Outflow boundary conditions for 3D simulations of non-periodic blood flow and pressure fields in deformable arteries

The simulation of blood flow and pressure in arteries requires outflow boundary conditions that incorporate models of downstream domains. We previously described a coupled multidomain method to couple analytical models of the downstream domains with 3D numerical models of the upstream vasculature. This prior work either included pure resistance boundary conditions or impedance boundary conditions based on assumed periodicity of the solution. However, flow and pressure in arteries are not necessarily periodic in time due to heart rate variability, respiration, complex transitional flow or acute physiological changes. We present herein an approach for prescribing lumped parameter outflow boundary conditions that accommodate transient phenomena. We have applied this method to compute haemodynamic quantities in different physiologically relevant cardiovascular models, including patient-specific examples, to study non-periodic flow phenomena often observed in normal subjects and in patients with acquired or congenital cardiovascular disease. The relevance of using boundary conditions that accommodate transient phenomena compared with boundary conditions that assume periodicity of the solution is discussed

Flow velocity mapping using contrast enhanced high-frame-rate plane wave ultrasound and image tracking: methods and initial in vitro and in vivo evaluation

Ultrasound imaging is the most widely used method for visualising and quantifying blood flow in medical practice, but existing techniques have various limitations in terms of imaging sensitivity, field of view, flow angle dependence, and imaging depth. In this study, we developed an ultrasound imaging velocimetry approach capable of visualising and quantifying dynamic flow, by combining high-frame-rate plane wave ultrasound imaging, microbubble contrast agents, pulse inversion contrast imaging and speckle image tracking algorithms. The system was initially evaluated in vitro on both straight and carotid-mimicking vessels with steady and pulsatile flows and in vivo in the rabbit aorta. Colour and spectral Doppler measurements were also made. Initial flow mapping results were compared with theoretical prediction and reference Doppler measurements and indicate the potential of the new system as a highly sensitive, accurate, angle-independent and full field-of-view velocity mapping tool capable of tracking and quantifying fast and dynamic flows

Simulated hemodynamics in human carotid bifurcation based on Doppler ultrasound data

Background: Atherosclerotic lesions commonly develop at arterial branch sites. Noninvasive carotid artery ultrasound is a well-established and effective method which allows real-time images and measurements of flow velocities. We aimed to develop a methodology for patient-specific computational 3D reconstruction and blood flow simulation based on ultrasound image data.Material and Methods: Subject-specific studies based on the acquisition of a set of longitudinal and sequential cross-sectional ultrasound images and Doppler velocity measurements at common carotid artery (CCA) bifurcation were performed at a university hospital. A developed simulation code of blood flow by the finite element method (FEM) that includes an adequate structured meshing of the common carotid artery bifurcation was used to investigate local flow biomechanics.Results: Hemodynamic simulations of CCA bifurcations for six individuals were analysed. Comparing pairs (Doppler, FEM) of velocity values, Lin's concordance correlation coefficient analysis demonstrated an almost perfect strength of agreement (c = 0.9911), in patients with different degrees of internal carotid artery (ICA) stenosis. Numerical simulations were able to capture areas of low wall shear stress correlated with stagnation zones.Conclusions: Simulated hemodynamic parameters can reproduce the disturbed flow conditions at the bifurcation of CCA and proximal ICA, which play an important role in the development of local atherosclerotic plaques. This novel technology might help to understand the relationship between hemodynamic environment and carotid wall lesions, and have a future impact in carotid stenosis diagnosis and management

Assessing the performance of ultrafast vector flow imaging in the neonatal heart via multiphysics modeling and In vitro experiments

Ultrafast vector flow imaging would benefit newborn patients with congenital heart disorders, but still requires thorough validation before translation to clinical practice. This paper investigates 2-D speckle tracking (ST) of intraventricular blood flow in neonates when transmitting diverging waves at ultrafast frame rate. Computational and in vitro studies enabled us to quantify the performance and identify artifacts related to the flow and the imaging sequence. First, synthetic ultrasound images of a neonate's left ventricular flow pattern were obtained with the ultrasound simulator Field II by propagating point scatterers according to 3-D intraventricular flow fields obtained with computational fluid dynamics (CFD). Noncompounded diverging waves (opening angle of 60 degrees) were transmitted at a pulse repetition frequency of 9 kHz. ST of the B-mode data provided 2-D flow estimates at 180 Hz, which were compared with the CFD flow field. We demonstrated that the diastolic inflow jet showed a strong bias in the lateral velocity estimates at the edges of the jet, as confirmed by additional in vitro tests on a jet flow phantom. Furthermore, ST performance was highly dependent on the cardiac phase with low flows (< 5 cm/s), high spatial flow gradients, and out-of-plane flow as deteriorating factors. Despite the observed artifacts, a good overall performance of 2-D ST was obtained with a median magnitude underestimation and angular deviation of, respectively, 28% and 13.5 degrees during systole and 16% and 10.5 degrees during diastole

