MRI follow-up of abdominal aortic aneurysms after endovascular repair

Abstract

Aneurysm size changes form the basis of the follow-up after endovascular abdominal aortic aneurysm repair, because aneurysm growth increases rupture risk. Aneurysm growth can be caused by endoleak (leakage of blood in the aneurysm sac). Therefore, accurate endoleak detection is important in growing aneurysms, because it can provide a treatment target to stop aneurysm growth. MRI has been shown to be more sensitive to endoleak than CT. In this thesis we report on the use of MRI in the follow-up of abdominal aortic aneurysms after endovascular repair. First, the use of MRI with a blood pool contrast agent for the detection of slow-flow endoleak is described. Gadofosveset trisodium, a gadolinium-based agent that binds to albumin, is used as contrast agent. The albumin binding causes a long retention time of the contrast agent in the vascular system, which allows for a longer delay between injection and imaging than conventional extracellular contrast agents. In Chapter 2 the value of blood pool contrast agent-enhanced MRI for endoleak detection is investigated in patients who present with nonshrinking aneurysms more than a year after EVAR, without evidence of endoleak on CTA and delayed CT. In 55 % of the MRI exams endoleaks were visualized. Especially the late phase postcontrast imaging 30 minutes after injection proved valuable for endoleak detection. In Chapter 3 graft porosity is visualized with this technique in patients with an original Excluder endograft. In vitro, the presumed porosity of this graft was demonstrated before. With blood pool contrast agent-enhanced MRI, graft porosity is visualized in vivo for the first time. A clear increase in signal intensity around the endograft can be observed in the postcontrast images which were acquired 30 minutes after injection. In the following chapters the superior soft tissue contrast of MRI is used for the visualization of aneurysm sac contents. In Chapter 4 we present a method to measure endoleak volume, unorganized thrombus volume and organized thrombus volume. In Chapter 5 the method is applied to patients with nonshrinking aneurysms more than a year after EVAR. This work demonstrates that – even years after EVAR – half of the nonshrinking aneurysm sac contents still consists of unorganized thrombus, irrespective of the presence of endoleaks. In Chapter 6 the longitudinal analysis of aneurysm sac contents in patients in the first year after EVAR is reported in relation with changes in aneurysm volume on long-term CT-follow up. Progressive organization of aneurysm sac contents is demonstrated which is reflected by an increase in the amount of organized thrombus volume and a decrease in unorganized thrombus volume in time. After one year still one third of the aneurysm sac consists of unorganized thrombus. No difference in the evolution of the aneurysm sac is found between stable, shrinking and growing aneurysms. Concludingly,slow-flow endoleaks can be detected with MRI with a blood pool contrast agent, and aneurysm sac contents can be visualized and monitored with MRI. Further longitudinal studies with MRI could eventually provide more clarity on the processes in the aneurysm sac after EVAR