211 research outputs found
A voxelized immersed boundary (VIB) finite element method for accurate and efficient blood flow simulation
We present an efficient and accurate immersed boundary (IB) finite element
(FE) method for internal flow problems with complex geometries (e.g., blood
flow in the vascular system). In this study, we use a voxelized flow domain
(discretized with hexahedral and tetrahedral elements) instead of a box domain,
which is frequently used in IB methods. The proposed method utilizes the
well-established incremental pressure correction scheme (IPCS) FE solver, and
the boundary condition-enforced IB (BCE-IB) method to numerically solve the
transient, incompressible Navier--Stokes flow equations. We verify the accuracy
of our numerical method using the analytical solution for the Poiseuille flow
in a cylinder, and the available experimental data (laser Doppler velocimetry)
for the flow in a three-dimensional 90{\deg} angle tube bend. We further
examine the accuracy and applicability of the proposed method by considering
flow within complex geometries, such as blood flow in aneurysmal vessels and
the aorta, flow configurations that would otherwise be difficult to solve by
most IB methods. Our method offers high accuracy, as demonstrated by the
verification examples, and high applicability, as demonstrated through the
solution of blood flow within complex geometry. The proposed method is
efficient, since it is as fast as the traditional finite element method used to
solve the Navier--Stokes flow equations, with a small overhead (not more than
5) due to the numerical solution of a linear system formulated for the IB
method.Comment: arXiv admin note: substantial text overlap with arXiv:2007.0208
Suite of Meshless Algorithms for Accurate Computation of Soft Tissue Deformation for Surgical Simulation
The ability to predict patient-specific soft tissue deformations is key for
computer-integrated surgery systems and the core enabling technology for a new
era of personalized medicine. Element-Free Galerkin (EFG) methods are better
suited for solving soft tissue deformation problems than the finite element
method (FEM) due to their capability of handling large deformation while also
eliminating the necessity of creating a complex predefined mesh. Nevertheless,
meshless methods based on EFG formulation, exhibit three major limitations: i)
meshless shape functions using higher order basis cannot always be computed for
arbitrarily distributed nodes (irregular node placement is crucial for
facilitating automated discretization of complex geometries); ii) imposition of
the Essential Boundary Conditions (EBC) is not straightforward; and, iii)
numerical (Gauss) integration in space is not exact as meshless shape functions
are not polynomial. This paper presents a suite of Meshless Total Lagrangian
Explicit Dynamics (MTLED) algorithms incorporating a Modified Moving Least
Squares (MMLS) method for interpolating scattered data both for visualization
and for numerical computations of soft tissue deformation, a novel way of
imposing EBC for explicit time integration, and an adaptive numerical
integration procedure within the Meshless Total Lagrangian Explicit Dynamics
algorithm. The appropriateness and effectiveness of the proposed methods is
demonstrated using comparisons with the established non-linear procedures from
commercial finite element software ABAQUS and experiments with very large
deformations. To demonstrate the translational benefits of MTLED we also
present a realistic brain-shift computation.Comment: Accepted for publication in Medical Image Analysi
Patient-specific computational modeling of subendothelial LDL accumulation in a stenosed right coronary artery: effect of hemodynamic and biological factors
Patient-specific computational modeling of subendothelial LDL accumulation in a stenosed right coronary artery: effect of hemodynamic and biological factors. Am J Physiol Heart Circ Physiol 304: H1455-H1470, 2013. First published March 15, 2013; doi:10.1152/ajpheart.00539.2012.-Atherosclerosis is a systemic disease with local manifestations. Low-density lipoprotein (LDL) accumulation in the subendothelial layer is one of the hallmarks of atherosclerosis onset and ignites plaque development and progression. Blood flow-induced endothelial shear stress (ESS) is causally related to the heterogenic distribution of atherosclerotic lesions and critically affects LDL deposition in the vessel wall. In this work we modeled blood flow and LDL transport in the coronary arterial wall and investigated the influence of several hemodynamic and biological factors that may regulate LDL accumulation. We used a three-dimensional model of a stenosed right coronary artery reconstructed from angiographic and intravascular ultrasound patient data. We also reconstructed a second model after restoring the patency of the stenosed lumen to its nondiseased state to assess the effect of the stenosis on LDL accumulation
Prevalence of scarred and dysfunctional myocardium in patients with heart failure of ischaemic origin: a cardiovascular magnetic resonance study
