123 results found
Johari NH, Wood NB, Cheng Z, et al., 2019, Disturbed flow in a stenosed carotid artery bifurcation: Comparison of RANS-based transitional model and LES with experimental measurements, International Journal of Applied Mechanics, Vol: 11, ISSN: 1758-8251
Blood flow in the carotid arteries is usually laminar, but can undergo laminar-turbulent transition in the presence of a high-grade stenosis. In this study, pulsatile flow in a patient-based stenosed carotid artery bifurcation was examined using both large eddy simulation (LES) with dynamic Smagorinsky eddy viscosity model, and a Reynolds-averaged Navier-Stokes (RANS) method with a transitional version of the shear stress transport (SST-Tran) model. In addition, an experimental phantom was built for the same bifurcation geometry and velocity measurements were made using particle image velocimetry (PIV). Comparisons with PIV measurements of axial velocity profiles demonstrated that both SST-Tran and LES predicted the experimental results fairly well, with LES being slightly superior. Furthermore, LES predicted cycle-to-cycle variations in the region where transition to turbulence occurred, indicating the unsteady nature of turbulence transition. On the other hand, the SST-Tran model was able to capture important flow features observed in the PIV experiment, demonstrating its potential as a cost-effective alternative to LES for haemodynamic analyses of highly disturbed flow in diseased arteries.
Cheng Z, Kidher E, Jarral OA, et al., 2016, Assessment of hemodynamic conditions in the aorta following root replacement with composite valve-conduit graft, Annals of Biomedical Engineering, Vol: 44, Pages: 1392-1404, ISSN: 0090-6964
This paper presents the analysis of detailed hemodynamics in the aortas of four patients following replacement with a composite bio-prosthetic valve-conduit. Magnetic resonance image-based computational models were set up for each patient with boundary conditions comprising subject-specific three-dimensional inflow velocity profiles at the aortic root and central pressure waveform at the model outlet. Two normal subjects were also included for comparison. The purpose of the study was to investigate the effects of the valve-conduit on flow in the proximal and distal aorta. The results suggested that following the composite valve-conduit implantation, the vortical flow structure and hemodynamic parameters in the aorta were altered, with slightly reduced helical flow index, elevated wall shear stress and higher non-uniformity in wall shear compared to normal aortas. Inter-individual analysis revealed different hemodynamic conditions among the patients depending on the conduit configuration in the ascending aorta, which is a key factor in determining post-operative aortic flow. Introducing a natural curvature in the conduit to create a smooth transition between the conduit and native aorta may help prevent the occurrence of retrograde and recirculating flow in the aortic arch, which is particularly important when a large portion or the entire ascending aorta needs to be replaced.
Singh SD, Xu XY, Wood NB, et al., 2015, Aortic flow patterns before and after personalised external aortic root support implantation in Marfan patients., Journal of Biomechanics, Vol: 49, Pages: 100-111, ISSN: 1873-2380
Implantation of a personalised external aortic root support (PEARS) in the Marfan aorta is a new procedure that has emerged recently, but its haemodynamic implication has not been investigated. The objective of this study was to compare the flow characteristics and hemodynamic indices in the aorta before and after insertion of PEARS, using combined cardiovascular magnetic resonance imaging (CMR) and computational fluid dynamics (CFD). Pre- and post-PEARS MR images were acquired from 3 patients and used to build patient-specific models and upstream flow conditions, which were incorporated into the CFD simulations. The results revealed that while the qualitative patterns of the haemodynamics were similar before and after PEARS implantation, the post-PEARS aortas had slightly less disturbed flow at the sinuses, as a result of reduced diameters in the post-PEARS aortic roots. Quantitative differences were observed between the pre- and post-PEARS aortas, in that the mean values of helicity flow index (HFI) varied by -10%, 35% and 20% in post-PEARS aortas of Patients 1, 2 and 3, respectively, but all values were within the range reported for normal aortas. Comparisons with MR measured velocities in the descending aorta of Patient 2 demonstrated that the computational models were able to reproduce the important flow features observed in vivo.
