基于流固耦合有限元模型评估颈椎旋转手法下颈动脉粥样硬化斑块的破裂风险

目的 评估颈椎旋转手法对颈动脉粥样硬化斑块的破裂风险。方法建立颈动脉粥样硬化斑块流固耦合（fluid-structure interaction, FSI）模型。采用FSI模型模拟颈椎旋转手法对颈动脉斑块和管腔的拉伸形变。记录斑块和管腔的血流最大剪切力（flow shear stress,FSS）、最大壁面切应力（wall shear stress,WSS）、最大斑块壁应力（plaque wall stress,PWS）、壁面拉应力（wall tensile stress,WTS）和壁面压强（wall pressure, WP）等力学参数。结果在颈动脉16%拉伸形变下,斑块最大WSS为40.54 Pa,此处斑块可能表皮损伤;最大PWS为66.16 kPa,远小于斑块破裂阈值;内部纤维帽最大WTS和最大应变分别为156.75 kPa和0.56,大于破裂应变范围,能达到断裂临界值;管腔最大WTS为1 040.30 kPa,已接近中膜破裂阈值,可能会引起血管损伤。结论 颈椎旋转至生理极限位后,颈动脉拉伸可能造成斑块表皮组织破坏导致脱落,斑块内部会形成损伤、溃疡和出血以及血管损伤,影响斑块稳定性。在颈动脉粥样硬化斑块的颈部疾病患者中应谨慎进行颈椎旋转手法治疗,手法前对斑块进行FSI评估可能是一种有效的安全性筛查方法。     

Coupled fluid-wall modelling of steady flow in stenotic carotid arteries

Arterial stenoses may cause critical blood flow and wall conditions leading to clinical complications. In this paper computational models of stenotic carotid arteries are proposed and the vessel wall collapse phenomenon is studied. The models are based on fluid-structure interactions (FSI) between blood and the arterial walls. Coupled finite element and computational fluid dynamics methods are used to simultaneously solve for stress and displacement in the solid, and for pressure, velocity and shear stress in the fluid domain. Results show high wall shear stress at the stenosis throat and low (negative) values accompanied by disturbed flow patterns downstream of the stenosis. The wall circumferential stress varies abruptly from tensile to compressive along the stenosis with high stress concentration on the plaque shoulders showing regions of possible plaque rupture. Wall compression and collapse are observed for severe cases. Post-stenotic collapse of the arterial wall occurs for stenotic severity as low as 50%, with the assumption that a given amount of blood flow needs to pass the stenotic artery; whereas if constant pressure drop should be maintained across a constriction, then collapse happens at severity of 75% and above. The former assumption is based on the requirement of adequate blood supply to the downstream organs/tissue, while the latter stems from the fact that the pumping mechanism of the body has a limited capacity in regulating blood pressure, in case a stenosis appears in the vasculature.     

Local Maximal Stress Hypothesis and Computational Plaque Vulnerability Index for Atherosclerotic Plaque Assessment

It is believed that atherosclerotic plaque rupture may be related to maximal stress conditions in the plaque. More careful examination of stress distributions in plaques reveals that it may be the local stress/strain behaviors at critical sites such as very thin plaque cap and locations with plaque cap weakness that are more closely related to plaque rupture risk. A “local maximal stress hypothesis” and a stress-based computational plaque vulnerability index (CPVI) are proposed to assess plaque vulnerability. A critical site selection (CSS) method is proposed to identify critical sites in the plaque and critical stress conditions which are be used to determine CPVI values. Our initial results based on 34 2D MRI slices from 14 human coronary plaque samples indicate that CPVI plaque assessment has an 85% agreement rate (91% if the square root of stress values is used) with assessment given by histopathological analysis. Large-scale and long-term patient studies are needed to further validate our findings for more accurate quantitative plaque vulnerability assessment.     

