Youjun Liu

Numerical study on thermal field of microwave ablation with water-cooled antenna

Purpose: This research was to reveal the thermal characteristics of a water-cooled microwave ablation antenna in phantom, and the influence of cooling water velocity on ablation pattern by numerical method. In addition, by comparing the numerical results with experimental results, the experimentally obtained SAR was proven to be correct. Methods: The temperature distribution in ablations was simulated by the finite element method. In the FEM, the cooling effect in the region of the water-cooled antenna was introduced by applying the convective coefficient to the related boundaries, and the experimental determined SAR in phantom was applied as heat generation of microwave generator. To study the effect of water flow rate on ablation pattern, three different water velocities were chosen. In addition, the phantom thermal properties changes were considered in simulation when the heating temperature was above 80°C. Results: The ablation pattern could be identified as a pear shape. The temperatures of monitoring points which were located near the antenna could rise more rapidly, and they were more likely to achieve steady heat transfer state within a short time compared with those far away from the antenna. The cooling water effectively decreased the temperature near the antenna, an under-temperature occurred in the cooling region, and the different cooling water flow rate did not affect significantly the ablation pattern. Conclusions: The numerical results compared reasonably well with the experimental results in both heating pattern and temperature of individual monitoring point over the same heating duration. The SAR measured by our previous experiment was also confirmed by this numerical simulation. This method could be employed to study combination thermal field of multi-antennas in future work.

Impact of Competitive Flow on Hemodynamics in Coronary Surgery: Numerical Study of ITA-LAD Model

Competitive flow from native coronary artery is considered as a major factor in the failure of the coronary artery bypass grafts. However, the physiological effects are not very clear. The aim is to research the impact of competitive flow caused by different left anterior descending (LAD) artery stenosis degrees on hemodynamics in internal thoracic artery (ITA) bypass graft. An idealized ITA-LAD model was built in CAD tools. The degree of the competitive flow was divided into five classes according to different LAD stenosis degrees: higher (no stenosis), secondary (30% stenosis), reduced (50% stenosis), lower (75% stenosis) and no competitive flow (fully stenosis). Finite volume method was employed for the numerical simulation. The flow velocity distributions, wall shear stress and oscillatory shear index were analyzed. Results showed that higher competitive flow in the bypass graft would produce unbeneficial wall shear stress distribution associating with endothelial dysfunction and subsequent graft failure. The coronary bypass graft surgery was preferred to be carried out when the LAD stenosis was higher than 75%.

Hemodynamics of the string phenomenon in the internal thoracic artery grafted to the left anterior descending artery with moderate stenosis

PURPOSEnThe internal thoracic artery is the choice of graft for coronary artery bypass grafting due to the excellent long-term patency. However internal thoracic artery graft failures still occur due to diffuse narrowing, known as the string phenomenon. Studies suggest that the string phenomenon is caused by competitive flow when the coronary stenosis is not serious, but the hemodynamics of the string phenomenon are still unclear. The purpose of this study is to clarify the hemodynamic characteristics of the string phenomenon.nnnMATERIALSnA patient-specific 3-dimensional model of the aortic arch and coronary arteries was reconstructed. A moderate stenosis was applied to the left anterior descending artery. The internal thoracic artery was used to bypass the stenosis. Two further 3D models were built to study the hemodynamics of the string phenomenon.nnnMETHODSnA numerical study was performed by coupling the 3D artery model with 0-dimensional lumped parameter model of the cardiovascular system.nnnRESULTSnThe graft flow, native coronary flow, wall shear stress and oscillatory shear index were calculated and illustrated. Inverse flow and high oscillatory shear index appeared on the internal thoracic artery graft when the stenosis was moderate.nnnCONCLUSIONnHigh oscillatory shear index might be the major hemodynamic characteristic of the string phenomenon in internal thoracic artery graft. The inverse graft flow and the difference in graft flow caused by clamping the stenosis can be used to evaluate the probability of observing the string phenomenon.

HEMODYNAMIC EFFECTS OF THE ANASTOMOSES IN THE MODIFIED BLALOCK–TAUSSIG SHUNT: A NUMERICAL STUDY USING A 0D/3D COUPLING METHOD

