Noninvasive methods of characterising local regional variations in aortic tissue to advance aneurysm rupture risk prediction

Abdominal Aortic Aneurysms (AAAs) is a permanent and irreversible dilation of the infrarenal section of the aorta. AAA’s are generally asymptomatic, until rupture of the AAA wall occurs. Rupture can lead to large abdominal bleeding and death within a short period of time. AAA formation affects the integrity of the aortic wall, leading to a decrease in compliance and tensile strength, increased wall stiffness and a progressive dilation of the wall. From a biomedical engineering perspective, rupture of an AAA occurs when, locally, the wall stress surpasses the strength of the wall. This suggests it is of importance to have wall property information and perform wall stress analysis which can assess the risk of rupture reliably. Noninvasive assessment of aneurysm wall properties would improve insight into the vascular changes, preceding rupture. This thesis aims to explore noninvasive methods of characterising aortic wall properties and the effectiveness of these techniques to aid in clinical assessment. In this study, the efficacy of acoustic radiation force impulse (ARFI) imaging for determination of aortic changes in vitro was reported. The study successfully developed an artificial aneurysm in excised tissue and the changes induced by aneurysm development were detected using ARFI. A feasibility case study demonstrated a method for estimation of in vivo tissue properties using ARFI and exhibited the viability of translation of this modality to AAA clinical use. Most preoperative imaging protocols use computerised tomography (CT) angiography with three dimensional (3D) reconstructions for sizing and planning. The resulting images are static images, despite the fact that the human aorta exists in a dynamic environment. The elastic properties of the aorta were examined to assess the changes in the dynamic environment using cardiac gated CT. Different regions of the aorta were shown to have different mechanical properties. High variation in mechanical behaviour was found to exist locally. A novel method which allowed these variances to be reflected in finite element reconstructions was established. This is believed to be an important step in the improvement and accuracy of finite element studies. The morphology and the regional variation in mechanical properties were both found to play a key role in accurate wall stress calculations. An index, described as Regional Prestress Rupture Index (RPRI), indicates that regional variations are important for accurate rupture prediction. The knowledge of regional distribution of mechanical behaviour and accurate wall dynamics has potential to be employed to improve the durability and long term clinical performance of stent-grafts used for treating AAA. A novel and elegant approach to compute the damage of the aorta using cardiac gated CT image data is also presented. This technique can also be applied to analyse image data of patients with cardiovascular disease and is not limited to the abdominal aorta. The study quantified tissue damage due to aneurysm formation. Due to the high variations between individual patients, this technique may represent a method of analysing patient-specific changes. The results and conclusions presented through this thesis may further contribute to the understanding of AAA biomechanics and rupture potential, and in the future may help provide improved clinical guidance on surgical intervention for AAA treatment.