RESUMEN
Respiratory diseases, which include diseases of the lung, pleura, bronchial tree, trachea, upper respiratory tract and of the respiratory muscles and nerves, are a common and important cause of illness and death among the population. Experimental evidences have shown that tissue lesions have different electrical properties compared with normal tissue. Therefore, lung tissues lesions may be differentiated from lung normal tissue by comparing the tissue passive electrical properties. The manuscript reports a feasibility study for minimally invasive in vivo human lung tissue tetrapolar bioimpedance measurements using a catheter during the bronchoscopy procedure based on multisine Electrical Impedance Spectroscopy (EIS) at 10 kHz - 1 MHz.
Asunto(s)
Broncoscopios , Broncoscopía/instrumentación , Broncoscopía/métodos , Pulmón , Impedancia Eléctrica , Femenino , Humanos , MasculinoRESUMEN
Electrical impedance tomography has been described as a new method of monitoring critically ill patients on mechanical ventilation. It has recently gained special interest because of its applicability for monitoring ventilation and pulmonary perfusion. Its bedside and continuous implementation, and the fact that it is a non-ionizing and non-invasive technique, makes it an extremely attractive measurement tool. Likewise, given its ability to assess the regional characteristics of lung structure, it could be considered an ideal monitoring tool in the heterogeneous lung with acute lung injury. This review explains the physical concept of bioimpedance and its clinical application, and summarizes the scientific evidence published to date with regard to the implementation of electrical impedance tomography as a method for monitoring ventilation and perfusion, mainly in the patient with acute lung injury, and other possible applications of the technique in the critically ill patient. The review also summarizes the limitations of the technique and its potential areas of future development.
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Lesión Pulmonar Aguda/diagnóstico , Tomografía , Impedancia Eléctrica , Humanos , Monitoreo Fisiológico/métodos , Tomografía/métodosRESUMEN
The current paper proposes a new indicator for well-balanced dehydration of patients undergoing a haemodialysis. It is based on an estimator for the extra-cellular volume and the ultrafiltration rate. The extra-cellular fluid was computed from continuous tetrapolar bio-impedance measurements taken on the lower leg in a frequency range of several kilohertz up to 500 kHz. Finite element simulations on different leg models with anisotropic conductivities calculated with Cole models were carried out in order to incorporate the significant anisotropy of human tissue into the estimation process. The indicator was tested on measurement data gathered from 25 persons during 150 haemodialysis sessions. Its performance was determined by computing ROC curves. Results of the data analysis are reported.
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Líquidos Corporales/fisiología , Análisis de Elementos Finitos , Diálisis Renal/métodos , Equilibrio Hidroelectrolítico/fisiología , Anisotropía , Electricidad , Femenino , Humanos , Masculino , Músculos/fisiología , Curva ROC , Factores de TiempoRESUMEN
Precision temperature measurements are required in the LTP, the LISA technology package, for various diagnostics objectives. In this article, we describe in detail the front-end electronics design and the associated temperature sensors to achieve the LTP requirements: noise equivalent temperature of 10 microK Hz(-12) in the frequency range from 1 to 30 mHz at room temperature. We designed an ac Wheatstone bridge and a subsequent digital demodulation to minimize 1/f noise. We show experimental results where the required sensitivity in the measurement bandwidth is fulfilled.
Asunto(s)
Electrónica/instrumentación , Vuelo Espacial/instrumentación , Termografía/instrumentación , Diseño de Equipo , Análisis de Falla de Equipo , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , TemperaturaRESUMEN
UNLABELLED: The investigation of the hydration process during the haemodialysis treatment sessions is very important for the development of methods for predicting the unbalanced fluid shifts and hypotension crisis hence improving the quality of the haemodialysis procedure. Bioimpedance measurements can give valuable information about the tissue under measurement, therefore characterizing the tissue. In this work we propose a non-invasive method based on local multifrequency bioimpedance measurements that allow us to determine the fluid distribution and variations during haemodialysis. METHODS: Clinical measurements were done using 10 HD patients during 60 HD sessions. Bioimpedance data, ultrafiltration volume, blood volume and blood heamatocrit variations were recorded continuously during the HD sessions. Bioimpedance of the local tissue was measured with a 4-elctrode impedance system using surface electrodes with sampling rate of 1meas./4min. at 6 different frequencies. The measured impedances were fitted into Cole-Cole model and the Cole-Cole parameters were continuously determined for each measurement point during the HD session. ANALYSIS: The 4 Cole-Cole parameters (R 00, R 0, Fc,alpha) and their variations were evaluated. Impedance values at infinite and zero (R 00, R 0) frequencies were extrapolated from Cole-Cole mathematical model. These values are assumed to represent the impedance of total tissue fluid and the impedance of the extracellular space respectively.
