[en] Introduction
The general context of this dissertation is to evaluate patient-specifi c approaches to the risk of rupture of abdominal aortic
aneurysm (AAA), using imaging techniques with ability to assess biological processes. Following a thorough description
of available imaging techniques, our work is divided in two main research objectives, namely: (i) to provide greater clinical
value to existing but unproven imaging concepts, and (ii) to suggest new concepts for improved AAA risk of rupture
assessment.
Methods
The fi rst research objective evaluated how far imaging biological activities using 18F-Fluorodeoxyglucose (FDG) Positron
Emission Tomography (PET) and modeling wall stress using fi nite element simulations (FES) may help clinical decisionmaking
in patients with AAA, and what would be their incremental value as compared to diameter-based patient management
algorithms. On a patient basis, clinical outcomes were evaluated with regard to FDG PET and FES signaling.
Further, the concept of AAA risk-equivalent diameter using FES was described and retrospectively validated using data
from large multicenter trials. The second research objective included the assessment of the biological activities of the
intraluminal thrombus (ILT) and the demonstration of its deleterious role in AAA using multimodality imaging. A special
emphasis was put on the ability of magnetic resonance imaging (MRI) to monitor the biological activities of ILT without
exogenous contrast, by evaluating its iron content.
Results
Increased FDG uptake was a diameter-independent marker of AAA-related events over 2 years. Missing dichotomy prevented
such a fi nding for increased wall stress, although its correlation with increased FDG uptake indicates a potentially comparable
value in terms of risk management. Wall metabolism is infl uenced by patient-specifi c susceptibility factors, indicating
hereditary or acquired alteration of the biological responses to wall stress. The concept of risk-equivalent diameters on FES
links biomechanical estimates to basic conclusions drawn from large diameter-based clinical AAA trials. Our retrospective
and diameter-adjusted validation analysis verifi ed that biomechanical risk indicators are higher in ruptured than non-ruptured
AAAs. Part of the FDG uptake is associated with biological activity along the luminal surface of the ILT, where we experimentally
demonstrated phagocytosis of superparamagnetic iron oxide on MRI , both ex vivo and in vivo. This phagocytosis is
correlated with the abundance of leukocytes and proteolytic activity. In addition, unenhanced MRI appearances resulting from
the endogenous iron distribution within ILT also relate to these biological activities. Lastly, multimodality imaging was used to
confi rm the concept of the deleterious role of the ILT in AAA growth in a model of AAA by infusion of elastase in the rat.
Conclusion
MRI and FDG PET are capable of evidencing and quantifying in vivo some of the notoriously deleterious biological processes
taking place in the aneurysmal sac, especially related to the entrapped phagocytes and red blood cells in ILT and
the periadventitial infl ammatory response. The central role played by ILT and its biological activities was demonstrated in
vivo using several imaging techniques. The clinical value of imaging these biological activities is epitomized by a diameterindependent
2-year increased risk of event in AAA with increased wall metabolism.
Disciplines :
Radiology, nuclear medicine & imaging
Author, co-author :
NCHIMI LONGANG, Alain ; Centre Hospitalier Universitaire de Liège - CHU > Service médical de radiodiagnostic
Language :
English
Title :
Imaging the mechanisms involved in abdominal aortic aneurysms rupture; a step towards patient-specific risk assessment
Defense date :
09 February 2015
Institution :
ULiège - Université de Liège
Degree :
Doctor in Medical Sciences
Promotor :
Sakalihasan, Natzi ; Université de Liège - ULiège > Département des sciences cliniques
President :
Defraigne, Jean-Olivier ; Centre Hospitalier Universitaire de Liège - CHU > Service de chirurgie cardio-vasculaire et thoracique