Li Z-Y, Tan FPP, Soloperto G, et al., 2015, Flow pattern analysis in a highly stenotic patient-specific carotid bifurcation model using a turbulence model, COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING, Vol: 18, Pages: 1099-1107, ISSN: 1025-5842
Wang Z, Wood NB, Xu XY, 2015, A viscoelastic fluid-structure interaction model for carotid arteries under pulsatile flow, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Vol: 31, ISSN: 2040-7939
Cheng Z, Wood NB, Gibbs RGJ, et al., 2015, Geometric and Flow Features of Type B Aortic Dissection: Initial Findings and Comparison of Medically Treated and Stented Cases, ANNALS OF BIOMEDICAL ENGINEERING, Vol: 43, Pages: 177-189, ISSN: 0090-6964
Cheng Z, Juli C, Wood NB, et al., 2014, Predicting flow in aortic dissection: Comparison of computational model with PC-MRI velocity measurements, MEDICAL ENGINEERING & PHYSICS, Vol: 36, Pages: 1176-1184, ISSN: 1350-4533
Wang Y, Downie S, Wood N, et al., 2013, Finite element analysis of the deformation of deep veins in the lower limb under external compression, MEDICAL ENGINEERING & PHYSICS, Vol: 35, Pages: 515-523, ISSN: 1350-4533
Cheng Z, Riga C, Chan J, et al., 2013, Initial findings and potential applicability of computational simulation of the aorta in acute type B dissection, JOURNAL OF VASCULAR SURGERY, Vol: 57, Pages: 35S-43S, ISSN: 0741-5214
Kousera CA, Wood NB, Seed WA, et al., 2013, A Numerical Study of Aortic Flow Stability and Comparison With <i>In</i> <i>Vivo</i> Flow Measurements, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 135, ISSN: 0148-0731
Borghi A, Wood NB, Mohiaddin RH, et al., 2012, Computational analysis of flow and stress patterns in patient specific thoracic aortic aneurysm models, Lecture Notes in Computational Vision and Biomechanics, Pages: 133-159
Thoracic Aortic Aneurysms (TAAs) are associated with low frequency in a given population but high mortality rate. No reliable surgical criterion is available at present but internal wall stress has proved to be more reliable as a predictor of rupture than the maximum diameter in case of Abdominal Aortic Aneurysms (AAAs). However, few studies have been reported on the role of biomechanical factors in the development and rupture of TAAs. This chapter describes a computational mechanics model of TAA based on patient-specific anatomical and flow conditions, acquired from Magnetic Resonance Imaging (MRI). The model has been applied to five patients with TAAs at different locations of the aorta. The results showed no correlation between peak stress and aneurysm size such as the maximum diameter. The effects of intra-luminal thrombus (ILT) and its mechanical properties on wall stress patterns were investigated. It has been found that the shape, size and location of ILT have a significant effect on wall stress patterns. Peak stress calculated using a fully coupled fluid-structure interaction simulation was similar to that predicted by a static solid simulation, in agreement with previous studies of AAA. The work presented here serves as a first step towards developing a reliable predictive tool to allow improved prognosis and surgical decision making for TAA patients.
Liu D, Wood NB, Witt N, et al., 2012, Assessment of Energy Requirement for the Retinal Arterial Network in Normal and Hypertensive Subjects, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 134, ISSN: 0148-0731
Wang Y, Pierce I, Gatehouse P, et al., 2012, Analysis of flow and wall shear stress in the peroneal veins under external compression based on real-time MR images, MEDICAL ENGINEERING & PHYSICS, Vol: 34, Pages: 17-27, ISSN: 1350-4533
Tan FPP, Xu XY, Torii R, et al., 2012, Comparison of Aortic Flow Patterns Before and After Transcatheter Aortic Valve Implantation, Cardiovascular Engineering and Technology, Vol: 3, Pages: 123-135
Abstract—Little is known of the likely changes in blood flowvelocity profiles and aortic wall shear stress (WSS) followingtranscatheter aortic valve implantation (TAVI). The objectiveof this study was to investigate the effects of TAVI onflow patterns in the thoracic aorta by using cardiovascularmagnetic resonance imaging (CMR) and computational fluiddynamics (CFD). An elderly patient with aortic stenosis wasexamined using MRI pre- and post-TAVI, and CFDsimulations were carried out incorporating MRI-derivedpatient-specific anatomy and upstream flow conditions. Pre-TAVI velocity profiles demonstrated the highly disturbedturbulent flow and jet impacting the wall of the arch owing tothe partial opening of the stenosed aortic valve, with likelypathological effects. In the Post-TAVI aorta, velocity profileswere similar to those of healthy aortas with spatially moreuniform WSS and lower turbulence levels, demonstrating thefavourable effects of the TAVI procedure in restoring normalaortic flow. This study has shown both the effectiveness ofTAVI on an individual patient and the advantage of thecombined CMR and CFD method for a comprehensivepatient-specific assessment of pre- and post-TAVI aortic flowpatterns and WSS over CMR alone.
