Numerical analysis of dynamic temperature in response to different levels of reactive hyperaemia in a three-dimensional image-based hand model

Vascular reactivity (VR) is considered as an effective index to predict the risk of cardiovascular events. A cost-effective alternative technique used to evaluate VR called digital thermal monitoring (DTM) is based on the response of finger temperature to vessel occlusion and reperfusion. In this work, a simulation has been developed to investigate hand temperature in response to vessel occlusion and perfusion. The simulation consists of image-based mesh generation and finite element analysis of blood flow and heat transfer in tissues. In order to reconstruct a real geometric model of human hand, a computer programme including automatic image processing for sequential MR data and mesh generation based on the transfinite interpolation method is developed. In the finite element analysis part, blood flow perfused in solid tissues is considered as fluid phase through porous media. Heat transfer in tissues is described by Pennes bioheat equation and blood perfusion rate is obtained from Darcy velocities. Capillary pressure, blood perfusion and temperature distribution of hand are obtained. The results reveal that fingertip temperature is strongly dependent on larger arterial pressure. This simulation is of potential to quantify the indices used for evaluating the VR in DTM test if it is integrated with the haemodynamic model of blood circulation in upper limb.     

椎动脉狭窄内血液两相流动数值模拟分析

目的:对应用三维重构得到的人体真实椎动脉进行血液两相流数值模拟,与经典单相流牛顿血液模型对比,分析动脉粥样硬化等病因与椎动脉狭窄处的血流动力学关系。方法:把考虑血细胞和血浆的两相流血液模型应用到逆向工程方法构建的基于人体生理解剖特征的椎动脉三维几何模型中去进行数值模拟,分析血细胞分布情况等血流动力学参数,并与单相流模型的模拟结果进行对比。结果:通过瞬态模拟计算,得到了椎动脉在心动周期内不同时刻的两相流和单相流模型的血流动力学参数。结论:通过对比单相流数值模拟结果,得出血管狭窄处血细胞出现聚集,血流更加复杂和低壁面切应力分布等与动脉粥样硬化及血栓的形成相关的结论。并且与两相流模型相比,单相流模型存在如无法获得如血细胞分布等不足,为进一步深入研究椎动脉等疾病的发病机理提供方法和理论支持。     

Computational analysis of blood flow in an integrated model of the left ventricle and the aorta

To study the effects of intraventricular flow dynamics on the aortic flow, we created an integrated model of the left ventricle and aorta and conducted a computer simulation of diastolic and systolic blood flow within this model. The results demonstrated that the velocity profile at the aortic annulus changed dynamically, and was influenced by the intraventricular flow dynamics. The profile was almost flat in early systole but became nonuniform as systole progressed, and was skewed toward the posterior side in midsystole and toward the anterior side in later systole. At a distance from the aortic annulus, a different velocity profile was induced by the twisting and torsion of the aorta. In the ascending aorta, the fastest flow was initially located in the posteromedial sector, and it moved to the posterior section along the circumference as systole progressed. The nonuniformity of the aortic inflow gave rise to a complex wall shear stress (WSS) distribution in the aorta. A comparison of the WSS distribution obtained in this integrated analysis with that obtained in flow calculations using an isolated aorta model with Poiseuille and flat inlet conditions showed that intraventricular flow affected the WSS distribution in the ascending aorta. These results address the importance of an integrated analysis of flow in the left ventricle and aorta.     

Numerical simulation of the flow field and mass transport pattern within the coronary artery

In this study, the steady and pulsatile flow field with mass transport analysis in an anatomically correct model of coronary artery is simulated numerically using a specific patient data from a 64-multislice computed tomography scanner. It is assumed that the blood flow is laminar and that the Navier-Stokes equations of motion are applied. Downstream of the bifurcation, a strong skewing occurs towards the flow divider walls as a result of branching. For the low-density lipoprotein (LDL) transport analysis where a specific boundary condition at the arterial walls is applied, LDL is generally elevated at locations where shear stress distribution is low, but it does not co-locate at whole domain. This numerical simulation gives an insight, as well as detailed quantitative data, of haemodynamic conditions in the left coronary artery as well as mass transfer patterns for a specific patient.     

Determination of the 3D temperature distribution during ferromagnetic hyperthermia under the influence of blood flow

A numerical simulation of tissue heating during thermo-seed ferromagnetic hyperthermia was performed to determine the temperature distribution of treated tumor tissues under the influence of three large blood vessels at different locations. The effects of the blood velocity waveform, blood vessel size, Curie point of the thermo-seeds and the thermo-seed number on temperature distributions were analyzed. The results indicate that the existence of a blood vessel inside the tumor has a significant cooling effect on the temperature distribution in a treated tumor tissue, which is enhanced with an increase in blood velocity. However, the pulsatile blood flow does not have apparently different effects on the outcomes of uniformly heating target tissues in comparison with the steady blood flow during the hyperthermia process. It is also concluded that a higher Curie point temperature and an increase in the number of thermo-seeds can result in profound increases in the temperature variations of the tumor tissue. In addition, tissue-equivalent phantom experiments were conducted to confirm the cooling effects of the blood vessels, and to validate the effectiveness and accuracy of the proposed heat transfer model for the ferromagnetic hyperthermia.     

