MRI-based Assessment of Endovascular Abdominal Aortic Aneurysm Repair

Abstract

Imaging techniques play a key role in the Endovascular Abdominal Aortic Aneurysm Repair (EVAR) follow-up. The most important parameters monitored after EVAR are the aneurysm size and the presence of endoleaks. Currently, computed tomographic angiography (CTA) is the most commonly used imaging modality for EVAR follow-up. Important advantages of CTA are its wide availability and relatively low costs. The most important disadvantages are the need for ionizing radiation and iodinated contrast agent. Magnetic Resonance Imaging (MRI) and Magnetic Resonance Angiography (MRA) have been advocated as promising alternatives to CTA for post-EVAR follow-up. MRI-techniques have evident theoretical advantages over CTA examination. Especially the high soft tissue contrast of MRI and the high sensitivity for non-nephrotoxic contrast agent give MRI an advantage over CTA evaluation. The research described in this thesis was conducted to evaluate MRI and Magnetic Resonance Angiography (MRA) techniques as an alternative to CTA in the follow-up of patients after EVAR, by comparing findings on MRI examination with those obtained in the same patients using CTA. The additional value of MRI's superior soft tissue contrast was also investigated. In order to address the issue of metal artifacts, seven of the most commonly used endografts were evaluated as to their suitability for MRI-based follow-up. Most metallic endo- or exoskeletons of endovascular grafts do not interfere with diagnostic imaging. Skeletons made out of Stainless steel however resulted in large artifacts making diagnostic evaluation impossible. We showed that with a dedicated MRI protocol, MRI is more sensitive for endoleak detection than CT is. We also showed that the addition of time-resolved or dynamic MRA increases MRI's accuracy for determining the origin of inflow of an endoleak. Furthermore, the additional information MRI inherently offers about thrombus composition cannot be obtained using CT. The exact processes taking place inside the aneurysm sac after exclusion from blood flow by placement of an endoprosthesis are not well understood. In the case of stable or non-shrinking aneurysms, it is important to know the etiology of non-shrinkage for rupture risk estimation and for treatment decisions. A newly developed method for evaluation of thrombus organization as a parameter for follow-up after EVAR was developed and tested. This method involves the classification of voxels based upon their relative signal intensity using manually set thresholds. Voxels are classified into tree groups; endoleak, fresh thrombus and old thrombus. It is demonstrated that new parameters describing aneurysm sac contents may provide additional information in EVAR follow-up. As expressed by several authors in the literature on EVAR follow-up, there is still a quest for the best set of parameters to be followed after EVAR: diameter change, volume change, the presence of endoleak, pressure inside the aneurysm, aneurysm wall motion or graft wall motion. Until now, each single parameter only offers part of the necessary information. Only long-term studies will be able to clarify what the essential parameters for follow-up after EVAR are and how they should be interpreted.