Tan FPP, Wood NB, Tabor G, et al., 2011, Comparison of LES of Steady Transitional Flow in an Idealized Stenosed Axisymmetric Artery Model With a RANS Transitional Model, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 133, ISSN: 0148-0731
Cheng Z, Tan FPP, Riga CV, et al., 2010, Analysis of Flow Patterns in a Patient-Specific Aortic Dissection Model, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 132, ISSN: 0148-0731
Soloperto G, Keenan NG, Sheppard MN, et al., 2010, Combined imaging, computational and histological analysis of a ruptured carotid plaque: A patient-specific analysis., Artery Research, Vol: 4, Pages: 59-65
Background: Rupture of carotid plaques is an important cause of cerebrovascularevents. Several factors, including wall shear stress (WSS), plaque morphology and peak capstress, have been associated with plaque vulnerability. The aim of this study was to investigatethe relationship between these factors in an in vivo human ruptured carotid plaque.Methods: A 74-year-old male presenting with a transient ischemic attack underwent carotidmagnetic resonance imaging (MRI), which indicated a ruptured plaque, followed by carotidendarterectomy, from which plaque histology was assessed. The carotid bifurcation was reconstructedfrom the MRI data, and three-dimensional flow simulations were performed usingcomputational fluid dynamics to determine WSS and related parameters. Plaque vulnerabilitywas assessed using a biomechanical method based on modified Glagov criteria.Results: The plaque rupture was just distal to the site of maximum stenosis in a region of lowWSS, where MRI and histology both demonstrated fibrous cap thinning, a large lipid pool andcalcification in the shoulder region. Plaque vulnerability analysis indicated critically vulnerableplaque at the rupture site by a wide margin.Conclusions: Both low and high WSS have been associated with plaque vulnerability, and highmechanical stress in the cap has been linked to plaque rupture, but these parameters are notroutinely assessed clinically. This study demonstrates a complete analysis by combiningimaging, histology and bio-fluid and biomechanical modelling.
Torii R, Keegan J, Wood NB, et al., 2010, MR image-based geometric and hemodynamic investigation of the right coronary artery withdynamic vessel motion, Annals of Biomedical Engineering, Vol: in press
The aim of this study was to develop a fully subject-specific model of the right coronaryartery (RCA), including dynamic vessel motion, for computational analysis to assess the effects ofcardiac-induced motion on hemodynamics and resulting wall shear stress (WSS). Vascular geometrieswere acquired in the right coronary artery (RCA) of a healthy volunteer using a navigator-gatedinterleaved spiral sequence at 14 time points during the cardiac cycle. A high temporal resolutionvelocity waveform was also acquired in the proximal region. Cardiac-induced dynamic vessel motionwas calculated by interpolating the geometries with an active contour model and a CFD simulationwith fully subject-specific information was carried out using this model. The results showed theexpected variation of vessel radius and curvature throughout the cardiac cycle, and also revealed thatdynamic motion of the right coronary artery consequent to cardiac motion had significant effects oninstantaneous WSS and oscillatory shear index (OSI). Subject specific MRI-based CFD is feasible and, ifscan duration could be shortened, this method may have potential as a non-invasive tool to investigatethe physiological and pathological role of hemodynamics in human coronary arteries.
Foin N, Segers D, Mohri Z, et al., 2010, Coronary Stenosis Imaging, Structure and Physiology, Part 1, Chapter 3: Haemodynamic effects of focal and diffuse coronary stenosis, Publisher: PCR Publishing, ISBN: 9782913628564
This book is an indispensable companion for all those involved in the diagnosis and treatment of ischemic heart disease. With a foreword by Morton J. Kern, MD, PhD.
