1,721,028 research outputs found
Acoustic waves in hydrogels: A bi-phasic model for ultrasound tissue-mimicking phantom
No full textIn the present paper a continuum poroelastic model for high frequency acoustic waves in hydrogels has been developed. The model has been used to obtain the acoustic longitudinal wave equation for ultrasound. In order to obtain a satisfactory model for hydrogels, a viscoelastic force describing the interaction between the polymer network of the matrix and the bounded water is introduced. The model is validated by means of ultrasound (US) wave speed and attenuation measurements in polyvinylalcohol (PVA) hydrogel samples as a function of their water volume fraction "β" and polymer matrix cross-linking. The model predicts that the law ∝ ν(1 + δ) for ultrasound attenuation can be applied as a function of the frequency ν, where δ is the frequency exponent of the polymer-bounded water viscosity. This outcome can well explain the attenuation of the US frequency in natural gels where δ is typically about 0.25÷0.50 while the value for pure water is 1. The theory and experiments show that US attenuation in hydrogels decreases steadily with the increase of its water volume fraction β in a linear. The new proposed dissipative mechanism leads to a US wave speed c that follows the law: c = cw(β - φsymbol)- 3/2, where cw is the US wave speed in water and φsymbol is the volume fraction of the bounded water. Since 0 0, the hydrogel US velocity is always higher than that of pure water. If β tends to 1 (100% water), then the US speed in hydrogels converges to a higher value than that of pure water. The US speed gap at β = 1, between hydrogels and water, is the direct consequence of the introduction of the polymer network-bounded water interaction. This is in line with the experimental results that show that the US speed gap at β = 1 decreases in the gel samples with a more cross-linked polymer matrix that has a lower bounded water volume fraction. On the contrary, if the water content is very low (i.e., β < 0.4), the measured US speed converges to that of the dry hydrogel matrix which increases in the samples with a higher degree of network cross-linking with greater elastic moduli. © 2008 Elsevier B.V. All rights reserved
Commercially Available Head-Mounted Displays Are Unsuitable for Augmented Reality Surgical Guidance: A Call for Focused Research for Surgical Applications
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The Right Mix of Visual and Mechanical Constraints to Guide the Relative Pose between Rigid Objects
No full textRestoring and/or obtaining the correct relative pose between rigid objects is a common precision task in many sectors, from industry to healthcare, as during the reduction of long-bones fractured. Augmented Reality (AR) can provide useful indications, showing how one bone should be positioned with respect to another, for example. However, moving a rigid body by manually managing its six degrees of freedom (DOF) until the desired pose is obtained can be difficult. At first, because AR visualization is commonly not able to provide an accurate depth perception. Second, because it is very challenging to control and handle all the six DOFs simultaneously. In this work, we evaluated whether the addition of adaptable and easily adjustable mechanical constraints, each constraining a single degree of freedom, allow increasing the feasibility, accuracy, and efficiency of an AR guidance. This preliminary study shows that handling simultaneously all the 6 DOFs of a rigid object is hardly feasible under hand-free AR guidance, and that we can therefore insert simple mechanical constraints to facilitate the achievement of the desired relative pose between rigid objects
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
Surgical simulators integrating virtual and physical anatomies
No full textAccording to literature evidences, simulation is of utmost importance for training purposes and for innovative surgical strategies assessment. Nowadays the market offers mainly two kind of simulators: rubber anatomies or virtual environments, each one with advantages and drawbacks. In this paper we describe a strategy to develop patientspecific simulators using a hybrid approach: silicone models of abdominal organs sensorized with electromagnetic coils, to acquire deformations, coupled with a virtual scene. As demonstrated, this approach allows to mix benefits of a real interaction with the physical replicas with the possibility to enrich the virtual visualization with add-ons and features difficult to obtain in the real environment. Copyright © 2011 for the individual papers by the papers' authors
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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