Computational Phlebology: The Simulation of a Vein Valve

We present a three-dimensional computer simulation of the dynamics of a vein valve. In particular, we couple the solid mechanics of the vein wall and valve leaflets with the fluid dynamics of the blood flow in the valve. Our model captures the unidirectional nature of blood flow in vein valves; blood is allowed to flow proximally back to the heart, while retrograde blood flow is prohibited through the occlusion of the vein by the valve cusps. Furthermore, we investigate the dynamics of the valve opening area and the blood flow rate through the valve, gaining new insights into the physics of vein valve operation. It is anticipated that through computer simulations we can help raise our understanding of venous hemodynamics and various forms of venous dysfunction.     

A digital model for the venous junctions

The venous network in the lower limbs is composed of a considerable number of confluent junctions. Each of these singularities introduces some blood flow disturbances. Each physiological junction is unique, in terms of its geometry as well as the blood flow rate. In order to account for this great variability, we developed a numerical model based on the use of the N3S code (a software package for solving Navier-Stokes equations). To test the validity of the model, one of the numerical simulations is compared with the data obtained in the corresponding experimental configuration. The velocity measurements were carried out with an ultrasonic pulsed Doppler velocimeter. We also measured pressure differences using differential sensors. The numerical computations were then used to obtain the values of the flow variables at any point, with various geometrical and flow configurations. As far as the velocity field is concerned, a very marked three-dimensional pattern with swirls was observed. The pressure evolution was also strongly disturbed, with a non-linear decrease. All these data indicate that confluence effects cannot be neglected when evaluating pressure decreases. With a tool of this kind, it is possible to accurately predict the disturbances associated with any geometrical configuration or any flow rate.     

A Digital Model for the Venous Junctions

The venous network in the lower limbs is composed of a considerable number of confluent junctions. Each of these singularities introduces some blood flow disturbances. Each physiological junction is unique, in terms of its geometry as well as the blood flow rate. In order to account for this great variability, we developed a numerical model based on the use of the N3S code (a software package for solving Navier-Stokes equations). To test the validity of the model, one of the numerical simulations is compared with the data obtained in the corresponding experimental configuration. The velocity measurements were carried out with an ultrasonic pulsed Doppler velocimeter. We also measured pressure differences using differential sensors. The numerical computations were then used to obtain the values of the flow variables at any point, with various geometrical and flow configurations. As far as the velocity field is concerned, a very marked three-dimensional pattern with swirls was observed. The pressure evolution was also strongly disturbed, with a non-linear decrease. All these data indicate that confluence effects cannot be neglected when evaluating pressure decreases. With a tool of this kind, it is possible to accurately predict the disturbances associated with any geometrical configuration or any flow rate.     

Venous stagnation induced by 7 dyas in HDT, in the cerebral, ophtalmic, renal and splancnhic territories

The scientific objectives was to quantify the vascular changes in the brain, eye fundus, renal parenchyma, and splanchnic network. Heart, Portal, Jugular, femoral veins were investigate by Echography. The cerebral mesenteric, renal and ophthalmic arteries were investigated by Doppler. Eye fundus vein an papilla were investigated by optical video eye fundus. The Left ventricle volume decreased as usual in HDT. The cerebral and ophthalmic vascular resistances did'nt change whereas the eye fundus papilla and vein, and the Jugular vein increased. These arterial and venous data confirm the existence of cephalic venous blood stasis without sign of intracranial hypertension. On the other hand the kidney volume increased which is in agreement with blood flow stagnation at this level. At last the Mesenteric vascular resistance decreased and the Portal vein section increased in HDT which is in favor of an increase in flow and flow volume through the splanchnic area.     

