1,721,854 research outputs found
Abdominal aortic aneurysm and thrombus modeling
No full textAAA disease, the local enlargement of the infrarenal aorta, is a serious condition that causes many deaths, especially in men exceeding 65years of age. Over the past quarter of a century, computational biomechanical models have been developed toward the assessment of AAA risk of rupture, technology that is now on the verge of being integrated within the clinical decision-making process. The modeling of AAA requires a holistic understanding of the clinical problem to set appropriate modeling assumptions and to draw sound conclusions from the simulation results. In this chapter, we summarize and critically discuss the state-of-the-art in AAA modeling, including the level of model validation. While most aspects concerning computational mechanics have already been settled, the exploration of the failure properties of the AAA wall and the acquisition of robust input data for simulations have the greatest potential for advancing this technology
Abdominal aortic aneurysm rupture prediction
No full textAbdominal aortic aneurysm (AAA) rupture is a local event in the aneurysm wall and global parameters, such as the maximum diameter and its expansion over time can only give very rough risk indications. It has a low specificity and often fails to predict individual risk for AAA rupture. This drawback stimulated the search for a more robust and individualized risk indicator, and the Biomechanical Rupture Risk Assessment (BRRA) is one among such approach. It attracted much interest in recent decades and could nowadays potentially complement the diameter indication for AAA repair. While Chapter 11 reviewed biomechanical modeling aspects, this chapter focuses on the clinical application of the BRRA. Workflow aspects and validation studies are summarized toward exploring BRRA clinical relevance. The last section discusses ongoing research concerning the integration of heterogenous risk factors, the exploration of vascular wall fracture mechanisms, and our understanding of the role played by AAA growth in rupture risk
An Integrated Fluid-Chemical Model Toward Modeling the Formation of Intra-Luminal Thrombus in Abdominal Aortic Aneurysms
No full textAbdominal Aortic Aneurysms (AAAs) are frequently characterized by the presence of an Intra-Luminal Thrombus (ILT) known to influence their evolution biochemically and biomechanically. The ILT progression mechanism is still unclear and little is known regarding the impact of the chemical species transported by blood flow on this mechanism. Chemical agonists and antagonists of platelets activation, aggregation, and adhesion and the proteins involved in the coagulation cascade (CC) may play an important role in ILT development. Starting from this assumption, the evolution of chemical species involved in the CC, their relation to coherent vortical structures (VSs) and their possible effect on ILT evolution have been studied. To this end a fluid-chemical model that simulates the CC through a series of convection-diffusion-reaction (CDR) equations has been developed. The model involves plasma-phase and surface-bound enzymes and zymogens, and includes both plasma-phase and membrane-phase reactions. Blood is modeled as a non-Newtonian incompressible fluid. VSs convect thrombin in the domain and lead to the high concentration observed in the distal portion of the AAA. This finding is in line with the clinical observations showing that the thickest ILT is usually seen in the distal AAA region. The proposed model, due to its ability to couple the fluid and chemical domains, provides an integrated mechanochemical picture that potentially could help unveil mechanisms of ILT formation and development
Synergy between shear-induced migration and secondary flows on red blood cells transport in arteries: considerations on oxygen transport
No full textShear-induced migration of red blood cells (RBCs) is a well-known
phenomenon characterizing blood flow in the small vessels (micrometre to
millimetre size) of the cardiovascular system. In large vessels, like the
abdominal aorta and the carotid artery (millimetre to centimetre size), the
extent of this migration and its interaction with secondary flows has not
been fully elucidated. RBC migration exerts its influence primarily on platelet
concentration, oxygen transport and oxygen availability at the luminal
surface, which could influence vessel wall disease processes in and adjacent
to the intima. Phillips’ shear-induced particle migration model, coupled to
the Quemada viscosity model, was employed to simulate the macroscopic
behaviour of RBCs in four patient-specific geometries: a normal abdominal
aorta, an abdominal aortic aneurysm (AAA), a normal carotid bifurcation
and a stenotic carotid bifurcation. Simulations show a migration of RBCs
from the near-wall region with a lowering of wall haematocrit (volume fraction
of RBCs) on the posterior side of the normal aorta and on the lateralexternal
side of the iliac arteries. A marked migration is observed on the
outer wall of the carotid sinus, along the common carotid artery and in
the carotid stenosis. No significant migration is observed in the AAA. The
spatial and temporal patterns of wall haematocrit are correlated with the
near-wall shear layer and with the secondary flows induced by the vessel
curvature. In particular, secondary flows accentuate the initial lowering in
RBC near-wall concentration by convecting RBCs from the inner curvature
side to the outer curvature side. The results reinforce data in literature showing
a decrease in oxygen partial pressure on the inner curvature wall of the
carotid sinus induced by the presence of secondary flows. The lowering of
wall haematocrit is postulated to induce a decrease in oxygen availability
at the luminal surface through a diminished concentration of oxyhaemoglobin,
hence contributing, with the reported lowered oxygen partial pressure,
to local hypoxia
A pull-back algorithm to determine the unloaded vascular geometry in anisotropic hyperelastic AAA passive mechanics
No full textBiomechanical studies on abdominal aortic aneurysms (AAA) seek to provide for better decision criteria to undergo surgical intervention for AAA repair. More accurate results can be obtained by using appropriate material models for the tissues along with accurate geometric models and more realistic boundary conditions for the lesion. However, patient-specific AAA models are generated from gated medical images in which the artery is under pressure. Therefore, identification of the AAA zero pressure geometry would allow for a more realistic estimate of the aneurysmal wall mechanics. This study proposes a novel iterative algorithm to find the zero pressure geometry of patient-specific AAA models. The methodology allows considering the anisotropic hyperelastic behavior of the aortic wall, its thickness and accounts for the presence of the intraluminal thrombus. Results on 12 patient-specific AAA geometric models indicate that the procedure is computational tractable and efficient, and preserves the global volume of the model. In addition, a comparison of the peak wall stress computed with the zero pressure and CT-based geometries during systole indicates that computations using CT-based geometric models underestimate the peak wall stress by 59 +/- A 64 and 47 +/- A 64 kPa for the isotropic and anisotropic material models of the arterial wall, respectively.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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