Luísa Costa Sousa

A novel automatic algorithm for the segmentation of the lumen of the carotid artery in ultrasound B-mode images

A novel algorithm is proposed for the segmentation of the lumen and bifurcation boundaries of the carotid artery in B-mode ultrasound images. It uses the image contrast characteristics of the lumen and bifurcation of the carotid artery in relation to other tissues and structures for their identification. The relevant ultrasound data regarding the artery presented in the input image is identified using morphologic operators and processed by an anisotropic diffusion filter for speckle noise removal. The information obtained is then used to define two initial contours, one corresponding to the lumen and the other one regarding the bifurcation boundaries, for the application of the Chan-Vese level set segmentation model. A set of longitudinal ultrasound B-mode grayscale images of the common carotid artery was acquired using a GE Healthcare Vivid-e ultrasound system. The results reveal that the new algorithm is effective and robust, and that its main advantage relies on the automatic identification of the carotid lumen, which overcomes the known limitations of the traditional algorithms.

Inverse methods in design of industrial forging processes

Abstract An approach to optimal shape design in forging is presented. The design problem is formulated as an inverse problem incorporating a finite element 3D analysis model and an optimisation technique conducted on the basis of design sensitivities. The mechanical analysis provides information to predict deformation, stresses and strains necessary to the shape optimisation problem. The objective is to minimise a function describing die underfill and excessive material waste. Analytical sensitivities of the objective function are required and the calculation of discrete derivatives based on the differentiation of the discrete problem equations is considered. Due to the time-dependent deformation process the direct differentiation method has been implemented. The capability of the proposed inverse approach to deal with optimal forging of industrial parts is demonstrated.

Eliminating Forging Defects Using Genetic Algorithms

Abstract In this article, an optimization method for metal forging process designs using finite element-based simulation is presented. Using as entry parameters the specifications of the final product the so-called inverse techniques developed for optimization problems allows the calculation of the optimal solution, the design parameters that produce the required product. An evolutionary genetic algorithm is proposed to calculate optimal shape geometry and temperature. An example demonstrating the efficiency of the developed method is presented considering a two-stage hot forging process. It considers optimization of the process parameters to reduce the difference between the realized and the prescribed final forged shape under minimal energy consumption, restricting the maximum temperature.

An efficient algorithm to estimate optimal preform die shape parameters in forging

An optimisation method for design of intermediate die shapes needed in some forging operations is presented. The basic problem consists of finding an optimal two‐step forging sequence by automatically designing the shape of the preforming tools. The optimisation problem is defined based on an inverse formulation. The objective function of the optimisation problem is a function describing the quality of the obtained part by measuring the die underfill. The finite element method is used to simulate the forging problem. The optimisation method is based on a modified sequential unconstrained minimisation technique and a gradient method. The sensitivity‐dependent algorithm requires computing the derivatives of the objective function with respect to the design variables defining the preform shapes. A direct differentiation method has been developed for this purpose. The optimisation scheme is demonstrated with two axisymmetric forging examples in which optimal preform dies are obtained.

Toward hemodynamic diagnosis of carotid artery stenosis based on ultrasound image data and computational modeling

AbstractThe ability of using non-expensive ultrasound (US) image data together with computer fluid simulation to access various severities of carotid stenosis was inquired in this study. Subject-specific hemodynamic conditions were simulated using a developed finite element solver. Individual structured meshing of the common carotid artery (CCA) bifurcation was built from segmented longitudinal and cross-sectional US images; imposed boundary velocities were based on Doppler US measurements. Simulated hemodynamic parameters such as velocities, wall shear stress (WSS) and derived descriptors were able to predict disturbed flow conditions which play an important role in the development of local atherosclerotic plaques. Hemodynamic features from six individual CCA bifurcations were analyzed. High values of time-averaged WSS (TAWSS) were found at stenosis site. Low values of TAWSS were found at the bulb and at the carotid internal and external branches depending on the particular features of each patient. High oscillating shear index and relative residence time values assigned highly disturbed flows at the same artery surface regions that correlate only moderately with low TAWSS results. Based on clinic US examinations, results provide estimates of flow changes and forces at the carotid artery wall toward the link between hemodynamic behavior and stenosis pathophysiology.