基于CTA的在体女性盆腔静脉血管网数字化三维模型构建的方法及意义

目的:探讨利用CTA原始数据集构建在体女性盆腔静脉血管网数字化三维模型的方法及意义。方法:基于双源CTA技术,获取1例宫颈癌患者的Dicom 3.0原始二维断层图像数据集。利用Mimics 10.01软件分别对骨盆、盆腔动脉血管网以及盆腔静脉血管网进行三维重建并配准融合。结果:构建的盆腔静脉血管网数字化三维模型可以清楚地显示下腔静脉、髂总静脉、髂外静脉、髂内静脉及其初级属支,以及双侧卵巢静脉等。与重建的骨盆、盆腔动脉血管网配准融合后,各支静脉血管的解剖走形及引流区域变得更加清晰明确。结论:基于CTA的计算机三维重建技术是一种研究女性盆腔静脉血管网的好方法,具有较大的运用价值。     

CRIMSON: An open-source software framework for cardiovascular integrated modelling and simulation

In this work, we describe the CRIMSON (CardiovasculaR Integrated Modelling and SimulatiON) software environment. CRIMSON provides a powerful, customizable and user-friendly system for performing three-dimensional and reduced-order computational haemodynamics studies via a pipeline which involves: 1) segmenting vascular structures from medical images; 2) constructing analytic arterial and venous geometric models; 3) performing finite element mesh generation; 4) designing, and 5) applying boundary conditions; 6) running incompressible Navier-Stokes simulations of blood flow with fluid-structure interaction capabilities; and 7) post-processing and visualizing the results, including velocity, pressure and wall shear stress fields. A key aim of CRIMSON is to create a software environment that makes powerful computational haemodynamics tools accessible to a wide audience, including clinicians and students, both within our research laboratories and throughout the community. The overall philosophy is to leverage best-in-class open source standards for medical image processing, parallel flow computation, geometric solid modelling, data assimilation, and mesh generation. It is actively used by researchers in Europe, North and South America, Asia, and Australia. It has been applied to numerous clinical problems; we illustrate applications of CRIMSON to real-world problems using examples ranging from pre-operative surgical planning to medical device design optimization.     

Numerical Modeling of Interstitial Fluid Flow Coupled with Blood Flow through a Remodeled Solid Tumor Microvascular Network

Modeling of interstitial fluid flow involves processes such as fluid diffusion, convective transport in extracellular matrix, and extravasation from blood vessels. To date, majority of microvascular flow modeling has been done at different levels and scales mostly on simple tumor shapes with their capillaries. However, with our proposed numerical model, more complex and realistic tumor shapes and capillary networks can be studied. Both blood flow through a capillary network, which is induced by a solid tumor, and fluid flow in tumor’s surrounding tissue are formulated. First, governing equations of angiogenesis are implemented to specify the different domains for the network and interstitium. Then, governing equations for flow modeling are introduced for different domains. The conservation laws for mass and momentum (including continuity equation, Darcy’s law for tissue, and simplified Navier–Stokes equation for blood flow through capillaries) are used for simulating interstitial and intravascular flows and Starling’s law is used for closing this system of equations and coupling the intravascular and extravascular flows. This is the first study of flow modeling in solid tumors to naturalistically couple intravascular and extravascular flow through a network. This network is generated by sprouting angiogenesis and consisting of one parent vessel connected to the network while taking into account the non-continuous behavior of blood, adaptability of capillary diameter to hemodynamics and metabolic stimuli, non-Newtonian blood flow, and phase separation of blood flow in capillary bifurcation. The incorporation of the outlined components beyond the previous models provides a more realistic prediction of interstitial fluid flow pattern in solid tumors and surrounding tissues. Results predict higher interstitial pressure, almost two times, for realistic model compared to the simplified model.     

The cerebrospinal venous system: anatomy, physiology, and clinical implications

There is substantial anatomical and functional continuity between the veins, venous sinuses, and venous plexuses of the brain and the spine. The term "cerebrospinal venous system" (CSVS) is proposed to emphasize this continuity, which is further enhanced by the general lack of venous valves in this network. The first of the two main divisions of this system, the intracranial veins, includes the cortical veins, the dural sinuses, the cavernous sinuses, and the ophthalmic veins. The second main division, the vertebral venous system (VVS), includes the vertebral venous plexuses which course along the entire length of the spine. The intracranial veins richly anastomose with the VVS in the suboccipital region. Caudally, the CSVS freely communicates with the sacral and pelvic veins and the prostatic venous plexus. The CSVS constitutes a unique, large-capacity, valveless venous network in which flow is bidirectional. The CSVS plays important roles in the regulation of intracranial pressure with changes in posture, and in venous outflow from the brain. In addition, the CSVS provides a direct vascular route for the spread of tumor, infection, or emboli among its different components in either direction.     

