Assessment Of Risk Factors For The Formation Of Thoracic Aorta Dissection Based On In-vitro Fluid-structure Experiments

Aortic dissection is a rare but dangerous cardiovascular emergency.With the trend toward younger age for patients with cardiovascular and cerebrovascular diseases and the change of population age structure in China,aortic dissection patients increase yearly.The initiation and propagation of aortic dissection are closely associated with the alterations of hemodynamic environment,local abnormal vascular wall deformation,and the initial damages of the vessel wall.The study of aortic dissection by hydrodynamic experimental research is helpful for screening and early intervention for people at high risk of dissection and uncovering the interaction mechanisms between hemodynamics and vascular reconstruction.Therefore,a series of numerical and experimental studies on the interplays among flow,wall deformation,and inner wall damages in the patient-specific aortic models have been carried out based on clinical computed tomography(CT)data.The main innovative research results are as follows:The clinical CT images of 25 cases of various types of the aorta were collected.The aortic tomography structures were segmented and reconstructed using Scan IP,an image toolkit of SIMPLEWARE for three-dimensional(3D)imaging reconstruction,among which the AD models were repaired to the state before tearing.The centerline of each segment of blood vessels was extracted,and the length measurement was also performed.By analyzing the changes of centerline structure,it is found that the length,diameter,and tortuosity of ascending aorta and aortic arch in patients with Type A aortic dissection have significant distinctions from those of healthy subjects.Therefore,based on the centerlines of the healthy and aging aortic arch and the geometric parameters of the three-dimensional aortic reconstruction model,the aortic models at different development stages are constructed.The hemodynamic characteristics in the models are numerically analyzed using fluid simulation software Ansys Fluent.In the numerical calculation,a novel auxiliary calculation domain is applied at the front of the aortic sinus,and the Windkessel three-element model is used to simulate the changes of terminal impedance and compliance.The simulated changes of the numerical flow field are highly consistent with the measured one using four-dimensional magnetic resonance imaging under a similar morphology.The simulation results indicate that the elongation of the ascending aorta can change the flow pattern from helical flow to local vortex flow and make the wall shear stress(WSS)distribution near the ascending aorta wall be abnormal.At the same time,high oscillatory shear index(OSI)area is observed in the front of the brachiocephalic artery bifurcation and the inner side of the ascending aorta.These two abnormal OSI areas exactly correspond to the ones where tears frequently occur,suggesting that the morphological changes of ascending aorta may have a long-term hemodynamic effect on the formation of aortic dissection.A novel method named ‘brush-spin-coating’ has been developed for making flexible vascular phantom by using fluid boundary layer effect and 3D printing to study the morphological changes of the aortic structure under peripheral pressurization conditions.The method dramatically reduces the influence of gravity and surface tension on the silicone layer attached to the vascular inner core so that the thickness of the silicone phantom is easily constable without the effect of the diameter of the inner core.The fabricated aortic phantoms with relative thickness changes of 3.2% have been achieved via the method.The phantoms were connected to the experimental circulation loop to investigate the alterations of flow and lumen in the aortic arch when the terminal impedance increased.The PIV(particle imaging velocity)measurement results show that with the expansion and elongation of the ascending aorta due to the increase of terminal impedance,the helical flow pattern in the ascending aorta changes to a local vortex flow structure,which is consistent with the results of numerical research.At the same time,the axial contraction deformation of the inner wall in the aortic arch and descending aorta are observed to be more than 10.0%,which is much higher than that of the adjacent wall(2.0%).Peripheral pressurization also leads to the abnormal widening of the diameter in the middle of ascending aorta as well as the beginning of descending aorta with increasing rates of 13.0% and 14.5%,respectively.At the same time,the higher axial deformation is also observed outside of the two locations,and the maximum axial deformation ratio reaches 6.9%.The above results show that these specific locations on the aortic arch are more sensitive to the increases of terminal impedance.The morphological parameters of these locations can be used as potential risk factors for the risk assessment of aortic dissection.The effect of internal wall damages on dissection initiation and propagation is observed.A specific range of loosening interlayer adhesion and inner wall breach has been made in the wall of silicone phantom by brush-spin-coating.Through the in vitro stepwise pressurization experiment,the morphological changes of the damaged structure in the inner wall during pressurization are observed by laser-enhanced tracking and CT scanning.It is found that the silicone phantom simply with an inner wall breach will break directly after overload without developing into a dissection structure.However,when there is interlayer adhesion damage around the breach,the inner layer around the inner wall breach will move toward the lumen,and the outer layer will move outward under pressurization,leading to the formation of a false lumen structure.This shows that pressurization changes the balance of forces on both sides of the inner layer at the interlayer loosening,resulting in a false lumen structure propagation.The in-vitro tear-inducing experiment reveals that the interlayer injury between the inner wall is a critical factor for the triggering and propagation of dissection.This conclusion provides an essential experimental basis for early diagnosis of patients with a high risk of dissection from intramural hematoma patients or other patients with aortic wall thickening.