Blood flow simulation and vascular reconstruction

In medical practice, bypass grafts are commonly used as an alternative route around strongly stenosed or occluded arteries. In contrast to arterial bifurcations, surgically created anastomosis can be modified with the objective of enabling optimal graft geometry to yield a flow environment that improves its longevity. This paper presents a three dimensional numerical study of blood flow through bypass systems with different geometries. Coupled with the finite element solver a shape optimization framework considering a genetic algorithm is presented. Numerical results show the benefits of understanding blood flow hemodynamic at anastomosis junctions achieving design improvements. Minimizing recirculation zones and flow stagnation can be useful in surgical planning.

Simulation model for hot and cold forging by mixed methods including adaptive mesh refinement

Uses a rigid viscoplastic formulation to simulate hot and cold forging processes. The finite element solution uses mixed methods in which the independent variables can be velocities, pressures and deviatoric stresses. Uses interface elements both in the mechanical and the thermal analysis, to take into account the effects of contact and friction, thermal conductivity of lubricants and heat generated by friction. The code developed includes an adaptive mesh refinement, triggered by an error estimator based on energy norms evaluated from nodal stress values, recovered from a local continuous polynomial expansion, and those given by the numerical solution. Assesses the code developed, using experimental results.

Blood Flow Simulation and Applications

In the vascular system altered flow conditions, such as separation and flow-reversal zones play an important role in the development of arterial diseases. Nowadays computational biomechanics modeling is still in the research and development stage. This chapter presents a numerical computational methodology for blood flow simulation using the Finite Element method outlining field equations and approaches for numerical solutions. Due to the complexity of the vascular system simplifying assumptions for the mathematical modeling process are made. Two applications of the developed tool to describe arterial hemodynamics are presented, a flow simulation in the human carotid artery bifurcation and a search for an optimized geometry of an artificial bypass graft.

Haemodynamic conditions of patient-specific carotid bifurcation based on ultrasound imaging

The purpose of this paper is to complement the characterisation of patient-specific carotid artery bifurcation haemodynamics based on image data obtained by Doppler ultrasound imaging. A methodology for patient-specific 3D luminal surface reconstruction followed by structured hexahedral meshing of the volume and blood flow simulation is presented. Quantitative descriptors of the flow based on wall shear stress (WSS) are used to compare healthy and stenosed carotid bifurcation haemodynamic disturbances. Independently on the presence of stenosis, the internal carotid artery has been identified as a region of abnormal high values of oscillating shear index and relative residence time and low values of time averaged WSS. For the healthy carotid bifurcation, WSS descriptors manage to capture flow disturbances at the external carotid artery. This work addresses the lack of quantitative analysis on anatomically realistic stenosed carotid bifurcations.

Automatic segmentation of the lumen of the carotid artery in ultrasound B-mode images

A new algorithm is proposed for the segmentation of the lumen and bifurcation boundaries of the carotid artery in B-mode ultrasound images. It uses the hipoechogenic characteristics of the lumen for the identification of the carotid boundaries and the echogenic characteristics for the identification of the bifurcation boundaries. The image to be segmented is processed with the application of an anisotropic diffusion filter for speckle removal and morphologic operators are employed in the detection of the artery. The obtained information is then used in the definition of two initial contours, one corresponding to the lumen and the other to the bifurcation boundaries, for the posterior application of the Chan-vese level set segmentation model. A set of longitudinal B-mode images of the common carotid artery (CCA) was acquired with a GE Healthcare Vivid-e ultrasound system (GE Healthcare, United Kingdom). All the acquired images include a part of the CCA and of the bifurcation that separates the CCA into the internal and external carotid arteries. In order to achieve the uppermost robustness in the imaging acquisition process, i.e., images with high contrast and low speckle noise, the scanner was adjusted differently for each acquisition and according to the medical exam. The obtained results prove that we were able to successfully apply a carotid segmentation technique based on cervical ultrasonography. The main advantage of the new segmentation method relies on the automatic identification of the carotid lumen, overcoming the limitations of the traditional methods.