In order for computational fluid dynamics to provide quantitative parameters to aid in the clinical assessment of type B aortic dissection, the results must accurately mimic the hemodynamic environment within the aorta. The choice of inlet velocity profile (IVP) therefore is crucial; however, idealised profiles are often adopted, and the effect of IVP on hemodynamics in a dissected aorta is unclear. This study examined two scenarios with respect to the influence of IVP—using (a) patient-specific data in the form of a three-directional (3D), through-plane (TP) or flat IVP; and (b) non-patient-specific flow waveform. The results obtained from nine simulations using patient-specific data showed that all forms of IVP were able to reproduce global flow patterns as observed with 4D flow magnetic resonance imaging. Differences in maximum velocity and time-averaged wall shear stress near the primary entry tear were up to 3% and 6%, respectively, while pressure differences across the true and false lumen differed by up to 6%. More notable variations were found in regions of low wall shear stress when the primary entry tear was close to the left subclavian artery. The results obtained with non-patient-specific waveforms were markedly different. Throughout the aorta, a 25% reduction in stroke volume resulted in up to 28% and 35% reduction in velocity and wall shear stress, respectively, while the shape of flow waveform had a profound influence on the predicted pressure. The results of this study suggest that 3D, TP and flat IVPs all yield reasonably similar velocity and time-averaged wall shear stress results, but TP IVPs should be used where possible for better prediction of pressure. In the absence of patient-specific velocity data, effort should be made to acquire patient’s stroke volume and adjust the applied IVP accordingly.
False lumen thrombosis (FLT) in type B aortic dissection has been associated with the progression of dissection and treatment outcome. Existing computational models mostly assume rigid wall behavior which ignores the effect of flap motion on flow and thrombus formation within the FL. In this study, we have combined a fully coupled fluid–structure interaction (FSI) approach with a shear-driven thrombosis model described by a series of convection–diffusion reaction equations. The integrated FSI-thrombosis model has been applied to an idealized dissection geometry to investigate the interaction between vessel wall motion and growing thrombus. Our simulation results show that wall compliance and flap motion can influence the progression of FLT. The main difference between the rigid and FSI models is the continuous development of vortices near the tears caused by drastic flap motion up to 4.45 mm. Flap-induced high shear stress and shear rates around tears help to transport activated platelets further to the neighboring region, thus speeding up thrombus formation during the accelerated phase in the FSI models. Reducing flap mobility by increasing the Young’s modulus of the flap slows down the thrombus growth. Compared to the rigid model, the predicted thrombus volume is 25% larger using the FSI-thrombosis model with a relatively mobile flap. Furthermore, our FSI-thrombosis model can capture the gradual effect of thrombus growth on the flow field, leading to flow obstruction in the FL, increased blood viscosity and reduced flap motion. This model is a step closer toward simulating realistic thrombus growth in aortic dissection, by taking into account the effect of intimal flap and vessel wall motion.
Purpose: To report a study that assesses the influence of the distance between the distal end of a thoracic stent-graft and the first reentry tear (SG-FRT) on the progression of false lumen (FL) thrombosis in patients who underwent thoracic endovascular aortic repair (TEVAR). Materials and Methods: Three patient-specific geometrical models were reconstructed from postoperative computed tomography scans. Two additional models were created by artificially changing the SG-FRT distance in patients 1 and 2. In all 5 models, computational fluid dynamics simulations coupled with thrombus formation modeling were performed at physiological flow conditions. Predicted FL thrombosis was compared to follow-up scans. Results: There was reduced false lumen flow and low time-averaged wall shear stress (TAWSS) in patients with large SG-FRT distances. Predicted thrombus formation and growth were consistent with follow-up scans for all patients. Reducing the SG-FRT distance by 30 mm in patient 1 increased the flow and time-averaged wall shear stress in the upper abdominal FL, reducing the thrombus volume by 9.6%. Increasing the SG-FRT distance in patient 2 resulted in faster thoracic thrombosis and increased total thrombus volume. Conclusion: The location of reentry tears can influence the progression of FL thrombosis following TEVAR. The more distal the reentry tear in the aorta the more likely it is that FL thrombosis will occur. Hence, the distal landing zone of the stent-graft should be chosen carefully to ensure a sufficient SG-FRT distance.
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