The modified Blalock–Taussig (BT) shunt is a palliative surgery which can help the tetralogy of Fallot (TOF) patient increase the blood oxygen saturation by interposing a systemic-to-pulmonary artery shunt. Two typical anastomotic shapes are frequently used in clinical practice: the end-to-side (ETS) and the side-to-side (STS) anastomosis. This paper examines the hemodynamic influence of the anastomotic shape in the modified BT shunt. Three models with different anastomotic shapes were reconstructed. The ETS anastomoses were applied in the first model. For the innominate artery (IA) and the pulmonary artery (PA) in the second model, the ETS and the STS anastomosis were applied, respectively. Finally, the STS anastomoses were applied in the third model. The 0D/3D coupling method was used to perform a numerical simulation by coupling the three-dimensional (3D) artery model with a zero-dimensional (0D) lumped parameter model for the cardiovascular system. The simulation results showed that the perfusion into the left and right PA in Model 1 was unbalanced. Swirling flow appeared in the shunt in Model 3, but the shunt flow rate in Model 3 was lower. The ETS anastomosis at the PA may cause unbalanced blood perfusion into the left and right PA. Conversely, the STS anastomosis can make the blood perfusion more balanced. Otherwise, the STS anastomosis at the IA could generate a swirling flow in the shunt which may provide a better hemodynamic environment while decreasing the pulmonary perfusion.

COMPUTATIONAL FLUID DYNAMICS OF TWO PATIENT-SPECIFIC SYSTEMIC TO PULMONARY SHUNTS

Tetralogy of Fallot is the most common cyanotic congenital heart defect. For severe cases, inserting a systemic to pulmonary shunt, which distributes part of systemic artery blood into the pulmonary artery, is the preferable palliative surgery. Based on the computed tomography images and three-dimensional geometry technologies, two patient-specific anatomical options of systemic to pulmonary shunts including the aorta to pulmonary shunt (APS) and innominate artery to pulmonary shunt (IPS) have been simulated for computational fluid dynamics. The objective of this study was to predict the hemodynamics within the shunts and confirm, through patient-specific simulations, the shunt with the optimal performance. Results indicated that both options created high velocity gradients and pressure gradients at the proximal end of the shunts. Obvious flow recirculation appeared at the inner region near the proximal end of the shunts. Part of the reverse flow from the descending aorta, left subclavian artery, left carotid artery and innominate artery was driven into the shunts during the diastolic period. The IPS provided better balanced and more adequate blood flow distributions between the systemic and pulmonary circulations. The APS provided slightly excessive pulmonary blood flow which can ultimately result in cardiac failure and pulmonary hypertension.

Hemodynamic Influence of Different Pulmonary Stenosis Degree in Glenn Procedure: A Numerical Study

Background. Single ventricle disease is treated by Glenn surgery. It is generally accompanied by stenosis on a pulmonary artery or its branches, which has great effect on hemodynamics. This study investigated the hemodynamic influence of different pulmonary stenosis degree in Glenn procedure. Materials. Four three-dimensional Glenn models with different left pulmonary artery stenosis rates as, respectively, 0% (model 1), 25% (model 2), 50% (model 3), and 75% (model 4) by the diameter were generated. Method. Geometric multiscale analysis method was used in the numerical simulations by coupling the lumped parameter model (LPM) and three-dimensional model. Results. During one cardiac cycle, the flow ratio between left pulmonary artery and superior vena cava was about 0.49 for models 1, 2, and 3, while the ratio decreased to 0.34 for model 4. On the other hand, hemodynamics parameters like power loss and oscillation shear index show complications of the stenosis to the postoperative development. Conclusion. When the stenosis rate is above 75%, it is suggested to treat stenosis before Glenn procedure, while when the stenosis rate is below 50%, there is no necessity to pay attention to it due to the little effect it makes.

HEMODYNAMICS-BASED LONG-TERM PATENCY OF DIFFERENT SEQUENTIAL GRAFTING: A PATIENT-SPECIFIC MULTI-SCALE STUDY

Background and aims: Sequential grafting is one of the common coronary artery bypass grafting (CABG) surgery. But the influence of the sequential grafting position on hemodynamics and the graft patency is still unclear. Materials and methods: The zero-dimensional/three-dimensional (0D/3D) coupling method was used to finalize the multi-scale simulation of two different sequential grafting models. First, a patient-specific 3D model was reconstructed based on coronary computed tomography angiography (CCTA) images. Two different sequential grafts were implemented on this patient-specific 3D model by using virtual surgery. Thus, two different postoperative 3D models were built. Then, a lumped parameter model (LPM; 0D) was built based on the patient physiological data to simulate the cardiovascular system. Finally, the 0D/3D coupling method was used to perform the numerical simulation by coupling a 0D LPM of the cardiovascular system and the patient-specific 3D models. Moreover, the long-term patency of these two different sequential grafts was discussed in this paper. Results: The coronary flow rate and the graft flow were calculated and illustrated. The instantaneous wave-free ratio (iFR) were calculated. Postoperative iFR values increase to over 0.90 for both sequential grafts. Some hemodynamics parameters were also illustrated, such as wall shear stress (WSS), oscillatory shear index (OSI). The area of low WSS in Model 1 was much less than that in Model 2. Two regions of high OSI exist in Model 2, while only one in Model 1. Conclusions: No significant differences exist on the short-term outcomes of two models. But the long-term patency of Model 2 was worse. The Model 1 may enhance long-term patency of grafting and should be priority when the sequential grafting need to be carried out.

