Simulation Studies On Doppler Ultrasound Signals In Abdominal Aortic Aneurysm

An abdominal aortic aneurysm (AAA) is a permanent focal dilatation of the abdominal aorta to 1.5 times its normal diameter. Although most AAA will not be ruptured, the ruptured AAA is a leading cause of sudden death. Thus, accurate evaluation of patients with AAA becomes a problem to be carefully studied. The purpose of this dissertation is trying to evaluate the patients with AAA by using the Doppler ultrasound technique. Through simulation experiments, the relationship between the characteristics of the Doppler ultrasound blood flow signals and the hemodynamic factors which influence on the development and rupture of AAA was built up, which makes the evaluation of patients with AAA more reliable.Disturbed flow conditions that develop inside AAA are contributed in various ways to the enlargement of AAA, so disturbed flow conditions can be used to access the development of AAA. In this dissertation, we built a complex Doppler ultrasound blood flow signal simulation platform which combined the computational fluid dynamics simulation results. By using this technique, the complex blood flow and the related Doppler ultrasound blood flow signal in various areas of different idealized AAA models were simulated. Two parameters derived from the Doppler ultrasound blood flow signals were proposed to evaluate the disturbed blood flow, the oscillating frequency index (OFI) and the averaged transient oscillating frequency index (TOFI). The simulation results showed that these two parameters worked well in evaluating the disturbed flow. This method was also validated by using a reconstructed AAA model from a real clinical case.The wall shear stress (WSS) directly acts on endothelial cells. The abnormal WSS will lead to endothelial cells dysfunction, which plays an important role in the formation and development of AAA. Thus the WSS related parameters can be used to reflect the AAA conditions. In this dissertation, the stress distributions in different AAA models were investigated by using the Fluid Structure Interaction (FSI). The Doppler ultrasound blood flow signal in the regions of interest were also simulated and carefully analyzed. We proposed two parameters to access the time averaged wall shear stress (TAWSS) and oscillatory shear index (OSI). The correlation analysis showed that the proposed parameters can qualitatively characterize the TAWSS and OSI, and it may be a new solution for non-contact accessing of the shear stress. This method was also validated by using the reconstructed AAA model from a real clinical case. Results showed that the proposed parameters worked well in the complex condition.In this dissertation, two special problems about the stent graft in the endovascular abdominal aortic aneurysm repair (EVAR) were also investigated. The impact of different types of bare-metal stents on the shear stress distribution was investigated by using the virtual-implant simulation method. Results showed that on the premise of fixation firmly, the less the number of the’V unit, the shorter the path length of bare-metal stents and the smaller the WSS rapid-changing area. Comparing the WSS and WSS gradient, we found that there were no notable differences between the type’A’and type’B’structure. The computation fluid dynamics simulation results showed that the size of the endoleak area had significant impact on the blood flow conditions in AAA when the type I endoleak existed at the proximal end. Thus, we proposed two parameters derived from the Doppler ultrasound blood flow signals to evaluate the severity of the endoleak. Simulation results showed that these two parameters would increase as the size of the leakage area increase.