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The Acoustics of Cerebral Aneurysms


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Authors

Lamarquette, Amélie 

Abstract

Stroke is not only the second leading cause of death worldwide, accounting for several million deaths per year, but also a leading factor of disability. Ruptured aneurysms are the most common cause of spontaneous subarachnoid haemorrhage, a devastating type of stroke. Aneurysms are localised expansions of blood vessels. Brain aneurysms most often form a bulge at a blood vessel bifurcation. The possible outcomes of ruptured aneurysms are disastrous, with a high death rate. The rupture of aneurysms is a complex phenomenon resulting from damage to walls of the blood vessel. Heamodynamics and vibrations are thought to be involved in the degradation of blood vessel structure. A sound, called bruit, is sometimes produced by unruptured brain aneurysms, which is a sign that these cerebral lesions are subject to wall vibrations. The aim of this thesis is to provide a better understanding of aneurysm sounds as they could be a crucial source of information to understand the reasons behind aneurysm rupture. Here we present an acoustic experiment investigating the source of aneurysm sound. In our simplified geometry, we identify that self-excited flow oscillations can generate sound at a distinct frequency. Periodic flow fluctuations have been observed in aneurysms by various studies. However, the potential of flow oscillations to cause aneurysm wall vibrations (hence sound) had not yet been proven. In our acoustic experiment we observe that, at critical flow rates, the frequency of the self-excited flow structures matches the structural natural frequency of the aneurysm model, leading to a resonance. We propose a theoretical model to predict the structural natural frequencies of our simplified model. Previously, the hypothesis that aneurysms vibrate at their preferred frequencies had been subject to debate as some researchers suggested that aneurysms are in fact stable (i.e. aneurysms do not vibrate at their natural frequency). This thesis constitutes an experimental proof that aneurysm are subject to resonance under certain conditions. More research is needed to link the mechanisms observed in this thesis and the rupture of aneurysms but, knowing if a cerebral lesion is subject to self-excited vibrations or resonance could be a valuable information in the estimation of the rupture risk, which in turn could help surgeons make more informed decisions.

Description

Date

2021-09-30

Advisors

Agarwal, Anurag

Keywords

aneurysm, cerebral, saccular, instability, self-excited, acoustics, flow, oscillations, vibrations, fluid mechanics

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
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