Asunto(s)
Impedancia Eléctrica , Diálisis Renal/instrumentación , Diálisis Renal/métodos , Fenómenos Biofísicos , Biofisica , Compartimentos de Líquidos Corporales , Líquidos Corporales , Peso Corporal , Electrodos , Diseño de Equipo , Humanos , Modelos Estadísticos , Modelos Teóricos , Programas Informáticos , Factores de TiempoRESUMEN
In this work we show the algorithms developed to extract the Cole parameters from multi-frequency EIT. With these parameters it is possible to obtain information about various different tissues and their pathologies. The algorithms developed obtain the Cole-model parameters from the real and imaginary parts of impedance, or using only the real part, without problems of convergence. A study of the influence of noise is performed with simulations. We find a correct solution in all cases with signal to noise ratio in the data higher than 40 dB. Finally, we show parametric images of the human abdomen obtained with these algorithms.
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Algoritmos , Impedancia Eléctrica , Tomografía/estadística & datos numéricos , Abdomen/anatomía & histología , Abdomen/fisiología , Simulación por Computador , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Procesamiento de Imagen Asistido por Computador/estadística & datos numéricos , Modelos Anatómicos , Modelos Biológicos , Tomografía/métodosRESUMEN
We report a numerical study of the induced electric fields and specific absorption rate (SAR) produced by microwave radiation from a half-wavelength dipole near tissue models, and the resulting transient and steady-state temperature rises. Several models were explored, including a uniform semi-infinite plane of tissue, uniform sphere, a phantom model of the head filled with tissue-equivalent material, a numerical model of the head with uniform dielectric properties (obtained from a digitized computed tomography image), and a numerical model of the head with different dielectric properties corresponding to various tissues. The electromagnetic calculations were performed for half-wave dipoles radiating at 900 and 1900 MHz at various distances from the model, using the finite-difference-time-domain (FDTD) method. The resulting temperature rises were estimated by finite element solution of the bioheat equation. The calculated SAR values agree well with an empirical correlation due to Kuster. If the limiting hazard of such exposures is associated with excessive temperature increase, present exposure limits are very conservative and guidelines that are easier to implement might provide adequate protection.
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Campos Electromagnéticos , Cabeza/fisiología , Calor , Modelos Biológicos , Absorción/fisiología , Cabeza/anatomía & histología , Cabeza/irrigación sanguínea , Humanos , Imagen por Resonancia Magnética , Modelos Químicos , Modelos Estadísticos , Fantasmas de Imagen , Flujo Sanguíneo RegionalRESUMEN
The investigation of processes of ischemia in different organ tissues is very important for the development of methods of protection and preservation during surgical procedures. Electrical impedance spectroscopy was used to distinguish between different tissues and their degree of ischemia. We describe mathematical methods used to adjust experimental data to Cole-Cole models for one-circle and two-circle impedance loci and a study of the main parameters for representing the behavior of ischemia in time. In vivo and in situ postmortem measurements of different tissues from pigs are shown in the 100 Hz to 1 MHz range. The Cole parameters that best characterize the ischemia are R0 and fc.
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Isquemia/fisiopatología , Animales , Impedancia Eléctrica , Enfermedades Renales/fisiopatología , Modelos Biológicos , Isquemia Miocárdica/fisiopatología , Miocardio/patología , PorcinosRESUMEN
The biomass density in biotechnological processes is often determined by indirect and manual methods. Electrical impedance spectroscopy can provide online viable biomass density estimators. In this work, we present two linear estimators obtained with this technique. Four different microorganisms were measured. The detection threshold was approximately 1 g/L (dry weight) for bacteria and 0.5 g/L for yeast. Liposome suspensions were also used to validate the methods. The monitoring of the continuous growth of a yeast culture is also presented.
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Biomasa , Reactores Biológicos , Candida/química , División Celular , Impedancia Eléctrica , Escherichia coli/química , Liposomas/química , Rhodobacter/química , Saccharomyces cerevisiae/química , Análisis Espectral/métodosRESUMEN
We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau1) pertains to heat convection by blood flow, and is of the order of 20-30 min for physiologically normal perfusion rates; the second (tau2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau1 and tau2. In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard.