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.     

Numerical simulation of local blood flow in the carotid and cerebral arteries under altered gravity

A computational fluid dynamics (CFD) approach was presented to model the blood flows in the carotid bifurcation and the brain arteries under altered gravity. Physical models required for CFD simulation were introduced including a model for arterial wall motion due to fluid-wall interactions, a shear thinning fluid model of blood, a vascular bed model for outflow boundary conditions, and a model for autoregulation mechanism. The three-dimensional unsteady incompressible Navier-Stokes equations coupled with these models were solved iteratively using the pseudocompressibility method and dual time stepping. Gravity source terms were added to the Navier-Stokes equations to take the effect of gravity into account. For the treatment of complex geometry, a chimera overset grid technique was adopted to obtain connectivity between arterial branches. For code validation, computed results were compared with experimental data for both steady-state and time-dependent flows. This computational approach was then applied to blood flows through a realistic carotid bifurcation and two Circle of Willis models, one using an idealized geometry and the other using an anatomical data set. A three-dimensional Circle of Willis configuration was reconstructed from subject-specific magnetic resonance images using an image segmentation method. Through the numerical simulation of blood flow in two model problems, namely, the carotid bifurcation and the brain arteries, it was observed that the altered gravity has considerable effects on arterial contraction/dilatation and consequent changes in flow conditions.     

Venous haemodynamics response to hypercapnia

The authors' aim was to evaluate the response of the intracranial venous system to application of functional tests. In 46 healthy volunteers, the basal veins were examined using venous transcranial color-coded duplex sonography. We evaluated angle-corrected venous peak-systolic flow velocities (V(vb)) in response to hypercapnia (I group: 29 subjects) and the orthostatic test (II group: 17 subjects). We have found that hypercapnia inducesa significant increase of the flow velocities in the basal veins, on average 60 +/- 22.7% (p < 0.00001). The reaction prevents venous hyperaemia, i. e., significant increase of the cerebral blood flow volume and the drainage of the brain. In contrast to hypercapnia, the orthostatic test led to decrease in the flow velocities in the basal veins. The percentage of velocity decrease ranged from 16to 32% (p < 0.001). Posturally induced reduction of the venous flow volume prevents excessive drainage from the brain. Thus, we have shown that both tests can be used for evaluation of reactivity of the intracranial venous system and they can complement each other.     

A patient-specific aortic valve model based on moving resistive immersed implicit surfaces

In this paper, we propose a full computational framework to simulate the hemodynamics in the aorta including the valve. Closed and open valve surfaces, as well as the lumen aorta, are reconstructed directly from medical images using new ad hoc algorithms, allowing a patient-specific simulation. The fluid dynamics problem that accounts from the movement of the valve is solved by a new 3D–0D fluid–structure interaction model in which the valve surface is implicitly represented through level set functions, yielding, in the Navier–Stokes equations, a resistive penalization term enforcing the blood to adhere to the valve leaflets. The dynamics of the valve between its closed and open position is modeled using a reduced geometric 0D model. At the discrete level, a finite element formulation is used and the SUPG stabilization is extended to include the resistive term in the Navier–Stokes equations. Then, after time discretization, the 3D fluid and 0D valve models are coupled through a staggered approach. This computational framework, applied to a patient-specific geometry and data, allows to simulate the movement of the valve, the sharp pressure jump occurring across the leaflets, and the blood flow pattern inside the aorta.     