A class of flow bifurcation models with lognormal distribution and fractal dispersion

We report a quantitative analysis of a simple dichotomous branching tree model for blood flow in vascular networks. Using the method of moment-generating function and geometric Brownian motion from stochastic mathematics, our analysis shows that a vascular network with asymmetric branching and random variation at each bifurcating point gives rise to an asymptotic lognormal flow distribution with a positive skewness. The model exhibits a fractal scaling in the dispersion of the regional flow in the branches. Experimentally measurable fractal dimension of the relative dispersion in regional flow is analytically calculated in terms of the asymmetry and the variance at local bifurcation; hence the model suggests a powerful method to obtain the physiological information on local flow bifurcation in terms of flow dispersion analysis. Both the fractal behavior and the lognormal distribution are intimately related to the fact that it is the logarithm of flow, rather than flow itself, which is the natural variable in the tree models. The kinetics of tracer washout is also discussed in terms of the lognormal distribution.     

Comparative study of Newtonian and non-Newtonian simulations of drug transport in a model drug-eluting stent

To elucidate the difference between Newtonian and shear thinning non-Newtonian assumptions of blood in the analysis of DES drug delivery, we numerically simulated the local flow pattern and the concentration distribution of the drug at the lumen-tissue interface for a structurally simplified DES deployed in a curved segment of an artery under pulsatile blood flow conditions. The numerical results showed that when compared with the Newtonian model, the Carreau (shear thinning) model could lead to some differences in the luminal surface drug concentration in certain areas along the outer wall of the curved vessel. In most areas of the vessel, however, there were no significant differences between the 2 models. Particularly, no significant difference between the two models was found in terms of the area-averaged luminal surface drug concentration. Therefore, we believe that the shear thinning property of blood may play little roles in DES drug delivery. Nevertheless, before we draw the conclusion that Newtonian assumption of blood can be used to replace its non-Newtonian one for the numerical simulation of drug transport in the DES implanted coronary artery, other more complex mechanical properties of blood such as its thixotropic behavior should be tested.     

Temperature evolution in tissues embedded with large blood vessels during photo-thermal heating

During laser-assisted photo-thermal therapy, the temperature of the heated tissue region must rise to the therapeutic value (e.g., 43 °C) for complete ablation of the target cells. Large blood vessels (larger than 500 micron in diameter) at or near the irradiated tissues have a considerable impact on the transient temperature distribution in the tissue. In this study, the cooling effects of large blood vessels on temperature distribution in tissues during laser irradiation are predicted using finite element based simulation. A uniform flow is assumed at the entrance and three-dimensional conjugate heat transfer equations in the tissue region and the blood region are simultaneously solved for different vascular models. A volumetric heat source term based on Beer–Lambert law is introduced into the energy equation to account for laser heating. The heating pattern is taken to depend on the absorption and scattering coefficients of the tissue medium. Experiments are also conducted on tissue mimics in the presence and absence of simulated blood vessels to validate the numerical model. The coupled heat transfer between thermally significant blood vessels and their surrounding tissue for three different tissue-vascular networks are analyzed keeping the laser irradiation constant. A surface temperature map is obtained for different vascular models and for the bare tissue (without blood vessels). The transient temperature distribution is seen to differ according to the nature of the vascular network, blood vessel size, flow rate, laser spot size, laser power and tissue blood perfusion rate. The simulations suggest that the blood flow through large blood vessels in the vicinity of the photothermally heated tissue can lead to inefficient heating of the target.     

A Simulation of the Flight Characteristics of the Deployable Hindwings of Beetle

An insect is an excellent biological object for the bio-inspirations to design and develop a MAV.This paper presents the simulation study of the flight characteristics of the deployable hindwings of beetle,Dorcustitanus platymelus.A 3D geometric model of the beetle was obtained using a 3D laser scanning technique.By studying its hindwings and flight mechanism,the mathematical model of the flapping motion of its hindwings was analyzed.Then a simulation analysis was carried out to analyze and evaluate the flapping flying aerodynamic characteristics.After that,the flow of blood in the hindwing veins was studied through simulation to determine the maximum pressure on a vein surface and the minimum blood flow in flight.A number of interesting bio-inspirations were obtained.It is believed that these findings can be used for the design and development of a MAV with similar flying capabilities to a natural beetle.     

