| dc.description.abstract | An occlusion in an artery (a stenosis) induces disturbances in the downstream flow and those disturbances produce mechanical waves that impinge on the vessel wall causing it to vibrate. These vibrations then travel through the soft tissues to the skin surface where they can be detected, thus providing a possible method for the non-invasive diagnosis of the underlying disease. Hence, in this study, the potential of measuring those disturbances was explored. Experiments were set-up to model the behaviour of carotid artery under different con-ditions. A 40:60 (by volume) glycerine-water solution was selected to simulate the vis-cosity of blood (approximately 4cP). A thin walled (250 μm - 350μm wall thickness) latex Penrose drain tube, with an external diameter of 6.35mm and a Young’s Modulus value of around 0.9MPa (for circumferential strains between 0.08 - 0.10), was used to mimic the carotid artery. Different severities (60%, 75% and 90% area reduction) of the stenoses, with a circular cross-section, were investigated. Two different types, axisym-metric and non-axisymmetric, were investigated to study the effects of asymmetry in the stenoses. The stenoses were 3D printed in an opaque VeroWhite material to mimic the occlusion. The stenosed artery was then embedded into a standardised neck phantom (filled with Parker Aquasonic 100 ultrasound gel) to mimic the soft tissues and the top of the phantom was sealed with a thin (50μm) polyurethane film (Platilon), with a stiffness value of approximately 21MPa, to simulate the skin. To detect the disturbance on the phantom surface, different equipment (including ac-celerometers, and a Laser Doppler Vibrometer [LDV]) were tested. The LDV proved to be the most reliable under all conditions and was chosen as the standard measurement method for all the phantom experiments. Having selected the appropriate materials and measurement techniques, the effects of flow rate, stenosis severity, stenosis symmetry and fluid viscosity were investigated. Both, steady and pulsatile flows were perfused through the phantom with flows ranging from 0-450ml/min for steady flow and 308-340ml/min for pulsatile flow. On modifying the viscosity of the fluid, it was seen that increasing the viscosity reduced the perturbations in the flow. This was expected due to the increased viscous forces in the flow, as the viscosity of the fluid increased. Furthermore, the experiments showed that, on increasing the flow rate, the stenosis severity, and/or introducing asymmetry in the stenosis, the post-stenotic perturbations in the flow were amplified and their zone of origin moved nearer to the stenosis. These features were confirmed by conducting bare tube experiments as well as some ultrasound scans in a modified phantom. On further investigation it was found that along with the positional dependence of these perturbations, their range of frequencies was increased with increasing flow rate, stenosis severity and/or stenosis asymmetry. In the phantom experiments the disturbances were barely detectable for an area reduc-tion of 60% and were weakly present at 75%. However, strong disturbances were seen for the highly (90%) stenosed tube. A possible cause of the unexpectedly small effect of the 75% stenosis was speculated to be the stenosis symmetry: in-vivo, atherosclerotic plaques are invariably not symmetrical. To show this, experiments were conducted with an asymmetric stenosis where higher level of disturbances were detected (even with the 75% stenosis severity), hence, emphasising the impact of stenosis symmetry. A preliminary computational simulation was also set-up to allow for future detailed modelling of the effects of changing the physical conditions on the signals arriving at the skin. The simulations (whose accuracy yet remains to be validated) showed similar effects of the increasing flow rates and the stenosis severity as the disturbances were amplified and moved nearer to the stenosis on increasing the value of either variable. Following this, an attempt was made to develop a fluid-structure interaction model to simulate the neck phantom and a sample simulation was set-up. This study developed a novel method for detecting the disturbances in the post-stenotic region, and the experimental results from this study suggest the feasibility of using LVD to infer the presence of a stenosis at an early stage before the symptoms are evident. | en_US |