HEMODYNAMIC COMPARISON BETWEEN NORMAL GRAFT AND Y-TYPE GRAFT IN CORONARY ARTERY BYPASS GRAFTING: A NUMERICAL STUDY USING 0D/3D COUPLING METHOD

Different surgical procedures in coronary artery bypass grafting (CABG) will cause different hemodynamic environments, which will affect the surgical result and long-term patency. In this study, the hemodynamic effect of the Y-type graft was discussed by comparing it with normal graft. A patient-specific 3-dimensional (3D) model of coronary artery with serious stenosis was reconstructed. Two different surgical procedures were applied on the 3D model through the virtual surgery, the normal graft and the Y-type graft. Thus, two 3D models with different grafts were built. The zero-dimensional (0D)/3D coupling method was used to perform the numerical simulation by coupling the 3D artery model with a 0D lumped parameter model of the cardiovascular system. The coronary flow rate and the graft flow were calculated and illustrated in this paper. The wall shear stress (WSS) and oscillatory shear index (OSI) were also calculated and depicted. The short-term outcomes of the normal graft and Y-type graft are almost the same. But the long-term patency of the Y-type graft is worse. Moreover, decreasing the diameter of the graft to LCX may get higher WSS and provide enough flow at the same time.

IMPACT OF COMPETITIVE FLOW ON HEMODYNAMICS OF LIMA-LAD GRAFTING WITH DIFFERENT STENOSIS: A NUMERICAL STUDY

Background and Aims: Competitive flow exists in the native coronary artery when using artery graft in the coronary artery bypass graft (CABG), and it is a major factor affecting the long-term patency of the arterial grafts. The purpose of this study is to investigate the effects of the competitive flow on hemodynamic characteristics of left internal mammary artery (LIMA). Materials and methods: Seven three-dimensional (3D) LIMA- left anterior decending (LAD) grafting models with different diameter stenosis at the trunk of LAD were reconstructed, including 25%, 40%, 50%, 60%, 75%, 90% and 100%. The boundary conditions of these models were provided by the same lumped parameter model (LPM; 0D sub-model) of cardiovascular circulatory system, which is based on 0D/3D coupling method. Results: The simulation results show that competitive flow decreases when the stenosis rate increases and backflow exists in LIMA during systole when the degree of stenosis is below 50%; the wall shear stress (WSS) of the graft increases from 0.85Pa to 1.79Pa with the increase of the native coronary stenosis degree, while the oscillatory shear index (OSI) decreases from 0.1557 to 0.00213. Conclusions: Competitive flow may produce adverse hemodynamic effects on artery graft, especially when the stenosis degree is below 50%, competitive flow will significantly lead the long-term patency of the artery graft worse because of the backflow, low WSS and high OSI, and the effect of OSI is more obvious than WSS.

Hemodynamic analysis of sequential graft from right coronary system to left coronary system

BackgroundSequential and single grafting are two surgical procedures of coronary artery bypass grafting. However, it remains unclear if the sequential graft can be used between the right and left coronary artery system. The purpose of this paper is to clarify the possibility of right coronary artery system anastomosis to left coronary system.MethodsA patient-specific 3D model was first reconstructed based on coronary computed tomography angiography (CCTA) images. Two different grafts, the normal multi-graft (Model 1) and the novel multi-graft (Model 2), were then implemented on this patient-specific model using virtual surgery techniques. In Model 1, the single graft was anastomosed to right coronary artery (RCA) and the sequential graft was adopted to anastomose left anterior descending (LAD) and left circumflex artery (LCX). While in Model 2, the single graft was anastomosed to LAD and the sequential graft was adopted to anastomose RCA and LCX. A zero-dimensional/three-dimensional (0D/3D) coupling method was used to realize the multi-scale simulation of both the pre-operative and two post-operative models.ResultsFlow rates in the coronary artery and grafts were obtained. The hemodynamic parameters were also showed, including wall shear stress (WSS) and oscillatory shear index (OSI). The area of low WSS and OSI in Model 1 was much less than that in Model 2.ConclusionsModel 1 shows optimistic hemodynamic modifications which may enhance the long-term patency of grafts. The anterior segments of sequential graft have better long-term patency than the posterior segments. With rational spatial position of the heart vessels, the last anastomosis of sequential graft should be connected to the main branch.