Idit Avrahami | Ariel University (original) (raw)

Papers by Idit Avrahami

Journal of Biomechanics, 2006

Journal of biomechanics, Jan 19, 2015

This paper presents a method for analyzing and comparing numerically Saphenous Vein Grafts (SVGs)... more This paper presents a method for analyzing and comparing numerically Saphenous Vein Grafts (SVGs) following Coronary Artery Bypass Graft surgery (CABG). The method analyses the flow dynamics inside vein grafts with and without supporting using Venous External Scaffolding Technology (VEST). The numerical method uses patients׳ specific computational fluid dynamics (CFD) methods to characterize the relevant hemodynamic parameters of patients׳ SVGs. The method was used to compare the hemodynamics of six patient׳s specific model and flow conditions of stented and non-stented SVGs, 12 months post-transplantation. The flow parameters used to characterize the grafts׳ hemodynamics include Time Averaged Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI) and Relative Residence Time (RRT). The effect of stenting was clearly demonstrated by the chosen parameters. SVGs under constriction of VEST were associated with similar spatial average of TAWSS (10.73 vs 10.29dyn/cm(2)), yet had fewer l...

International Journal of Hydrogen Energy

Computational and mathematical methods in medicine, 2018

The carotid bifurcation tends to develop atherosclerotic stenoses which might interfere with cere... more The carotid bifurcation tends to develop atherosclerotic stenoses which might interfere with cerebral blood supply. In cases of arterial blockage, the common clinical solution is to remove the plaque via carotid endarterectomy (CEA) surgery. Artery closure after surgery using primary closures along the cutting edge might lead to artery narrowing and restrict blood flow. An alternative approach is patch angioplasty which takes longer time and leads to more during-surgery complications. The present study uses numerical methods with fluid-structure interaction (FSI) to explore and compare the two solutions in terms of hemodynamics and stress and strain fields developed in the artery wall.

Biomechanics and Modeling in Mechanobiology

During diastole, coronary perfusion depends on the pressure gradient between the myocardial tissu... more During diastole, coronary perfusion depends on the pressure gradient between the myocardial tissue and the coronary origin located at the aortic root. This pressure gradient is influenced by the flow field near the closing valve leaflets. Clinical evidence is conclusive that patients with severe aortic stenosis (AS) suffer from diastolic dysfunction during hyperemia, but show increased coronary blood flow during rest. Physiological or pathological factors do not provide an explanation of this effect. Transcatheter aortic valve implantation (TAVI) was shown to decrease the coronary flow along with its main purpose of improving the aortic flow and reducing the risk of heart failure. There is no clear physiological or pathological explanation for the increase of coronary flow due to AS and its decrease immediately after TAVI. In this manuscript, we present a numerical study that examines the impact of AS and valve replacement on the coronary perfusion during rest conditions. The study compares the hemodynamics of five different 2D numerical models: a baseline “healthy valve” case, two AS cases and two TAVI cases. The analysis used time-dependent computational fluid-structure interaction (FSI) simulations of the blood flow in the aortic root including the dynamics of the flexible passive valve leaflets and the varying resistance of the coronary arteries. Despite its simplifications, our 2D model succeeded to capture the major effects that dominant the blood hemodynamics in the aortic root, and to explain the hemodynamic effect that leads to the changes in coronary flow found in other in-vitro and clinical studies. It provides an important insight and emphasizes the need for more comprehensive studies on coronary's hemodynamics after valve replacement. There are numerous variations of TAVI designs and it is very difficult and time consuming to cover all the possible models using complex 3D models. The use of a simple 2D model with FSI and varying coronary resistance can be of great help to demonstrate the main features of the design.

Aortic Aneurysm, 2017

Aortic arch aneurysm is a complex aortic pathology which affects one or more aortic arch vessels.... more Aortic arch aneurysm is a complex aortic pathology which affects one or more aortic arch vessels. In this chapter, we explore the hemodynamic behavior of the aortic arch in aneurysmatic and treated cases with three currently available treatment approaches: surgery graft, hybrid stent-graft and chimney stent-graft. The analysis included time-dependent experimental and numerical models of aneurysmatic arch and of the surgery, hybrid and chimney endovascular techniques. Dimensions of the models are based on typical anatomy, and boundary conditions are based on typical physiological flow. Flexible and transparent experimental models were used on a mock circulation in vitro experimental system to allow both visualization and time-dependent flow and pressure measurements. The simulations used computational fluid dynamics (CFD) methods to delineate the time-dependent flow dynamics in the four geometric models. Results of velocity vectors, flow patterns, pressure and wall shear stress distributions are presented. Both the numerical and experimental results agree on the poor hemodynamics of the aortic arch aneurysm and present the hemodynamic advantages of the surgery technique, implying the possible advantage of fenestrated stent-graft for the aortic arch. Out of the two minimally invasive procedures, the hybrid procedure clearly exhibits better hemodynamic performances. The chimney graft technique is based on off-the-shelf devices; thus, it is low in cost and requires less pre-operation preparations. However, it is associated with higher risks for complications, such as endoleaks and stroke. This chapter may give some insight into the hemodynamic characteristics of the different procedures.

