Search

A formulation in cylindrical coordinates of the nonlinear torsional wave propagation on a hyperelastic material characterized by Hamilton's strain energy function is proposed. The objective of this formulation is to study and assess soft tissues, taking into account both geometrical and physical nonlinearity. Specifically, this work analyzes the propagation of torsional shear waves through an isotropic axisymmetric medium, so the only non-zero velocity component is associated with the angular coordinate. To transform the equations from Cartesian to cylindrical coordinates, the covariant and contravariant transformations are employed. A transverse torsional wave propagating through a quasi-incompressible hydrogel from the emitter to the receiver is considered. As the close form solution is not straightforward, a numerical simulation using the Finite Difference Time Domain method is performed. The results are obtained for a realistic range of wave frequencies and nonlinear parameters for medical applications.

There is a challenge in characterizing the soft tissue mechanical functionality in cylindrical geometries by current elastography techniques applied to small organs. Torsional waves are a type of shear elastic waves that propagate through soft tissue radially and in depth in a curled geometry, ideally suited to explore structures such as the cervix. Here, a sensor based on a novel arrangement of concentric sandwiches of piezo- and electromechanical elements is prototyped and tested to quantitatively assess stiffness in human cervix. The purpose is to determine the robustness of a torsional wave elastography (TWE) sensor prototype to quantify cervical stiffness, as a first step to validate a reliable measurement protocol using TWE. An array of tests designed to validate a set of hypothesis about the contact conditions were performed on up to 3 different nonpregnant voluntary women. The mechanical properties were reconstructed from the recorded TWE signals, and the outcomes were statistically analyzed to validate the contact condition hypothesis. The results suggest that, although future tests need to be performed to fully assess the repeatability and quality of the measurements, some characteristics of the measurement protocol become clear; in particular: (1) the use of speculum is recommended since it not only stabilizes the measurements, but also corrects and stabilizes the orientation of the cervical neck, and allows to visually verify the positioning and alignment of the probe, while not significantly increasing discomfort to women during this test; (2) the use of lubricant gel has no significant effect on the measurement quality; (3) 1cm off-centering from external cervical os is not acceptable; (4) a range of applied force between 500-2000 N does not significantly vary measurement signal quality, but the reconstructed value of the shear modulus does vary, probably due to its constitutive nonlinearity; (5) breathing movements affects measurements, but short duration of TWE pulse (0.3 s) allows to take measurements in apnea.

This paper presents a continuous finite element model for shallow water equations coupled with a bedload sediment conservation law. The model incorporates a sign-preserving conservative correction based on the flux correction transport methodology. Complete algorithm permits simulation of flows with evolutionary boundaries and erodible–non erodible interfaces under severe conditions without spurious mass exchanges and oscillations. Numerical experiments explore the efficacy of the procedure for dam-break problems on channels with dry beds, for partially and totally erodible beds, and for migration of small bedforms. Computation of real river morphodynamics under intense flooding illustrates practical application of the model.

A novel torsional wave sensor designed to characterize mechanical properties of soft tissues is presented in this work. Elastography is a widely used technique since the 1990s to map tissue stiffness. Moreover, quantitative elastography uses the velocity of shear waves to achieve the shear stiffness. This technique exhibits significant limitations caused by the difficulty of the separation between longitudinal and shear waves and the pressure applied while measuring. To overcome these drawbacks, the proposed torsional wave sensor can isolate a pure shear wave, avoiding the possibility of multiple wave interference. It comprises a rotational actuator disk and a piezoceramic receiver ring circumferentially aligned. Both allow the transmission of shear waves that interact with the tissue before being received. Experimental tests are performed using tissue mimicking phantoms and cervical tissues. One contribution is a sensor sensitivity study that has been conducted to evaluate the robustness of the new proposed torsional wave elastography (TWE) technique. The variables object of the study are both the applied pressure and the angle of incidence sensor–phantom. The other contribution consists of a cervical tissue characterization. To this end, three rheological models have fit the experimental data and a static independent testing method has been performed. The proposed methodology permits the reconstruction of the mechanical constants from the propagated shear wave, providing a proof of principle and warranting further studies to confirm the validity of the results., This research was supported by the Ministry of Education DPI2014-51870-R, DPI2010-17065 and UNGR15-CE-3664, Ministry of Health DTS15/00093 and PI16/00339, and Junta de Andalucía P11-CTS-8089 projects. Modesto T. López-López acknowledges financial support by the project FIS2013-41821-R (Ministry of Economy and Competitiveness, co-funded by the ERDF, European Union). We acknowledge the collaboration of the members of the Anatomical Pathology Unit of the University Hospital Complex of Granada. Finally, Antonio Callejas is grateful to the University of Granada for the award of an PhD fellowship.