Ultrasound image-based computer model of a common carotid artery with a plaque

Ultrasound scans were acquired from a common carotid artery in a patient with an early atherosclerotic plaque forming a mild asymmetrical stenosis. The 3D vascular geometry of the diseased arterial segment was reconstructed from a series of 2D cross-sectional images, and computational meshes for the flow and wall domains were developed. Numerical flow simulations incorporating coupled fluid–solid interaction were implemented using flow and pressure waveforms measured in vivo. The effects of wall distensibility were investigated by comparing the predictions obtained with different wall compliance, one with ‘natural’ compliance and another with a stiffer wall. Limited flow separation was predicted in the post-stenotic zone. The non-uniform thickness of the diseased wall restricted the wall motion locally and re-distributed the stress, giving raised concentrations at the plaque shoulders.     

Measurement of the 3D arterial wall strain tensor using intravascular B-mode ultrasound images: a feasibility study

Intravascular ultrasound (IVUS) elastography is a promising tool for studying atherosclerotic plaque composition and assessing plaque vulnerability. Current IVUS elastography techniques can measure the 1D or 2D strain of the vessel wall using various motion tracking algorithms. Since biological soft tissue tends to deform non-uniformly in 3D, measurement of the complete 3D strain tensor is desirable for more rigorous analysis of arterial wall mechanics. In this paper, we extend our previously developed method of 2D arterial wall strain measurement based on non-rigid image registration into 3D strain measurement. The new technique registers two image volumes acquired from the same vessel segment under different levels of luminal pressure and longitudinal stress. The 3D displacement field obtained from the image registration is used to calculate the local 3D strain tensor. From the 3D strain tensor, radial, circumferential and longitudinal strain distributions can be obtained and displayed. This strain tensor measurement method is validated and evaluated using IVUS images of healthy porcine carotid arteries subjected to a luminal pressure increase and longitudinal stretch. The ability of the algorithm to overcome systematic noise was tested, as well as the consistency of the results under different longitudinal frame resolutions.     

法洛四联症根治术后对左肺动脉狭窄影响机理的数值分析

目的 利用计算流体力学(CFD)方法分析法洛四联症(TOF)根治术后不同的左肺动脉狭窄率（50%,20%,0%）模型的局部血流动力学改变。方法 通过对患者CT数据处理,完成3组左肺肺动脉狭窄几何的全三维数字化重构;结合主肺动脉血流量等临床数据,对3组模型中肺动脉分叉区的血液流动速度场、压力、壁面剪切应力等进行记录。结果 在3组模型的非定常模拟中,左肺动脉起始端均存在与狭窄率相关的反流和血流分布,右肺动脉血流分布规则。3组模型的静态压力、压力损失、壁面剪切力等也不同。结论 肺动脉分支的合理扩大在TOF根治术中有重要意义;术后左肺动脉狭窄是造成肺动脉反流的早期和重要因素;通过CFD模拟术后患者的三维模型可以对手术的效果提供早期参考。     

A computational model to predict aortic wall stresses in patients with systolic arterial hypertension

Computational cardiovascular mechanics has allowed scientists to create complex 3D models for the simulation of cardiovascular problems. Mechanical stress plays a crucial role in the function of the cardiovascular system; stress analysis is a useful tool for the understanding of vascular pathophysiology. By using the spiral CT imaging and computational structural analysis, we present a noninvasive method of wall stress analysis in the normal aorta. The aortic segment was extended from the origin of the inferior mesenteric artery to the aortic bifurcation. The length of this segment was 12 cm, while the maximum transverse diameter was 2.075+/-0.129 cm. A 3D aortic model was constructed based on the CT scan images. The aorta was assumed to have a uniform wall thickness of 1.5mm. The generated unstructured grid, which was used for the structural analysis, consisted of 14,440 hexahedral elements. The wall material was assumed to be hyperelastic, homogeneous, isotropic and nearly incompressible (Poisson ratio=0.45). According to experimental studies, the Young modulus of aortic wall was set equal to 4.66 MPa. The shear stress induced by the blood flow was neglected. A finite-element static structural analysis was performed. Three different cases were examined applying constant intraluminal systolic blood pressures of 120, 180 and 240 mmHg, respectively. The von Mises stress distribution and the displacements of the aortic wall were calculated. Peak wall stress for the normal load case of 120 mmHg was 22.5 N/cm2, while the max displacement was 0.44 mm. The case with the intraluminal pressure of 180 mmHg resulted into peak wall stress of 32 N/cm2 with max displacement 0.59 mm, while for 240 mmHg was 40.6N/cm2, max displacement 0.72 mm. The rise in blood pressure caused all stresses to increase. The pattern of stress distribution and the orientation of the stress were similar for all test cases. A quantitative evaluation of the aortic wall stresses under systolic hypertension is presented. The calculated values of peak wall stress are far lower to those of failure strength of healthy aortic wall specimens estimated by ex vivo mechanical testing (121.0 N/cm2). Our values are consistent with prior stress values predicted by experimental studies. The described methodology offers a significant advancement in incorporating biomechanical principles in the clinical assessment of hypertensive patients with normal or aneurysmatic aortas and can be applied in a patient-specific basis in both conditions in order to detect the vulnerable high stressed regions and the resultant risk of aortic dissection or rupture. We hypothesize that this could assist in deciding the timing of surgical intervention, especially in high-risk patients with abdominal aortic aneurysms.     

