Particle image velocimetry measurements of blood flow in aneurysms using 3D printed flow phantoms

Abstract

Cardiovascular diseases (CVD) remain one of the leading causes of deaths worldwide. The formation and presence of aneurysm is a very important question in the study of this CVDs. An aneurysm is a balloon-like bulge on a blood vessel which forms over time. An aneurysm is usually considered to be a result of weakening of the blood vessel walls, this definition has stood over many years without being conclusively proven. Eventually, the aneurysm could clot or burst due to degradation of the aneurysm wall and accumulation of blood. The latter would lead to internal bleeding and result in a stroke. Local hemodynamics have been found to be very important in the study of the evolution of an aneurysm. In this study, a steady flow experimental investigation was conducted using planar Particle Image Velocimetery (PIV) on a rigid flow phantom of an idealised geometry consisting of a curve parent artery and a spherical aneurysm located on the outer convex side of the curvature. The flow phantom was fabricated directly using a commercially available desktop Stereolithography (STL) 3D printer instead of the more conventional investment casting method using a core. Although 3D printing technologies have been around for many years, the fabrication of flow phantoms by direct printing is still largely under-explored. This thesis details the results of investigation into the optimal printing and post-printing procedures required to produce a flow phantom of suitable clarity and transparency. Other important areas of concern such as the geometric accuracy, surface topography and refractive index of the final model are also investigated. A planar PIV is conducted to study the impact of flow rates on the local flow field in and around the aneurysm and their impact on the wall shear stress. It was found that direct 3D printing is appropriate for the fabrication of flow phantoms suitable for PIV or other flow visualisation techniques. It reduces the complexities and time needed compared to the conventional investment casting methods. It was observed that the optical properties of the printed material such as the high refractive index (RI) and the transmittivity of light could cause a problem in large models. From the PIV measurements it was found that flow rates affect the flow field in both the parent artery and the aneurysm. First, high velocities were observed on the outer curvature of the parent artery. Secondly the centre of rotation in the aneurysm is not at the geometric centre but is displaced slightly in the direction of the flow. Finally, the flow rate affects the angle in which flow enters the aneurysm from the parent vessel. This change in the flow angle affects the flow within the aneurysm. A higher flow rate in the parent artery increases the incident angle which brings the centre of rotation closer to the geometric centre of the aneurysm, this changes the location and magnitude of high velocities and hence the local wall shear stress (WSS) on the wall of the aneurysm. This may have implications in the evolution of aneurysms.