variability

Abstract Introduction. Conventional postoperative surveillance after endovascular repair of abdominal aortic aneurysms (AAA) relies on computed tomography to identify endoleaks. This study sought to evaluate the potential use of magnetic resonance imaging (MRI) to determine the success of endovascular AAA exclusion by correlating MRI signal intensity with intraaneurysmal thrombus histology and luminosity in a canine model of endovascular AAA repair. Methods. Infrarenal AAA were created in 12 mongrel dogs: 6 with persistently patent retrograde (type II) endoleaks and 6 with no endoleak. T1, T2, and cine-MRA gadolinium-enhanced images were obtained using a fast spoiled gradient recalled protocol in a 3.0 Tesla MR system at 2 days, and 1, 2, and 4 weeks after aneurysms were excluded from antegrade perfusion by a stent graft. T1- and T2-weighted signal intensities of the intraaneurysmal thrombus were analyzed and quantified as signal:noise ratio. Animals were euthanized at a mean of 71 days after aneurysm exclusion for those with multiple side branch endoleaks and at 28 days for control animals with no endoleak. Quantitative analysis of fibrin and intact RBC content as well as total luminosity were analyzed using computerized image analysis. Results were correlated with distance from the endoleak channel for aneurysms with patent endoleaks and correlated with time following exclusion in aneurysms without endoleak. Results. Cine-MRA confirmed endoleak patency and demonstrated pulsatile perfusion of the intraaneurysmal endoleak channel. T1- and T2-weighted MR signal intensity evolved progressively over time in completely excluded aneurysms. Signal intensity varied as a function of distance from patent endoleaks. Intra-aneurysmal thrombus appearance and signal intensity correlated to histologic findings (Tables). The percentage of RBC decreased from 69.5 to 2.7%, while the percentage of fibrin increased from 5.7 to 93.3% with progressively increasing distance from endoleak (Graph). Computerized image analysis revealed increase in luminosity of the visual spectrum evidenced by luminosity of 127 ± 15.59 for areas within 5 mm of the endoleak versus 155 ± 5.7 further than 10 mm from the endoleak, indicating a more homogenous deposition with minimal remodeling and minimal cellularity. The correlation between histologic and MR findings evolved with time in a manner similar to distance from the endoleak channel with increasing luminosity, increasing fibrin content (10 → 87%) and RBC fragments and decreasing intact RBC (61 → 7%). Conclusions. MRI signal intensity corresponds to the histological evolution of thrombus over time in fully excluded aneurysms. A similar pattern of MRI and histological appearance of thrombus organization occurs with increasing distance from persistently patent retrograde endoleaks. Further studies are necessary to determine the utility of MRI assessment of intraaneurysmal thrombus as a technique for postoperative surveillance. TABLE I—ABSTRACT P62 . No Endoleak (n = 6). Chronologic MR and Histologic Characteristics of Intraaneurysmal Thrombus Over Time. Time Histology T1 signal:noise ratio T2 signal:noise ratio Luminosity 2 Days Intact RBC ∗ 1.28:1 0.96:1 N/A 1 Week RBC ∗ lysis 4.14:1 24.3:1 127 2 Weeks Met-Hgb ∗ 10.4:1 16.7:1 143 4 Weeks Ferritin & fibrin 6.6:1 4.8:1 155 ∗ RBC: red blood cells; Met-Hgb: met-hemoglobin. TABLE II—ABSTRACT P62 . Retrograde (type II) endoleak (n = 6). Anatomic MR and Histologic Analysis of Intraaneurysmal Thrombus as a Function of Distance from Endoleak Channel. Distance to endoleak channel Histology T1 signal:noise ratio T2 signal:noise ratio Luminosity Within channel Intact RBC ∗ 1.15:1 1.01:1 N/A RBC ∗ lysis 4.26:1 25.8:1 130 5–10 mm Met-Hgb ∗ 9.3:1 14.0:1 141 > 10 mm Ferritin & fibrin 7.3:1 4.4:1 157 ∗ RBC: red blood cells; Met-Hgb: met-hemoglobin. Download full-size image