Stress analysis of carotid atheroma in transient ischemic attack patients: evidence for extreme stress-induced plaque rupture

Plaque rupture has been considered to be the result of its structural failure. The aim of this study is to suggest a possible link between higher stresses and rupture sites observed from in vivo magnetic resonance imaging (MRI) of transient ischemic attack (TIA) patients, by using stress analysis methods. Three patients, who had recently suffered a TIA, underwent in vivo multi-spectral MR imaging. Based on plaque geometries reconstructed from the post-rupture status, six pre-rupture plaque models were generated for each patient dataset with different reconstructions of rupture sites to bridge the gap of fibrous cap from original MRI images. Stress analysis by fluid structure interaction simulation was performed on the models, followed by analysis of local stress concentration distribution and plaque rupture sites. Furthermore, the sensitivity of stress analysis to the pre-rupture plaque geometry reconstruction was examined. Local stress concentrations were found to be located at the plaque rupture sites for the three subjects studied. In the total of 18 models created, the locations of the stress concentration regions were similar in 17 models in which rupture sites were always associated with high stresses. The local stress concentration region moved from circumferential center to the shoulder region (slightly away from the rupture site) for a case with a thick fibrous cap. Plaque wall stress level in the rupture locations was found to be much higher than the value in non-rupture locations. The good correlation between local stress concentrations and plaque rupture sites, and generally higher plaque wall stress level in rupture locations in the subjects studied could provide indirect evidence for the extreme stress-induced plaque rupture hypothesis. Local stress concentration in the plaque region could be one of the factors contributing to plaque rupture.     

基于冠状动脉CT造影的生物机械应力在斑块评估及不良心血管事件预测中的应用

冠状动脉长期暴露于危险因素会引起粥样硬化,进而导致斑块形成与进展。通过早期识别高危斑块特征将有助于预防斑块破裂或糜烂,从而避免急性心血管事件的发生。而生物机械应力(biomechanical stress)在动脉粥样硬化斑块进展及破裂中发挥重要的作用。近些年,已经可以通过无创冠脉CT血管造影(coronary computed tomography angiography, CCTA)利用计算流体力学(computational fluid dynamic, CFD)进行建模,从而得到相应的生物机械应力参数,尤其是壁面剪切应力(wall shear stress, WSS)将有助于更好地构建临床模型从而预测斑块进展及主要不良心血管事件(major adverse cardiac events, MACE)。本文重点介绍生物机械应力以及CCTA所计算得出的WSS在动脉粥样硬化中的作用,并讨论有关CCTA生物机械应力与冠心病相关的研究。     

Effect of Stenosis Asymmetry on Blood Flow and Artery Compression: A Three-Dimensional Fluid-Structure Interaction Model