Soloperto G, Keenan NG, Chan C, et al., 2009, Comparison between ruptured and intact atherosclerotic plaques: flow- modelling study of in vivo carotid arteries by cardiovascular magnetic resonance, EUROPEAN HEART JOURNAL, Vol: 30, Pages: 763-763, ISSN: 0195-668X
Torii R, Wood NB, Hadjiloizou N, et al., 2009, Fluid-structure interaction analysis of a patient-specific right coronary artery with physiological velocity and pressure waveforms, Communications in Numerical Methods in Engineering, Vol: 25, Pages: 565-580, ISSN: 1069-8299
Coupled fluid-structure interaction (FSI) analysis of the human right coronary artery (RCA) has been carried out to investigate the effects of wall compliance on coronary hemodynamics. A 3-D model of a stenosed RCA was reconstructed based oil multislice computerized tomography images. A velocity waveform in the proximal RCA and a pressure waveform in the distal RCA of a patient with a severe stenosis were acquired with a catheter delivered wire probe and applied as boundary conditions. The arterial wall was modeled as a Mooncy-Rivlin hyperelastic material. The predicted maximum wall displacement (3.85 mm) was comparable with the vessel diameter (similar to 4 mm), but the diameter variation was much smaller, 0.134 mm at the stenosis and 0.486 mm in the distal region. Comparison of the computational results between the FSI and rigid-wall models showed that the instantaneous wall shear stress (WSS) distributions were affected by diameter variation in the arterial walk increasing systolic blood pressure dilated the vessel and consequently lowered WSS, whereas the opposite occurred When pressure started to decrease. However. file effects of wall compliance on time-averaged WSS (TAWSS) and oscillatory shear index (OSI) were insignificant (4.5 and 2.7% difference in maximum TAWSS and OSI. respectively). Copyright (C) 2009 John Wiley & Sons, Ltd.
Torii R, Wood NB, Hadjiloizou N, et al., 2009, Stress phase angle depicts differences in coronary artery hemodynamics due to changes in flow and geometry after percutaneous coronary intervention, American Journal of Physiology - Heart and Circulatory Physiology, Vol: 296, Pages: H765-H776, ISSN: 0363-6135
Torii R, Wood NB, Hadjiloizou N, Dowsey AW, Wright AR, Hughes AD, Davies J, Francis DP, Mayet J, Yang G, Thom SA, Xu XY. Stress phase angle depicts differences in coronary artery hemodynamics due to changes in flow and geometry after percutaneous coronary intervention. Am J Physiol Heart Circ Physiol 296: H765-H776, 2009. First published January 16, 2009; doi:10.1152/ajpheart.01166.2007.-The effects of changes in flow velocity waveform and arterial geometry before and after percutaneous coronary intervention (PCI) in the right coronary artery (RCA) were investigated using computational fluid dynamics. An RCA from a patient with a stenosis was reconstructed based on multislice computerized tomography images. A nonstenosed model, simulating the same RCA after PCI, was also constructed. The blood flows in the RCA models were simulated using pulsatile flow waveforms acquired with an intravascular ultrasound-Doppler probe in the RCA of a patient undergoing PCI. It was found that differences in the waveforms before and after PCI did not affect the time-averaged wall shear stress and oscillatory shear index, but the phase angle between pressure and wall shear stress on the endothelium, stress phase angle (SPA), differed markedly. The median SPA was -63.9 degrees (range, -204 degrees to -10.0 degrees) for the pre-PCI state, whereas it was 10.4 degrees (range, -71.1 degrees to 25.4 degrees) in the post-PCI state, i.e., more asynchronous in the pre-PCI state. SPA has been reported to influence the secretion of vasoactive molecules (e. g., nitric oxide, PGI(2), and endothelin-1), and asynchronous SPA (approximate to -180 degrees) is proposed to be proatherogenic. Our results suggest that differences in the pulsatile flow waveform may have an important influence on atherogenesis, although associated with only minor changes in the time-averaged wall shear stress and oscillatory shear index. SPA may be a useful indicator in predicting sites prone to atherosclerosis.