Non-Newtonian Blood Flow in Left Coronary Arteries with Varying Stenosis: A Comparative Study

This paper presents Computational fluid dynamic (CFD) analysis of blood flow in three different 3-D models of left coronary artery (LCA). A comparative study of flow parameters (pressure distribution, velocity distribution and wall shear stress) in each of the models is done for a non-Newtonian (Carreau) as well as the Newtonian nature of blood viscosity over a complete cardiac cycle. The difference between these two types of behavior of blood is studied for both transient and steady states of flow. Additionally, flow parameters are compared for steady and transient boundary conditions considering blood as non-Newtonian fluid. The study shows that the highest wall shear stress (WSS), velocity and pressure are found in artery having stenosis in all the three branches of LCA. The use of Newtonian blood model is a good approximation for steady as well as transient blood flow boundary conditions if shear rate is above 100 s-1. However, the assumption of steady blood flow results in underestimating the values of flow parameters such as wall shear stress, pressure and velocity.     

Modelling hand skin temperature in relation to body composition

Skin temperature is a challenging parameter to predict due to the complex interaction of physical and physiological variations. Previous studies concerning the correlation of regional physiological characteristics and body composition showed that obese people have higher hand skin temperature compared to the normal weight people. To predict hand skin temperature in a different environment, a two-node hand thermophysiological model was developed and validated with published experimental data. In addition, a sensitivity analysis was performed which showed that the variations in skin blood flow and blood temperature are most influential on hand skin temperature. The hand model was applied to simulate the hand skin temperature of the obese and normal weight subgroup in different ambient conditions. Higher skin blood flow and blood temperature were used in the simulation of obese people. The results showed a good agreement with experimental data from the literature, with the maximum difference of 0.31 °C. If the difference between blood flow and blood temperature of obese and normal weight people was not taken into account, the hand skin temperature of obese people was predicted with an average deviation of 1.42 °C. In conclusion, when modelling hand skin temperatures, it should be considered that regional skin temperature distribution differs in obese and normal weight people.     

Integration of geometric separation mechanisms by implementing curved constrictions in a biochip microchannel fluidic separator

This paper investigates the effectiveness of using curved constrictions in the bifurcation region of T-type fluid separators for promoting flow development in the intervals between bifurcations. A design of biofluid separator is proposed and a mathematical analysis and a numerical simulation of the blood flow in microchannels are conducted. The design is based on a modification of an existing T-shaped biochip device which consists of a main channel and a series of perpendicularly positioned side channels. By means of bifurcation effect, the blood is separated into plasma concentration flow from the side channels and blood cell concentration flow from the main channel. In this design, curved constrictions are inserted between bifurcations to replace the original straight channel section, so that the constriction and curved channel effects can be induced apart from the existing bifurcation effect. The mathematical analysis is aimed to the flow field and shear stress of the blood fluid in the microchannel geometries employed in the current design, including bifurcation, constriction and curved channel. The numerical simulation and mathematical analysis result in agreed conclusions, giving some insights into the importance of the relevant geometries in promoting biofluid separation. The main results can be summarised as follows: (i) the constrictions can largely increase the shear stress by the ratio of square of the reduction of the sections between the constriction and parent main channel. (ii) The curved channel intervals can induce centrifugal force, smoothly transit the flow field and increase the chances depleting fluid from the cell-free layer. (iii) The thickness of the boundary layer skimmed into the side channels from the main channel is decreased in this design and can be controlled, falling into the cell-free layer region by adjusting the geometry of the side channels.     

Hemodynamic investigation of a patient-specific abdominal aortic aneurysm with iliac artery tortuosity

Abstract

This paper describes a systematic investigation on the hemodynamic environment in a patient-specific AAA with tortuous common iliac artery(CIA) and external iliac artery (EIA). 3D reconstructions from CT scans and subsequent computational simulation are carried out. It is found out that the Newtonian and non-Newtonian models have very similar flow field and WSS distribution. More importantly, it is revealed that the torturous CIA maintained its helical flow. It is concluded that the assumption of Newtonian blood is adequate in capturing the intra-aneurysmal hemodynamics. Moreover, it is speculated that the physiological spiral flow protects the twisted CIA from the thrombosis formation.     

Effect of posture on regional gas exchange in pigs.

