| Aerospace medical research has confirmed that astronauts had different physiological alterations in cardiac structure and function,including cephalic fluid shift,decreased heart output,cardiac atrophy,decreased heart rate and arrhythmia,due to the microgravity and/or simulated microgravity.When subsequent return to full gravity on the earth,they suffer orthostatic intolerance and syncope.Therefore,it is essential to investigate and evaluate the cardiac systolic function following microgravity and/or simulated microgravity.Due to the limited space for space experiment,ultrasound system,as the relatively small medical image equipment,is performed to detect the cardiac structure and function changes in the space capsule.Some studies reported that myocardial ultrasound elastography is a potential tool to evaluate myocardial systolic.Therefore,this study is aimed to investigate the effect of myocardial elastography on cardiac remodeling induced by simulated microgravity,to evaluate myocardial mechanical properties which reflect the contractile excitation conduction.Myocardial elastography may provide a new visual method to explore the mechanism of cardiac remodeling under microgravity and to assess the recovery of heart following rehabilitation training after spaceflight.Firstly,in order to mimic realistic hemodynamics of left ventricle(LV)under weightlessness,the fluid-structure coupled simulation of LV based on finite element method was performed.The fluid and structure model of LV was constructed from short-axis Magnetic Resonance images of heart of healthy male.The simulation results show that microgravity induces the changes of blood flow pattern,strain and stress distribution of LV wall,as well as the decrease of strain and stress in the middle and basal segments of LV.Then,since the accuracy of displacement computed by myocardial elastography is the key to quantitative evaluation of cardiac function,this study verified two elastography algorithm,Vectorized Normalized Cross-Correlation(VNCC)based elastography algorithm,and Dynamic Programming and Analytic Minimization(DPAM)based elastography algorithm,by using the simulated ultrasound radio frequency(RF)data.These RF data were obtained from a 2D simplified LV model,which was simulated the contraction of LV,and were created by the Field Ⅱ.Compared with the truth value of the axial of the LV model,the accuracy of axial displacement of both 6 segments and node points of VNCC based elastography algorithm is higher than that of DPAM elastography algorithm.Finally,in order to investigate the cardiac alterations to simulated microgravity,mice subjected to tail suspension for 28 days were used to mimics the redistribution of body fluid and hindlimb unloading.The structure and function alterations of heart induced by simulated microgravity were detected through VNCC based myocardial elastography,M-mode echocardiography,and the measurements of the images of hematoxylin-eosin(HE)staining.The results of the myocardial elastography suggest that the average axial displacement of LV basal segments significantly decreased.The M-mode recordings showed that the LV posterior wall thickness and LV mass significantly decreased.,but no pathological changes were found at the cellular level from the HE images.In conclusion,myocardial ultrasound elastography can provide a new method for astronauts to visualize and evaluate regional myocardial contractility.This study provides theoretical basis and methodological support for finding effective protective measures or medical intervention for cardiac remodeling induced by microgravity in the future,and is of great significance for the development of aerospace medical ultrasound. |
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