A nonlinear three-dimensional thick-wall model with fluid-structure interactions is introduced to simulate blood flow in carotid arteries with an asymmetric stenosis to quantify the effects of stenosis severity, eccentricity, and pressure conditions on blood flow and artery compression (compressive stress in the wall). Mechanical properties of the tube wall are measured using a thick-wall stenosis model made of polyvinyl alcohal hydrogel whose mechanical properties are close to that of carotid arteries. A hyperelastic Mooney–Rivlin model is used to implement the experimentally measured nonlinear elastic properties of the tube wall. A 36.5% pre-axial stretch is applied to make the simulation physiological. The Navier–Stokes equations in curvilinear form are used for the fluid model. Our results indicate that severe stenosis causes critical flow conditions, high tensile stress, and considerable compressive stress in the stenosis plaque which may be related to artery compression and plaque cap rupture. Stenosis asymmetry leads to higher artery compression, higher shear stress and a larger flow separation region. Computational results are verified by available experimental data. © 2003 Biomedical Engineering Society. PAC2003: 8719Uv, 8710+e     

Stress analysis using anatomically realistic coronary tree

Plaque rupture with superimposed thrombosis is the main cause of the acute coronary syndromes of unstable angina, myocardial infarction, and sudden death. Endothelial disruption leading to plaque rupture may relate to mechanical fatigue associated with cyclic flexion of plaques. A novel method is proposed to assess stress and strain distribution using the finite element (FE) analysis and in vivo patient-specific dynamic 3D coronary arterial tree reconstruction from cine angiographic images. The local stresses were calculated on the diseased arterial wall which was modeled as consisting of a central fibrotic cap subjected to the cyclic flexion from cardiac contraction. Various parameters characterizing the plaque were chosen including vessel diameter, percentage narrowing, and lesion length. According to the FEA simulations, the results show that the smaller vessel diameter, greater percentage narrowing, and/or larger lesion size may result in higher stress on the plaque cap, with the vessel diameter as the dominant factor.     

3D flow study in a mildly stenotic coronary artery phantom using a whole volume PIV method

Blood flow dynamics has an important role in atherosclerosis initiation, progression, plaque rupture and thrombosis eventually causing myocardial infarction. In particular, shear stress is involved in platelet activation, endothelium function and secondary flows have been proposed as possible variables in plaque erosion. In order to investigate these three-dimensional flow characteristics in the context of a mild stenotic coronary artery, a whole volume PIV method has been developed and applied to a scaled-up transparent phantom. Experimental three-dimensional velocity data was processed to estimate the 3D shear stress distributions and secondary flows within the flow volume. The results show that shear stress reaches values out of the normal and atheroprotective range at an early stage of the obstructive pathology and that important secondary flows are also initiated at an early stage of the disease. The results also support the concept of a vena contracta associated with the jet in the context of a coronary artery stenosis with the consequence of higher shear stresses in the post-stenotic region in the blood domain than at the vascular wall.     

冠状动脉斑块分型及分叉角度对斑块影响的血液动力学模拟

目的 分析斑块分布分型和血管分叉角度对冠状动脉分叉血管内血液动力学的影响,进一步探讨对斑块易损性发展的影响规律。方法 基于人体冠状动脉分叉血管的平均几何参数,构建不同斑块分布分型和血管分叉角度情况下的流固耦合模型,研究关键部位处的血流速度、压力以及剪切应力分布。结果 斑块上游的肩部是斑块表面剪切应力最大的部位,容易发生溃疡或破裂并进一步发展;当分叉血管单侧有斑块时,分叉脊处的剪切应力大于双侧有斑块的情况。血管分叉脊处的压力和剪切应力随分叉角度减小而逐渐增大。结论 分叉处单侧有斑块时,斑块溃疡或破裂的概率更大。主支血管内斑块的存在会促进分叉处斑块的形成和发展。血管分叉的角度越小,分叉脊处的血管内壁越易受损。研究结果可为易损斑块治疗方案的设计与优化提供理论参考。     

The finite element analysis of stresses in atherosclerotic arteries during balloon angioplasty

Finite element models of diseased arteries subject to balloon dilation are used to study the stresses in the arterial wall and plaque at peak balloon pressures. These models incorporate large strain, simulate contact between the balloon and the inner artery, and the constituent materials are modeled as nonlinear elastic. In these analyses, balloon dilation pressures range from 300 to 450 kPa. The stress in the diseased artery is recorded under different stenosis levels dilated using different balloons and possible causes of restenosis and plaque rupture are discussed.     