Sun N, Torii R, Wood NB, et al., 2009, Computational modeling of LDL and albumin transport in an in vivo CT image-based human right coronary artery, Journal of Biomechanical Engineering, Vol: 131, Pages: 1-1-1-10
Sun N, Torii R, Wood NB, et al., 2009, Computational Modeling of LDL and Albumin Transport in an In Vivo CT Image-Based Human Right Coronary Artery, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 131, ISSN: 0148-0731
Torii R, Keegan J, Wood NB, et al., 2009, The effect of dynamic vessel motion on haemodynamic parameters in the right coronary artery: a combined MR and CFD study, BRITISH JOURNAL OF RADIOLOGY, Vol: 82, Pages: S24-S32, ISSN: 0007-1285
Sun N, Leung JH, Wood NB, et al., 2009, Computational analysis of oxygen transport in a patient-specific model of abdominal aortic aneurysm with intraluminal thrombus, BRITISH JOURNAL OF RADIOLOGY, Vol: 82, Pages: S18-S23, ISSN: 0007-1285
Jackson MJ, Wood NB, Zhao SZ, et al., 2009, Low wall shear stress predicts subsequent development of wall hypertrophy in lower limb bypass grafts, Artery Research, Pages: 1-7
Background: Venous grafts commonly develop myointimal hyperplasia, which can lead to stenoses and, ultimately, with expression of adhesion molecules, lumenal occlusion. The aim of the present study was to investigate whether wall shear stress measured post-operatively would predict subsequent myointimal hypertrophy in lower limb venous bypass grafts. Methods: Magnetic resonance imaging and ultrasound were performed in a cohort of patients following lower limb venous bypass graft surgery for peripheral arterial disease at baseline (1e2 weeks) and at follow-up (9e12 months). Wall shear stress was determined at baseline using computational fluid dynamics techniques and intima-media thickness along the length of the graft was measured by ultrasound at baseline and follow up. Results: Complete follow-up was possible in eight patients, in whom low wall shear stress at baseline predicted high intima-media thickness. The relationship between wall shear stress (WSS) and intima-media thickness (IMT)was curvilinear with IMTincreasing sharply at lower levels ofWSS (IMT >1.0 mm at <0.3 Pa). Conclusions: Low wall shear stress is associated with subsequent increase in myointimal thickness in lower limb venous bypass grafts. This is believed to be the first prospective study in humans to demonstrate the relationship between low wall shear stress and myointimal thickening and indicates a likely causative role for lowwall shear stress in the development ofmyointimal hyperplasia. ª 2009 Association for Research into Arterial Structure and Physiology. Published by Elsevier B.V. All rights reserved.
Liu D, Wood NB, Witt N, et al., 2009, Computational Analysis of Oxygen Transport in the Retinal Arterial Network, CURRENT EYE RESEARCH, Vol: 34, Pages: 945-956, ISSN: 0271-3683
Tan FPP, Torii R, Borghi A, et al., 2009, Analysis of flow patterns in a patient-specific thoracic aortic aneurysm model, Publisher: Elsevier, Pages: 680-690
In this study, a newly developed two-equation transitional model was employed for the prediction of blood flow patterns in a thoracic aortic aneurysm (TAA) where the growth and progression are closely linked to low and oscillating wall shear stresses. Laminar–turbulent transition in the dilated vessel can alter the flow structure, shear stress and pressure distribution within the aneurysm. A patient-specific TAA model was reconstructed from magnetic-resonance (MR) images and measured velocity waveform was used as the inflow condition. Laminar flow and a correlation-based transitional version of Menter’s hybrid k /k x Shear Stress Transport (SST Tran) model were implemented in pulsatile simulations from which WSS distribution was obtained throughout a cardiac cycle and velocity profiles were compared with MR measurements. The correlation-based transitional model was found to produce results in closer agreement with the MR data than the laminar flow simulation.
Jackson MJ, Wood NB, Zhao S, et al., 2009, Low wall shear stress predicts subsequent development of wall hypertrophy in lower limb bypass grafts, Artery Research, Vol: 3, Pages: 32-38, ISSN: 1872-9312
Background: Venous grafts commonly develop myointimal hyperplasia, which can lead to stenoses and, ultimately, with expression of adhesion molecules, lumenal occlusion. The aim of the present study was to investigate whether wall shear stress measured post-operatively would predict subsequent myointimal hypertrophy in lower limb venous bypass grafts. Methods: Magnetic resonance imaging and ultrasound were performed in a cohort of patients following lower limb venous bypass graft surgery for peripheral arterial disease at baseline (1e2 weeks) and at follow-up (9e12 months). Wall shear stress was determined at baseline using computational fluid dynamics techniques and intima-media thickness along the length of the graft was measured by ultrasound at baseline and follow up. Results: Complete follow-up was possible in eight patients, in whom low wall shear stress at baseline predicted high intima-media thickness. The relationship between wall shear stress (WSS) and intima-media thickness (IMT)was curvilinear with IMTincreasing sharply at lower levels ofWSS (IMT >1.0 mm at <0.3 Pa). Conclusions: Low wall shear stress is associated with subsequent increase in myointimal thickness in lower limb venous bypass grafts. This is believed to be the first prospective study in humans to demonstrate the relationship between low wall shear stress and myointimal thickening and indicates a likely causative role for lowwall shear stress in the development ofmyointimal hyperplasia.
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