Examinando por Autor "Osses, Axel"

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    A simplified homogenization model applied to viscoelastic behavior of cortical bone at ultrasonic frequencies
    (Elsevier, 2022) Aróstica, Reidmen; Aguilera, Ana; Osses, Axel; Minonzio. Jean-Gabriel
    Cortical bone is a complex multiscale medium and its study is of importance for clinical fracture prevention. In particular, cortical attenuation is known to be linked with shock energy absorption and ability to resist fracture. However, the links between cortical bone absorption and its multiscale structure are still not well understood. This work is about the use of homogenized tensors in order to characterize the viscoelastic behavior of cortical bone at ultrasonic frequencies, i.e., about 0.1 to 10 MHz. Such tensors are derived from the cell problem via two-scale homogenization theory for linear elastic and Kelvin–Voigt viscoelastic descriptions. The elliptic formulations obtained from the cell problems are implemented within the range of medically-observed porosities. Microstructure is assessed considering cubic cells with cylindrical inclusion and transverse isotropic assumption. A simplified model, adding one temporal parameter per phase, allows a good agreement with experimental data. The corresponding attenuation is proportional to the square of the frequency, in agreement with Kramer–Kronig relations. This development is proposed in the context of robust clinical inverse problem approaches using a restricted number of parameter. Two main properties for the material filling the pores are adjusted and discussed: absorption and shear contribution. Best agreement with experimental data is observed for material inside the pores being solid and highly attenuating.
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    Real Time Waveguide Parameter Estimation Using Sparse Multimode Disperse Radon Transform
    (IEEE, 2021) Araya, Claudio; Martinez, Alejandro; Ramiandrisoam Donatien; Ta, Dean; Xu, Kailiang; Osses, Axel; Minonzio, Jean-Gabriel
    Osteoporosis and associated fragility fractures are still a societal problem. Several quantitative ultrasound approaches have been proposed to overcome limitations of the current gold standard DXA. Bi Directional Axial Transmission (BDAT) is based on the measurement of waves guided by the cortical bone shell. Cortical thickness (Ct.Th) and porosity (Ct.Po) estimates correspond to the maxima of the objective function Proj(Ct.Th,Ct.Po), initially defined as the projection of a tested model in the singular vector basis (method 1). Each model matrix has the same dimension, i.e., Nf=124 x Nk=256, 512, 1024 or 2048 pixels, of an ultrasonic guided wave spectrum experimental image Norm(f,k). The total number of models is equal to Nth,=38 x Npo=25, i.e., the number of cortical thickness and porosity taken into account, ranging respectively from 0.8 to 4.5 mm and 1 to 25%. Finally, each pixel of the alternative objective function (NthxNpo pixels) corresponds to the pixel-wise image multiplication between one model and the experimental guided wave spectrum image (method 2) or a sparse matrix multiplication between experimental and model reshaped vectors (method 3). The three methods were tested on data obtained on 400 measurements. It was observed that methods 2 and 3 provided the same Ct.Th Ct.Po values while differences with method 1 decreased with Nk. Acceptable differences, i.e., lower than the typical measurement resolution (0.2 mm for Ct.Th and 1% for Ct.Po) were achieved for Nk=2048. Using Matlab on a standard desktop, this calculation took 20, 4 and 0.3 s, for the methods 1 to 3, respectively. Method 3 calculation was achieved in 5 ms using C++. This last value opens perspective toward guiding interface improvement using real time objective function.
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    Validation of 4D Flow based relative pressure maps in aortic flows
    (Elsevier, 2021) Noltea, David; Urbina, Jesús; Sotelo, Julio; Sok, Leo; Montalba, Cristian; Valverde, Israel; Osses, Axel; Uribe, Sergio; Bertoglio, Cristóbal
    While the clinical gold standard for pressure difference measurements is invasive catheterization, 4D Flow MRI is a promising tool for enabling a non-invasive quantification, by linking highly spatially resolved velocity measurements with pressure differences via the incompressible Navier–Stokes equations. In this work we provide a validation and comparison with phantom and clinical patient data of pressure difference maps estimators. We compare the classical Pressure Poisson Estimator (PPE) and the new Stokes Estimator (STE) against catheter pressure measurements under a variety of stenosis severities and flow intensities. Specifically, we use several 4D Flow data sets of realistic aortic phantoms with different anatomic and hemodynamic severities and two patients with aortic coarctation. The phantom data sets are enriched by subsampling to lower resolutions, modification of the segmentation and addition of synthetic noise, in order to study the sensitivity of the pressure difference estimators to these factors. Overall, the STE method yields more accurate results than the PPE method compared to catheterization data. The superiority of the STE becomes more evident at increasing Reynolds numbers with a better capacity of capturing pressure gradients in strongly convective flow regimes. The results indicate an improved robustness of the STE method with respect to variation in lumen segmentation. However, with heuristic removal of the wall-voxels, the PPE can reach a comparable accuracy for lower Reynolds’ numbers.