Modelling of flow and wall behaviour in a mildly stenosed tube

In the present computational analysis, pulsatile flow and vessel wall behaviour in a simplified model of a stenosed vessel were investigated. Geometry of a 45% axisymmetrically stenosed (by area) cylindrical tube and a sinusoidal inflow waveform were simulated, with the fluid being assumed to be incompressible and Newtonian. The vessel wall was treated as a thick-walled, incompressible and isotropic material with uniform mechanical properties across the normal as well as the constricted segment. The study of fluid flow and wall motion was initially carried out separately using two commercial codes CFX4.2 and ABAQUS7 respectively. Their combined effects and interactions were later investigated through an iteratively coupled algorithm. Model validations on the rigid-wall fluid and static no-flow solid models were satisfactory, with Root Mean Square deviations of around 7% in centreline axial velocity between the prediction and measurement values for the rigid wall stenosis model, and 5% in circumferential stress for a cylindrical tube model under static loading when compared with the analytical solution. Results on velocity profiles, wall shear stress, intramural strain and stress for the rigid and compliant cases were all presented. Comparison between the rigid and compliant models revealed that, the flow separation layer distal to the stenosis was thicker and longer, and wall shear stress was slightly lower in the compliant model by less than 7.2%. Results obtained from the static wall model (with uniform pressure loading) and coupled fluid/wall interaction modelling of pulsatile flow showed qualitatively similar wall strain and stress patterns but considerable differences in magnitude. The radial and axial stresses were reduced by 31 and 8%, while the circumferential stress was increased by 13% due to the presence of pulsatile flow. Under the flow and structural conditions investigated, the effects of wall compliance were small, and did not change the flow and solid behaviours qualitatively in this case.     

A nonlinear finite element simulation of balloon expandable stent for assessment of plaque vulnerability inside a stenotic artery

The stresses induced on plaque wall during stent implantation inside a stenotic artery are associated with plaque rupture. The stresses in the plaque–artery–stent structure appear to be distinctly different for different plaque types in terms of both distribution and magnitude. In this study, a nonlinear finite element simulation was executed to analyze the influence of plaque composition (calcified, cellular, and hypocellular) on plaque, artery layers (intima, media, and adventitia), and stent stresses during implantation of a balloon expandable coronary stent into a stenosed artery. The atherosclerotic artery was assumed to consist of a plaque and normal arterial tissues on its outer side. The results revealed a significant influence of plaque types on the maximum stresses induced within plaque wall and artery layers during stenting, but not when calculating maximum stress on stent. The stress on stiffer calcified plaque wall was in the fracture level (2.21 MPa), whereas cellular and hypocellular plaques play a protective role by displaying less stress on their wall. The highest von Mises stresses were observed on less stiff media layer. The findings of this study suggest a lower risk of arterial vascular injury for calcified plaque, while higher risk of plaque ruptures for cellular and hypocellular plaques.     