BACKGROUND: Cardiovascular magnetic resonance (CMR) with late gadolinium enhancement (LGE) can provide unique data on the transmural extent of scar/viability. We assessed the prevalence of dysfunctional myocardium, including partial thickness scar, which could contribute to left ventricular contractile dysfunction in patients with heart failure and ischaemic heart disease who denied angina symptoms. METHODS: We invited patients with ischaemic heart disease and a left ventricular ejection fraction < 50% by echocardiography to have LGE CMR. Myocardial contractility and transmural extent of scar were assessed using a 17-segment model. RESULTS: The median age of the 193 patients enrolled was 70 (interquartile range: 63-76) years and 167 (87%) were men. Of 3281 myocardial segments assessed, 1759 (54%) were dysfunctional, of which 581 (33%) showed no scar, 623 (35%) had scar affecting ā¤50% of wall thickness and 555 (32%) had scar affecting > 50% of wall thickness. Of 1522 segments with normal contractile function, only 98 (6%) had evidence of scar on CMR. Overall, 182 (94%) patients had ā„1 and 107 (55%) patients had ā„5 segments with contractile dysfunction that had no scar or ā¤50% transmural scar suggesting viability. CONCLUSIONS: In this cohort of patients with left ventricular systolic dysfunction and ischaemic heart disease, about half of all segments had contractile dysfunction but only one third of these had > 50% of the wall thickness affected by scar, suggesting that most dysfunctional segments could improve in response to an appropriate intervention
The Safety and Feasibility of Transitioning From Transfemoral to Transradial Access Left Ventricular Endomyocardial Biopsy
BACKGROUND: Left ventricular endomyocardial biopsy (LVEMB) is commonly performed via the transfemoral route. Radial access may help reduce vascular access complications, but there are few data on the safety and feasibility of transradial LVEMB. OBJECTIVE: Describe the safety and feasibility of transitioning from transfemoral to transradial access LVEMB. METHODS: This is a single-center, prospective, observational cohort study. Fifty procedures in 49 patients were included, 25 (50%) via the femoral route and 25 (50%) via the radial route. RESULTS: The cohort had a mean age of 47 Ā± 13 years and the most common indication for LVEMB was myocarditis. From June 2015 until September 2016, all procedures (n = 21) were performed via the femoral approach; thenceforth, there was a gradual transition to the radial approach. More tissue samples were obtained when the procedure was performed via the femoral approach (P<.01). The minimum sampling target of 3 specimens was not met in 4 patients (16%) via the radial approach and in 1 patient (4%) via the femoral approach. Complications occurred in 3/25 transradial procedures (12%; 2 cardiac perforations and 1 forearm hematoma) and 3/25 transfemoral procedures (12%; 1 cardiac perforation, 1 femoral artery pseudoaneurysm, and 1 ventricular fibrillation). Cardiac perforations via the transradial approach occurred during the early transition period. There were no deaths. CONCLUSIONS: Transradial LVEMB is feasible, with a similar complication profile to femoral procedures, but associated with a smaller number of specimens. Transitioning from transfemoral to transradial procedures may initially be associated with a higher risk of complications and potentially a lower diagnostic yield
The link between job satisfaction and organizational commitment:differences between public and private sector employees
Employees in the public and private sectors experience different working conditions and employment relationships. Therefore, it can be assumed that their attitudes toward their job and organizations, and relationships between them, are different. The existing literature has identified the relationship between organizational commitment and job satisfaction as interesting in this context. The present field study examines the satisfactionācommitment link with respect to differences between private and public sector employees. A sample of 617 Greek employees (257 from the private sector and 360 from the public sector) completed standardized questionnaires. Results confirmed the hypothesized relationship differences: Extrinsic satisfaction and intrinsic satisfaction are more strongly related to affective commitment and normative commitment for public sector employees than for private sector ones. The results are discussed, limitations are considered, and directions for future research are proposed
Three-dimensional reconstruction of coronary arteries and plaque morphology using CT angiography - comparison and registration with IVUS
BACKGROUND: The aim of this study is to present a new methodology for three-dimensional (3D) reconstruction of coronary arteries and plaque morphology using Computed Tomography Angiography (CTA). METHODS: The methodology is summarized in six stages: 1) pre-processing of the initial raw images, 2) rough estimation of the lumen and outer vessel wall borders and approximation of the vesselās centerline, 3) manual adaptation of plaque parameters, 4) accurate extraction of the luminal centerline, 5) detection of the lumen - outer vessel wall borders and calcium plaque region, and 6) finally 3D surface construction. RESULTS: The methodology was compared to the estimations of a recently presented Intravascular Ultrasound (IVUS) plaque characterization method. The correlation coefficients for calcium volume, surface area, length and angle vessel were 0.79, 0.86, 0.95 and 0.88, respectively. Additionally, when comparing the inner and outer vessel wall volumes of the reconstructed arteries produced by IVUS and CTA the observed correlation was 0.87 and 0.83, respectively. CONCLUSIONS: The results indicated that the proposed methodology is fast and accurate and thus it is likely in the future to have applications in research and clinical arena
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