Aps Division of Fluid Dynamics Meeting Abstracts, 2000

ABSTRACT Arteriovenous fistulae (AVF) serve as vascular access for hemodialysis. Failure of AVF o... more ABSTRACT Arteriovenous fistulae (AVF) serve as vascular access for hemodialysis. Failure of AVF often occurs due to stenosis. A previous numerical study, employing pulsatile CFD with an idealized geometry (IG) model, showed a correlation between local flow disturbance and typical sites of stenosis in radiocephalic AVF, using the relative residence time (RRT) index. However, the employment of IG gives rise to the question whether or not these findings hold true for more general and patient-specific geometries. Therefore, in the current work five different AVF geometries were modeled, of which four are IG with different anastomosis angles, and the fifth involves three geometric irregularities. The results of this work show that variations in the anastomosis angle have some effect on the span and distribution of the RRT field, but the correlation between high RRT zones and typical stenosis sites is maintained. In contrast, geometric irregularities have a more significant impact on the RRT values. Moreover, these results show that for an AVF model with geometric irregularities, an additional zone of high RRT values emerges, which might imply that the RRT index, when applied with non-idealized geometry, does not capture the typical spread of stenosis in radiocephalic AVF.

Computers in Biology and Medicine, Jun 23, 2014

Introduction: Restenosis is strongly attributed to stresses caused by stent-artery interactions g... more Introduction: Restenosis is strongly attributed to stresses caused by stent-artery interactions generated in the artery after balloon angioplasty. Numerical methods are often used to examine the stent-artery mechanical interactions. To overcome the extensive computational requirements demanded by these simulations, simplifications are needed. Objective: We introduce simplified models to calculate the mechanical interactions between netstructured stents and arteries, and discuss their validity and implications. Methods: 2D simplified numerical models are suggested, which allow cost effective assessment of arterial stresses and the potential damage factor (DF). In these models, several contact problems were solved for arteries with hyper elastic mechanical properties. Stresses were calculated for a large range of cases and for different numerical model types. The effects of model simplifications, oversizing mismatch and stenosis rate and length and symmetry on the resulting stresses were analyzed. Results & conclusions: Results obtained from planar 2D models were found in good agreement with results obtained from complex 3D models for cases with axisymmetric constant or varying stenosis. This high correlation between the results of 3D cases with varying stenosis and the more simple 2D cases can be used as a simplified and convenient tool for calculating the arterial wall stresses in complex cases. Maximal stresses obtained by the 2D model with an asymmetric stenosis are lower than the maximal stresses obtained in the axisymmetric case with the same stenosis percentage. Therefore, axisymmetric models may provide the worst-case estimation values for a stent of interest.

The presented study is focused on the hemodynamics aspects of thoracic aortic aneurysm and approa... more The presented study is focused on the hemodynamics aspects of thoracic aortic aneurysm and approaches for restoring hemodynamics in the aortic arch. The study includes numerical investigation of the aortic arch hemodynamics of a healthy aorta, aorta with aneurysm, and of two endovascular repairing procedures. The first endovascular repair approach is the total aortic arch hybrid debranching. The second implantation uses chimney graft technique. The analysis includes the fluid dynamics in the aorta and branching arteries under time-dependent physiological conditions. The results show the effect of aneurysm on blood flow in the descending aorta and in aortic arch side branches. In the aneurysmatic case, the aneurysm provokes a highly disturbed flow and large recirculation regions, especially during diastole. Out of the two endovascular techniques, the hybrid procedure was found preferred from hemodynamics point of view, with less disturbed and recirculating regions. Although the chimney procedure requires less manufacturing times and cost, it is associated with higher risks rate, and therefore, it is recommended only for emergency cases. This study may shade light on the hemodynamic factors for these complications, and provide insights on ways to improve the procedure.

Biomedicine & Pharmacotherapy, 2015

Patients with coronary heart disease demonstrate changes in skin microcirculation and a decrease ... more Patients with coronary heart disease demonstrate changes in skin microcirculation and a decrease in cutaneous blood mass. The goal of this study was to assess the feasibility of diagnosing myocardial ischemia based on peripheral microcirculatory variables. The skin microcirculatory measurements were monitored using an LPT system comprising a Laser Doppler Flowmeter (LDF), a photoplethysmograph (PPG) and a transcutaneous oxygen tension device (tc-PO2). Concurrently, heart rate and blood pressure were monitored. Measurements were performed before and after exercise stress test. Subjects were divided into ischemic (20) and nonischemic (27) patients based on myocardial perfusion imaging (MPI). The results indicate differences in LPT variables between ischemic and nonischemic patients following exercise, while no differences in the central variable values were observed between the two groups. Peripheral microcirculatory variables may be useful for non-invasive assessment of myocardial ischemia. The system has clinical potential for sensitive and noninvasive monitoring of vital variables during medical procedures in clinics, as well as in home care for patients who suffer from ischemic cardiac diseases.