出口边界条件和壁厚对冠状动脉壁面剪切力和冯米塞斯应力的影响

探讨基于个体化患者计算机断层血管造影(CTA)影像的冠状动脉流固耦合(FSI)数值分析中,不同出口边界条件和血管壁模型对时间平均壁面剪切力(TAWSS)和冯米塞斯应力(VMS)的影响。根据患者CTA影像重建右冠状动脉(RCA)管腔流域三维几何;将管腔流域表面向外扩张0.5 mm,形成均匀厚度血管壁;运用类似虚拟去除斑块的方法建立不均匀厚度血管壁模型。FSI分析时,分别给予zero和impedance两种出口边界条件;获得从舒张末期开始心动周期主要时间点TAWSS和VMS的分布,比较不同模型结果的差异。结果表明, 两种出口边界条件下,TAWSS空间分布基本一致,且血管狭窄段均高于其他部位;zero条件下峰值VMS出现在压强最大时刻点0.42 s,而impedance条件下峰值VMS出现在入口血流速度最大时刻点0.64 s,并且达到前者的20倍。同是impedance出口边界条件时,TAWSS分布基本一致,没有显著性差异;两种血管壁模型中,VMS的分布一致,血管狭窄段比其他部位低,但是不均匀厚度血管壁模型中局部位置VMS绝对值高于均匀壁厚血管壁模型。医学影像技术的发展可以提供更高精度的冠脉结构以及出入口速度和压强边界条件,不但对研究血流动力学以及结构力学因素与心血管疾病的关系具有重要意义,也可以更好的服务于患者个体化诊断与治疗。     

基于个性化颈内动脉瘤模型的流固耦合分析

目的在考虑血管壁弹性条件下,分析颈内动脉血液流动和壁面切应力的分布特性,探讨动脉瘤破裂的生物力学因素。方法依据二维医学扫描图像构建三维个性化颈内动脉瘤模型。依据人体生理统计数据构建出血管壁模型。根据人体颈内动脉生理流动条件,利用有限体积法和有限元法模拟分析流固耦合作用下颈内动脉瘤中的血流动力学。结果在动脉瘤腔中有一个明显的涡旋存在,此涡旋流动的方向在心动周期内没有改变;在动脉瘤颈和动脉瘤壁面处存在一个壁面切应力值相对较大区域;在动脉瘤颈和动脉瘤顶有两个区域的Von Mises应力处于局部最大值。从材料强度角度考虑,这几个区域都是动脉瘤容易破裂的地方。结论通过流固耦合计算可以获得血管壁面应力分布特性,进而推断动脉瘤破裂的可能位置。     

双向双侧格林手术的数值研究

目的利用数值模拟方法预测左上腔静脉与肺动脉的连接位置改变对双向双侧格林(BBDG)手术的影响。方法首先,根据拥有左上腔静脉的单心室心脏缺陷综合征病人的医学图像重建出三维几何模型。其次,基于力反馈器创建其他数值模拟所需的模型,并利用有限体积法进行流体力学的数值模拟。最后,分析与评估获得的血流动力学参数。结果血液在左、右上腔静脉与肺主动脉中进行再循环。左上腔静脉到右上腔静脉之间的距离由右上腔静脉直径的2倍逐渐变化到右上腔静脉直径的3.5倍,发现当距离为3倍直径时,能量损失最少,而距离为2倍时,能量损失最大。血液分流比(左肺动脉流量/右肺动脉流量)的计算结果范围为0.65～1.11。结论在BBDG手术的治疗中,左、右上腔静脉之间距离太近会导致不适宜的分流比以及消耗更多的能量。本研究结果对于评价伴随有左上腔静脉的单心室心脏缺陷综合征的治疗手术是非常有意义的。     

Mechanical,biological and structural characterization of in vitro ruptured human carotid plaque tissue

Recent experimental studies performed on human carotid plaques have focused on mechanical characterization for the purpose of developing material models for finite-element analysis without quantifying the tissue composition or relating mechanical behaviour to preoperative classification. This study characterizes the mechanical and biological properties of 25 human carotid plaques and also investigates the common features that lead to plaque rupture during mechanical testing by performing circumferential uniaxial tests, Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) on each specimen to relate plaque composition to mechanical behaviour. Mechanical results revealed large variations between plaque specimen behaviour with no correlation to preoperative ultrasound prediction. However, FTIR classification demonstrated a statistically significant relationship between stress and stretch values at rupture and the level of calcification (P = 0.002 and P = 0.009). Energy-dispersive X-ray spectroscopy was carried out to confirm that the calcium levels observed using FTIR analysis were accurate. This work demonstrates the potential of FTIR as an alternative method to ultrasound forpredicting plaque mechanical behaviour. SEM imaging at the rupture sites of each specimen highlighted voids created by the nodes of calcifications in the tissue structure which could lead to increased vulnerability of the plaque.