A Review of Electrical Impedance Tomography in Lung Applications: Theory and Algorithms for Absolute Images.

Electrical Impedance Tomography (EIT) is under fast development, the present paper is a review of some procedures that are contributing to improve spatial resolution and material properties accuracy, admitivitty or impeditivity accuracy. A review of EIT medical applications is presented and they were classified into three broad categories: ARDS patients, obstructive lung diseases and perioperative patients. The use of absolute EIT image may enable the assessment of absolute lung volume, which may significantly improve the clinical acceptance of EIT. The Control Theory, the State Observers more specifically, have a developed theory that can be used for the design and operation of EIT devices. Electrode placement, current injection strategy and electrode electric potential measurements strategy should maximize the number of observable and controllable directions of the state vector space. A non-linear stochastic state observer, the Unscented Kalman Filter, is used directly for the reconstruction of absolute EIT images. Historically, difference images were explored first since they are more stable in the presence of modelling errors. Absolute images require more detailed models of contact impedance, stray capacitance and properly refined finite element mesh where the electric potential gradient is high. Parallelization of the forward program computation is necessary since the solution of the inverse problem often requires frequent solutions of the forward problem. Several reconstruction algorithms benefit by the Bayesian inverse problem approach and the concept of prior information. Anatomic and physiologic information are used to form the prior information. An already tested methodology is presented to build the prior probability density function using an ensemble of CT scans and in vivo impedance measurements. Eight absolute EIT image algorithms are presented.

Image reconstruction using interval simulated annealing in electrical impedance tomography.

Electrical impedance tomography (EIT) is an imaging technique that attempts to reconstruct the impedance distribution inside an object from the impedance between electrodes placed on the object surface. The EIT reconstruction problem can be approached as a nonlinear nonconvex optimization problem in which one tries to maximize the matching between a simulated impedance problem and the observed data. This nonlinear optimization problem is often ill-posed, and not very suited to methods that evaluate derivatives of the objective function. It may be approached by simulated annealing (SA), but at a large computational cost due to the expensive evaluation process of the objective function, which involves a full simulation of the impedance problem at each iteration. A variation of SA is proposed in which the objective function is evaluated only partially, while ensuring boundaries on the behavior of the modified algorithm.

Electrical impedance tomography reconstruction through Simulated Annealing with incomplete evaluation of the objective function.

The EIT reconstruction problem is approached as an optimization problem where the difference between a simulated impedance domain and the observed one is minimized. This optimization problem is often solved by Simulated Annealing (SA), but at a large computational cost due to the expensive evaluation process of the objective function. We propose here, a variation of SA applied to EIT where the objective function is evaluated only partially, while ensuring upper boundaries on the deviation on the behavior of the modified SA. The reconstruction method is evaluated with simulated and experimental data.

Análise da resistência mecânica de 13 cimentos ósseos acrílicos / Analysis of mechanical resistance of 13 acrylics bone cements

Objetivos: Analisar a resistência mecânica de 13 tipos de cimentos ósseos acrílicos comercializados em nosso país, testando suas propriedades físicas de resistência às forças de tração e de flexão e comparar a resistência dos cimentos ósseos sem antibiótico com seus similares com antibiótico. Método: Foram confeccionados corpos de prova de acordo com a norma regulamentadora vigente (ISO 5833 e ISO 527), utilizando os parâmetros Weibull, onde, para se obter resultados dentro do limite mínimo de erro aceitável de 5%, o número de espécimes testados deve ser de 7 a 11 corpos de provas. Foram selecionamos nove corpos de prova para cada marca. Os testes de tração e de flexão foram realizados no Departamento de Materiais da Faculdade de Odontologia da Universidade de São Paulo - SP com a máquina de ensaio universal Riehle modelo Fs-5, com capacidade de 2.400 kg e com velocidade de 0,02 a 2 polegadas/segundo. Foram anotadas a força (MPa) no momento de fratura dos nove corpos de prova para cada cimento, assim como a média e desvio padrão para o teste de tração e flexão. O estudo estatístico foi realizado pelo teste de contraste de Tukey e pelo teste da análise de variância comparando-se as médias das amostras de cada cimento utilizado. Resultados: Observamos diferença significante entre as amostras selecionadas neste trabalho e diminuição significante na resistência mecânica das amostras que continham antibiótico na sua fórmula original em comparação com seus similares sem antibiótico. Conclusões: Os cimentos ósseos apresentaram diferenças significantes quanto à resistência mecânica à tração e à flexão. A presença de antibiótico na composição original dos cimentos ósseos diminuiu significantemente a resistência mecânica nos testes de tração e flexão.

Dynamic imaging in electrical impedance tomography of the human chest with online transition matrix identification.