Although recent high-resolution studies demonstrate the importance of nongravitational determinants for both pulmonary blood flow and ventilation distributions, posture has a clear impact on whole lung gas exchange. Deterioration in arterial oxygenation with repositioning from prone to supine posture is caused by increased heterogeneity in the distribution of ventilation-to-perfusion ratios. This can result from increased heterogeneity in regional blood flow distribution, increased heterogeneity in regional ventilation distribution, decreased correlation between regional blood flow and ventilation, or some combination of the above (Wilson TA and Beck KC, J Appl Physiol 72: 2298-2304, 1992). We hypothesize that, although repositioning from prone to supine has relatively small effects on overall blood flow and ventilation distributions, regional changes are poorly correlated, resulting in regional ventilation-perfusion mismatch and reduction in alveolar oxygen tension. We report ventilation and perfusion distributions in seven anesthetized, mechanically ventilated pigs measured with aerosolized and injected microspheres. Total contributions of pulmonary structure and posture on ventilation and perfusion heterogeneities were quantified by using analysis of variance. Regional gradients of posture-mediated change in ventilation, perfusion, and calculated alveolar oxygen tension were examined in the caudocranial and ventrodorsal directions. We found that pulmonary structure was responsible for 74.0 +/- 4.7% of total ventilation heterogeneity and 63.3 +/- 4.2% of total blood flow heterogeneity. Posture-mediated redistribution was primarily oriented along the caudocranial axis for ventilation and along the ventrodorsal axis for blood flow. These mismatched changes reduced alveolar oxygen tension primarily in the dorsocaudal lung region.     

射频消融中温度场建立的探讨

提出了建立在射频电流组织加热和热传导基础上的射频消融中温度场建立的理论模型,初步分析了血流对温度分布提影响,得出稳定后的温度场在径向的分布基本上与ｒ及血流速度成反比;近场的温度场的建立过程的时间常数与血流速度成反比。     

Stochastic Simulation of Human Pulmonary Blood Flow and Transit Time Frequency Distribution Based on Anatomic and Elasticity Data

The objective of our study was to develop a computing program for computing the transit time frequency distributions of red blood cell in human pulmonary circulation, based on our anatomic and elasticity data of blood vessels in human lung. A stochastic simulation model was introduced to simulate blood flow in human pulmonary circulation. In the stochastic simulation model, the connectivity data of pulmonary blood vessels in human lung was converted into a probability matrix. Based on this model, the transit time of red blood cell in human pulmonary circulation and the output blood pressure were studied. Additionally, the stochastic simulation model can be used to predict the changes of blood flow in human pulmonary circulation with the advantage of the lower computing cost and the higher flexibility. In conclusion, a stochastic simulation approach was introduced to simulate the blood flow in the hierarchical structure of a pulmonary circulation system, and to calculate the transit time distributions and the blood pressure outputs.     

Blood flow distribution in dog gastrocnemius muscle at rest and during stimulation

The distribution of blood flow within the isolated perfused dog gastrocnemius muscle (weight 100-240 g) was studied by intra-arterial injection of radioactively labeled microspheres (diameter 15 micron) at rest and during supramaximal stimulation to rhythmic isotonic tetanic contractions of varied frequency against varied loads. After the experiment the muscle was cut into 180-250 pieces of approximately 0.75 g each, and the blood flow to each muscle piece was determined from its radioactivity. The inhomogeneity of blood flow was represented as the frequency distribution of the ratios of regional specific blood flow, i.e., blood flow per unit tissue weight of the piece, QR, to the overall specific blood flow of the muscle, Q. The QR/Q values for the individual pieces of a muscle were found to vary widely both at rest and during stimulation. With rising work load the frequency distribution had a tendency to broaden and flatten, indicating increasing perfusion inhomogeneity. On the average of the experiments, there was no significant difference in specific blood flow between the three anatomic components of the gastrocnemius (lateral and medial heads of gastrocnemius and flexor digitorum superficialis) nor between the superficial and deep portions within these anatomic components, only the distal third of the muscle was relatively less perfused compared with the proximal two-thirds. The considerable inhomogeneity of blood flow as revealed by microsphere embolization and by other methods is expected to exert important limiting effects on local O2 supply, particularly during exercise. Its neglect would lead to serious errors in the analysis of O2 supply to muscle tissue.     

Multiscale modeling of blood flow: from single cells to blood rheology

Mesoscale simulations of blood flow, where the red blood cells are described as deformable closed shells with a membrane characterized by bending rigidity and stretching elasticity, have made much progress in recent years to predict the flow behavior of blood cells and other components in various flows. To numerically investigate blood flow and blood-related processes in complex geometries, a highly efficient simulation technique for the plasma and solutes is essential. In this review, we focus on the behavior of single and several cells in shear and microcapillary flows, the shear-thinning behavior of blood and its relation to the blood cell structure and interactions, margination of white blood cells and platelets, and modeling hematologic diseases and disorders. Comparisons of the simulation predictions with existing experimental results are made whenever possible, and generally very satisfactory agreement is obtained.