Hemodynamics in the Stenosed Carotid Bifurcation with Plaque Ulceration

The presence of irregular plaque surface morphology or ulceration of the atherosclerotic lesion has been identified as an independent risk factor for ischemic stroke. Doppler ultrasound (DUS) is the most commonly performed non-invasive technique used to assess patients suspected of having carotid artery disease, but currently does not incorporate the diagnosis of plaque ulceration. Advanced Doppler analyses incorporating quantitative estimates of flow disturbances may result in diagnostic indices that identify plaque ulcerative conditions. A technique for the fabrication of DUS-compatible flow phantoms was developed, using a direct-machining method that is amenable to comprehensive DUS investigations. In vitro flow studies in an ensemble of matched model vessel geometries determined that ulceration as small as 2 mm can generate significant disturbances in the downstream flow field in a moderately stenosed carotid artery, which are detectable using the DUS velocity-derived parameter turbulence intensity (TI) measured with a clinical system. Further experimental results showed that distal TI was significantly elevated (P \u3c 0.001) due to proximal plaque ulceration in the mild and moderately stenosed carotid bifurcation (30%, 50%, 60% diameter reduction), and also increased with stenosis severity. Pulsatile computational fluid dynamics (CFD) models, with simulated particle tracking, demonstrated enhanced flow disruption of the stenotic jet and slight elevations in path-dependent shear exposure parameters in a stenosed carotid bifurcation model with ulceration. In addition, CFD models were used to evaluate the DUS index TI using finite volume sampling

PIV-based Investigation of Hemodynamic Factors in Diseased Carotid Artery Bifurcations with Varying Plaque Geometries

Ischemic stroke is often a consequence of complications due to clot formation (i.e. thrombosis) at the site of an atherosclerotic plaque developed in the internal carotid artery. Hemodynamic factors, such as shear-stress forces and flow disturbances, can facilitate the key mechanisms of thrombosis. Atherosclerotic plaques can differ in the severity of stenosis (narrowing), in eccentricity (symmetry), as well as inclusion of ulceration (wall roughness). Therefore, in terms of clinical significance, it is important to investigate how the local hemodynamics of the carotid artery is mediated by the geometry of plaque. Knowledge of thrombosis-associated hemodynamics may provide a basis to introduce advanced clinical diagnostic indices that reflect the increased probability of thrombosis and thus assist with better estimation of stroke risk, which is otherwise primarily assessed based on the degree of narrowing of the lumen. A stereoscopic particle image velocimetry (stereo-PIV) system was configured to obtain instantaneous full-field velocity measurements in life-sized carotid artery models. Extraction of the central-plane and volumetric features of the flow revealed the complexity of the stenotic carotid flow, which increased with increasing stenosis severity and changed with the symmetry of the plaque. Evaluation of the energy content of two models of the stenosed carotid bifurcation provided insight on the expected level of flow instabilities with potential clinical implications. Studies in a comprehensive family of eight models ranging from disease-free to severely stenosed (30%, 50%, 70% diameter reduction) and with two types of plaque symmetry (concentric or eccentric), as well as a single ulcerated stenosed model, clearly demonstrated the significance of plaque geometry in marked alteration of the levels and patterns of downstream flow disturbances and shear stress. Plaque eccentricity and ulceration resulted in enhanced flow disturbances. In addition, shear-stress patterns in those models with eccentric stenosis were suggestive of increased thrombosis potential at the post-stenotic recirculation zone compared to their concentric counterpart plaques

In Vitro and Computational Analyses of Blood Flow at Aortoiliac Bifurcation for Patients with Atherosclerotic Plaque Treated with Endovascular Procedures

This research has developed an appropriate approach allowing for more accurate assessment of haemodynamic changes following implantation of endovascular stent graft to treat patients with occlusive aortoiliac disease. Two different endovascular techniques involving the use of different types of stent grafts were analysed and compared with regard to haemodynamics associated with these techniques. Results improved understanding of the flow characteristics of these endovascular techniques

The influence of anesthesia and fluid-structure interaction on simulated shear stress patterns in the carotid bifurcation of mice

Background: Low and oscillatory wall shear stresses (WSS) near aortic bifurcations have been linked to the onset of atherosclerosis. In previous work, we calculated detailed WSS patterns in the carotid bifurcation of mice using a Fluid-structure interaction (FSI) approach. We subsequently fed the animals a high-fat diet and linked the results of the FSI simulations to those of atherosclerotic plaque location on a within-subject basis. However, these simulations were based on boundary conditions measured under anesthesia, while active mice might experience different hemodynamics. Moreover, the FSI technique for mouse-specific simulations is both time- and labor-intensive, and might be replaced by simpler and easier Computational Fluid Dynamics (CFD) simulations. The goal of the current work was (i) to compare WSS patterns based on anesthesia conditions to those representing active resting and exercising conditions; and (ii) to compare WSS patterns based on FSI simulations to those based on steady-state and transient CFD simulations. Methods: For each of the 3 computational techniques (steady state CFD, transient CFD, FSI) we performed 5 simulations: 1 for anesthesia, 2 for conscious resting conditions and 2 more for conscious active conditions. The inflow, pressure and heart rate were scaled according to representative in vivo measurements obtained from literature. Results: When normalized by the maximal shear stress value, shear stress patterns were similar for the 3 computational techniques. For all activity levels, steady state CFD led to an overestimation of WSS values, while FSI simulations yielded a clear increase in WSS reversal at the outer side of the sinus of the external carotid artery that was not visible in transient CFD-simulations. Furthermore, the FSI simulations in the highest locomotor activity state showed a flow recirculation zone in the external carotid artery that was not present under anesthesia. This recirculation went hand in hand with locally increased WSS reversal. Conclusions: Our data show that FSI simulations are not necessary to obtain normalized WSS patterns, but indispensable to assess the oscillatory behavior of the WSS in mice. Flow recirculation and WSS reversal at the external carotid artery may occur during high locomotor activity while they are not present under anesthesia. These phenomena might thus influence plaque formation to a larger extent than what was previously assumed. (C) 2016 Elsevier Ltd. All rights reserved