One of the electrical impedance tomography objectives is to estimate the electrical resistivity distribution in a domain based only on electrical potential measurements at its boundary generated by an imposed electrical current distribution into the boundary. One of the methods used in dynamic estimation is the Kalman filter. In biomedical applications, the random walk model is frequently used as evolution model and, under this conditions, poor tracking ability of the extended Kalman filter (EKF) is achieved. An analytically developed evolution model is not feasible at this moment. The paper investigates the identification of the evolution model in parallel to the EKF and updating the evolution model with certain periodicity. The evolution model transition matrix is identified using the history of the estimated resistivity distribution obtained by a sensitivity matrix based algorithm and a Newton-Raphson algorithm. To numerically identify the linear evolution model, the Ibrahim time-domain method is used. The investigation is performed by numerical simulations of a domain with time-varying resistivity and by experimental data collected from the boundary of a human chest during normal breathing. The obtained dynamic resistivity values lie within the expected values for the tissues of a human chest. The EKF results suggest that the tracking ability is significantly improved with this approach.

Electrical impedance tomography.

PURPOSE OF REVIEW: Electrical impedance tomography (EIT) is a noninvasive, radiation-free monitoring tool that allows real-time imaging of ventilation. The purpose of this article is to discuss the fundamentals of EIT and to review the use of EIT in critical care patients. RECENT FINDINGS: In addition to its established role in describing the distribution of alveolar ventilation, EIT has been shown to be a useful tool to detect lung collapse and monitor lung recruitment, both regionally and on a global basis. EIT has also been used to diagnose with high sensitivity incident pneumothoraces during mechanical ventilation. Additionally, with injection of hypertonic saline as a contrast agent, it is possible to estimate ventilation/perfusion distributions. SUMMARY: EIT is cheap, noninvasive and allows continuous monitoring of ventilation. It is gaining acceptance as a valuable monitoring tool for the care of critical patients.

Three-dimensional electrical impedance tomography: a topology optimization approach.

Electrical impedance tomography is a technique to estimate the impedance distribution within a domain, based on measurements on its boundary. In other words, given the mathematical model of the domain, its geometry and boundary conditions, a nonlinear inverse problem of estimating the electric impedance distribution can be solved. Several impedance estimation algorithms have been proposed to solve this problem. In this paper, we present a three-dimensional algorithm, based on the topology optimization method, as an alternative. A sequence of linear programming problems, allowing for constraints, is solved utilizing this method. In each iteration, the finite element method provides the electric potential field within the model of the domain. An electrode model is also proposed (thus, increasing the accuracy of the finite element results). The algorithm is tested using numerically simulated data and also experimental data, and absolute resistivity values are obtained. These results, corresponding to phantoms with two different conductive materials, exhibit relatively well-defined boundaries between them, and show that this is a practical and potentially useful technique to be applied to monitor lung aeration, including the possibility of imaging a pneumothorax.

Shear versus micro-shear bond strength test: a finite element stress analysis.

OBJECTIVES: This study aimed at comparing the stress distribution in shear and micro-shear test set-ups using finite element analysis, and suggesting some parameter standardization that might have important influence on the results. METHODS: Two-dimensional plane strain finite element analysis was performed using MSCPatran and MSCMarc softwares. Model configurations were based on published experimental shear and micro-shear test set-ups and material properties were assumed to be isotropic, homogeneous and linear-elastic. Typical values of elastic modulus and Poisson's ratios were assigned to composite, dentin and adhesive. Loading conditions considered a single-node concentrated load at different distances from the dentin-adhesive interface, and proportional geometry (1:5 scale, but fixed adhesive layer thickness in 50microm) with similar calculated nominal strength. The maximum tensile and shear stresses, and stress distribution along dentin-adhesive interfacial nodes were analyzed. RESULTS: Stress distribution was always non-uniform and greatly differed between shear and micro-shear models. A pronounced stress concentration was observed at the interfacial edges due to the geometric change: stress values farther exceeded the nominal strength and tensile stresses were much higher than shear stresses. For micro-shear test, the relatively thicker adhesive layer and use of low modulus composites may lead to relevant stress intensification. An appropriate loading distance was established for each test (1mm for shear and 0.1mm for micro-shear) in which stress concentration would be minimal, and should be standardized for experimental assays. SIGNIFICANCE: The elastic modulus of bonded composites, relative adhesive layer thickness and load application distance are important parameters to be standardized, once they influence stress concentration.

Análise do desgaste do polietileno do componente acetabular da prótese total do quadril, utilizando o método de elementos finitos de dimulação computadorizada / Wearing of polyethylene of the acetabular component of total hip prothesis: analysis using the method of finite elements computerized simulation

Este trabalho apresenta os resultados obtidos na simulação computadorizada da artroplastia total do quadril. Estuda o desgaste do polietileno do componente acetabular, utilizando o método de elementos finitos, com o objetivo de verificar a validade deste método de simulação. Foi simulado o desgaste do polietileno produzido pelo contato da cabeça femoral de cromo-cobalto, tendo o componente acetabular orientação espacial de 45° de inclinação lateral e anteversão neutra. Os resultados encontrados foram semelhantes àqueles observados em estudos clínicos.