The effects of external compression on venous blood flow and tissue deformation in the lower leg

External pneumatic compression of the lower legs is effective as prophylaxis against deep vein thrombosis. In a typical application, inflatable cuffs are wrapped around the patient's legs and periodically inflated to prevent stasis, accelerate venous blood flow, and enhance fibrinolysis. The purpose of this study was to examine the stress distribution within the tissues, and the corresponding venous blood flow and intravascular shear stress with different external compression modalities. A two-dimensional finite element analysis (FEA) was used to determine venous collapse as a function of internal (venous) pressure and the magnitude and spatial distribution of external (surface) pressure. Using the one-dimensional equations governing flow in a collapsible tube and the relations for venous collapse from the FEA, blood flow resulting from external compression was simulated. Tests were conducted to compare circumferentially symmetric (C) and asymmetric (A) compression and to examine distributions of pressure along the limb. Results show that A compression produces greater vessel collapse and generates larger blood flow velocities and shear stresses than C compression. The differences between axially uniform and graded-sequential compression are less marked than previously found, with uniform compression providing slightly greater peak flow velocities and shear stresses. The major advantage of graded-sequential compression is found at midcalf. Strains at the lumenal border are approximately 20 percent at an external pressure of 50 mmHg (6650 Pa) with all compression modalities.     

The use of smoothed particles approach in realistic mathematical model of intracardiac blood flow: Simulation of a self-organizing tornado-like flow

We performed computer simulation and visualization of blood flow in the left ventricle by the method of smooth particle hydrodynamics (SPH). This visualization qualitatively describes the evolution of twisted stream and graphically demonstrates the direction of velocity field at each moment of time. The geometrical features of the left ventricle are approximated by three-dimensional segmentation of experimental clinical images obtained from multispiral computer tomography (MSCT). The model adequately describes the possible configuration of swirling flow in the left ventricle and is a part of a comprehensive study of swirling flows in different compartments of heart, which comply with a family of the exact solutions of hydrodynamic Navier–Stokes equations for the class of quasipotential¹ swirling flows. Computer visualization shows how simulated by SPH method jet of a model liquid, which is placed in limited space, remains continuity and keeps its clockwise vorticity along the direction of the flow propagation during the whole cycle. Then it turns on approximately 120° by the time of ejection into the aorta. Such structure of the flow provides more effective pumping of blood as a model liquid through the ventricle as compared to a lamellar flow mode.     

Enhanced global mathematical model for studying cerebral venous blood flow

Here we extend the global, closed-loop, mathematical model for the cardiovascular system in Müller and Toro (2014) to account for fundamental mechanisms affecting cerebral venous haemodynamics: the interaction between intracranial pressure and cerebral vasculature and the Starling-resistor like behaviour of intracranial veins. Computational results are compared with flow measurements obtained from Magnetic Resonance Imaging (MRI), showing overall satisfactory agreement. The role played by each model component in shaping cerebral venous flow waveforms is investigated. Our results are discussed in light of current physiological concepts and model-driven considerations, indicating that the Starling-resistor like behaviour of intracranial veins at the point where they join dural sinuses is the leading mechanism. Moreover, we present preliminary results on the impact of neck vein strictures on cerebral venous hemodynamics. These results show that such anomalies cause a pressure increment in intracranial cerebral veins, even if the shielding effect of the Starling-resistor like behaviour of cerebral veins is taken into account.     

Vascular anatomy of the anterolateral thigh flap

Arterial and venous anatomy and their relation to the anterolateral thigh flap were examined in 10 specimens of six fresh cadavers in which radiopaque materials were injected into both the arterial and venous systems. Territories and positions of individual perforating arteries were measured, and the venous drainage pathway of the flap was analyzed. All specimens were radiographed stereoscopically to observe the three-dimensional structure of the arteries and veins. The territory of each perforating artery was smaller than expected. Most of the venous blood that had perfused the dermis was considered to pool in a polygonal venous network located in the skin layer and to enter the descending branch of the lateral circumflex femoral artery through large descending veins. The venous territories were considered different from the arterial territories. The findings in this study suggest that the design of the anterolateral thigh flap should be based on the venous architecture rather than on the arterial architecture and that the flap survival rate might be improved if thinning is performed appropriately.     

手术前后三维动脉瘤模型的血液动力学模拟对比

本研究目的在于从临床患者的医学影像出发,在手术前后分别对三维动脉瘤模型进行了定常模拟,观察动脉瘤模型内的流场形态以及各血液动力学参数的变化并进行讨论。由于三维的数值模拟比二维的跟接近实际,而且更加直观形象,所以研究结果对于分析动脉瘤的破裂机理具有重要的临床应用价值。