The application of the Boundary Element Method (BEM) to elastostatic problems involving 3D non-homogeneous materials like Functionally Graded Materials (FGMs) is presented in this paper. The Analog Equation Method (AEM) is used to transform the original problem into a new problem with unknown fictitious source but known Fundamental Solution. By means of this transformation a system of uncoupled Domain/Boundary Integral Equations (D/BIEs) is first obtained, combining standard Boundary Element discretization and Radial Basis Functions (RBFs) approximation for the fictitious source. The application of the original differential operator to the displacement BIE provides the extra equations to compute the unknown fictitious source. The boundary character of the method is maintained since the integrals involved in the equations are limited only to the boundary since the RBFs are selected in such a way that the corresponding analog equation could be solved analytically. Numerical examples for three-dimensional problems in continuously non-homogeneous, isotropic and linear elastic FGMs are presented and discussed.

A sensor based on torsional waves is proposed, designed and validated to quantify elastic constants of soft tissue. The proposed transducer design is able to deploy a shear wave elastography technique that accurately quantifies the shear viscoelastic moduli of the tissue, which, as opposed to compression waves, are strongly dependent on the stroma architecture, thus serving as a new biomarker.

The WHO estimates that 15 million babies yearly (1 in 10) will be born preterm. Worldwide, complications of preterm births have supplanted pneumonia as the primary cause of child mortality [1]. The biology of cervical ripening that leads to birth is poorly understood, and there is no clinical tool to quantitatively evaluate the cervical biomechanical state, which in words of Feltovich [2]“… likely contributes to the reason the singleton spontaneous preterm birth rate has not changed appreciably in more than 100 years.” Towards this problem, we work on enabling new sensor technologies sentient to soft tissue biomechanics, to endow a new class of biomarkers that quantify the mechanical functionality of the cervix, and indeed any soft tissue, ranging pathologies from tumors, atherosclerosis, liver fibrosis to osteoarticular syndromes. Ultrasonic characterization of soft tissue has been developed as a clinical diagnostic tool [3] and evolved through different technologies including our torsional wave principle [4]. Our recent advances covering (a) torsional waves (shear elastic waves that propagate in quasifluids radially and in depth in a curled geometry), (b) sensors (based on a novel arrangement of concentric sandwiches of elements), (c) propagation models and (d) patient testing, are allowing to quantify the mechanical functionality beyond linear parameters: dispersive and nonlinear. [1] WHO, 2012; [2] Feltovich. AJOG207(2012):345–354 [3] Barr. Ultras Quart28(2012):13–20; [4] Melchor. Ultrasonics, 54(2014):1950–1962.

Dissertação de mestrado em Gestão, Universidade Lusíada de Lisboa, 2014. Exame público realizado em 10 de Fevereiro de 2014. O objetivo deste trabalho é questionar sobre o futuro do mercado imobiliário. Com esta finalidade, apresentou-se um trajeto possível para o futuro deste setor. Para além de se fazer um levantamento histórico do setor imobiliário em Portugal, pretendeu-se ressalvar a sua importância na economia do país. Com este intuito, fez-se a sua contextualização, tanto no domínio nacional, como ainda internacional. As múltiplas facetas do mercado imobiliário, subdividido em vários segmentos, a complexidade que este apresenta, fruto dos inúmeros fatores que concorrem para a sua operacionalidade, são uma questão difícil de trabalhar, uma vez que não se podem descurar os elementos intrínsecos que o constituem e que contribuem para a sua unidade, sob pena de se fazer uma interpretação errónea. A crise mundial, iniciada nos setores imobiliários e bancários dos Estados Unidos da América, é um dado referencial para o início de uma perturbação económica mais profunda. Contudo a suas consequências na economia nacional são indiretas, pois esta sofre mais dos efeitos da falta de apoios e de confiança e da dívida acumulada do que dos investimentos realizados pela banca no campo internacional. A falta de investimento no setor e a dificuldade em adquirir crédito para comprar imóveis fizeram com que a possibilidade do arrendamento começasse a ganhar peso. Este poderá ser o elemento que despoletará a viragem em direção a uma política de sucesso no mercado imobiliário e uma via importante que deverá ser explorada pela administração de condomínios.