Numerical Investigation of Rupture Potential of Abdominal Aortic Aneurysm

UoM administered thesis: Phd

  • Authors:
  • Wisam Al-Obaidi

Abstract

Abstract Numerical investigation of rupture potential of Abdominal Aortic Aneurysm Wisam Jasim Kadhim Al-Obaidi, 2019 Doctor of Philosophy, the University of Manchester Abdominal aortic aneurysm (AAA) is a life-threatening cardiovascular condition, especially if the rupture occurs. Previous investigations have shown that low wall shear stress, high oscillatory shear index distributions may play a significant role in the formation and progression of AAA. The location of the rupture has been observed to coincide with the location of maximum wall stress. This thesis aims to investigate the rupture potential of the abdominal aortic aneurysm using a comprehensive numerical simulation involving fluidstructure interaction. The main objectives are to (i) Investigate the suitability of using a computational model derived from 3D ultrasound (3D-US) images in the finite element stress analysis instead of Computed Tomography (CT) images which have the risk of ionising radiation; (ii) Develop a numerical model using extended finite element method based on the principles of fracture mechanics to predict the initiation/propagation of rupture in AAA; (iii) Study the effect of minor geometrical differences induced by segmentation process, and their role on the hemodynamic metrics; (iv) Study the effect of different patterns of inlet and outlet boundary; (v) Develop a finite element model to investigate the effect of the surrounding organs on the stress profiles of AAA. Finite element analyses were conducted by applying a static pressure of 120mmHg on the computational models of AAA derived from 3D-US and CT images, with and without the surrounding organs. The predicted wall stress profiles have been analysed and compared quantitatively and qualitatively. In general, the 3D-US models generated higher wall stress compared to the CT models. However, the difference in magnitude and location is greatly reduced if characteristic stress is considered instead of the maximum stress. The magnitude of wall stress was found to be higher in the absence of surrounding organs. An extended finite element model was developed to investigate the effect of three different levels of pressure and strength. The stress profiles, rupture length, and location were obtained and compared between 3D-US and CT models. Despite the overestimation of stress magnitude, the 3D-US models showed comparable locations of rupture. Two fluid-structure interaction simulations were conducted using STAR CCM+ and ABAQUS by applying boundary conditions of standard velocity/pressure waves, and mass flow wave with 3-elements Windkessel model. The flow patterns, hemodynamic metrics, and wall stress distribution were compared between models obtained from two different segmentation software. In addition, the type of boundary conditions was seen to affect the prediction of hemodynamic distributions. All numerical simulations were validated against experimental works that are available in the literature. Overall, it was concluded that 3D-ultrasound models are feasible to be used in the stress analysis if the characteristic stress is taken into account in the evaluation of the rupture. The extended finite element method was shown to be suitable for rupture risk assessment. The hemodynamic patterns and wall stress profiles of abdominal aortic aneurysm are sensitive to minor geometrical differences induced by the segmentation process, and the boundary conditions used in the simulation. Finally, the presence of the surrounding organs in the model was shown to significantly affect the magnitude and distribution of the wall stress of the abdominal aortic aneurysm.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2020