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Calefacción , Microondas , Humanos , Modelos Biológicos , Dolor , Factores de TiempoRESUMEN
Human thresholds for skin sensations of warmth were measured at frequencies from 2.45 to 94 GHz. By solving the one-dimensional bioheat equation, we calculated the temperature increase at the skin surface or at a depth of 175 microm at incident power levels corresponding to the observed thresholds. The thermal analysis suggests that the thresholds correspond to a localized temperature increase of about 0.07 degrees C at and near the surface of the skin. We also found that, even at the highest frequency of irradiation, the depth at which the temperature receptors are located is not a relevant parameter, as long as it is within 0.3 mm of the surface. Over the time range of the simulation, the results of the thermal model are insensitive to blood flow, but sensitive to thermal conduction; and this sensitivity increases strongly with frequency. We conclude with an analysis of the effect of thermal conduction on surface temperature rise, which becomes a dominant factor at microwave frequencies over 10 GHz.
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Regulación de la Temperatura Corporal/efectos de la radiación , Microondas , Modelos Biológicos , Umbral Sensorial/efectos de la radiación , Adulto , Humanos , Masculino , Sensibilidad y Especificidad , Piel/efectos de la radiación , TemperaturaRESUMEN
This paper deals with the design, implementation and performance of TIE-4sys, an electrical impedance tomograph. This instrument is a parallel broad-band real-time system. It measures impedance using an array of 16 electrodes and reconstructs the images using a weighted back-projection technique. The objective of this development is to enable multifrequency EIT clinical studies to be undertaken. The system is capable of acquiring 25 frames/s and makes multifrequency cardiac-gated images. The frequency range is from 10 kHz to 250 kHz and the signal to noise ratio for the real component is better than 60 dB.
Asunto(s)
Impedancia Eléctrica , Tomografía/instrumentación , Electrodos , Electrónica Médica/instrumentación , Diseño de Equipo , Corazón/fisiología , HumanosRESUMEN
Dynamic and multifrequency imaging methods have been demonstrated both theoretically and experimentally. Multifrequency methods are able to produce images of static structures inside the measured object. Data collection systems, however, are affected by errors due to their non-ideal frequency behaviour. If the frequencies used in the measurement were close enough, the system would behave in almost the same way. In this case, however, the impedance change displayed by biological tissues is small, so the situation is similar to dynamic imaging. We call this method the quasi-differential imaging method. We have designed and built an instrument able to apply signals from 1 kHz to 1 MHz, with frequency increments of 10 Hz. Patient interface circuits and demodulators were designed to display a flat response in the full frequency range of operation. Signals are digitized with 16 bit resolution and sent to the host computer using a high-speed serial interface. This allows a maximum measurement speed of about 8 images/s. All the system parts were full characterized out of the system and the results of these measurements are given as an indication of the limits of its use as a quasi-static imaging or quasi-differential imaging data collection system.
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Impedancia Eléctrica , Tomografía/instrumentación , Electrodos , Electrónica Médica/instrumentación , Diseño de Equipo , Humanos , Procesamiento de Imagen Asistido por Computador/instrumentaciónRESUMEN
This study was designed to perform multifrequency impedance measurements on the pig's eye. On one hand, impedance of the ocular tissues was measured from 10 kHz to 10 MHz. The aqueous and vitreous humours and the cornea showed no relaxation in this range of frequencies, whereas the lens and its parts (the cortex and the nucleus) did. On the other hand, multifrequency EIT and dynamic imaging were performed on the lens and on the whole eye. Data and images obtained after thermal and chemical injuries are presented and discussed.
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Córnea/fisiología , Impedancia Eléctrica , Cristalino/fisiología , Tomografía/métodos , Animales , Humor Acuoso/fisiología , Córnea/fisiopatología , Lesiones de la Cornea , Oftalmopatías/diagnóstico , Oftalmopatías/fisiopatología , Humanos , Técnicas In Vitro , Corteza del Cristalino/fisiología , Núcleo del Cristalino/fisiología , Porcinos , Tomografía/instrumentación , Cuerpo Vítreo/fisiologíaRESUMEN
Static images of the human body using electrical impedance tomography techniques can be obtained by measuring at two or more different frequencies. The frequencies used depend on the application, and their selection depends on the frequency behaviour of the impedance for the target tissue. An analysis using available data and theoretical models for tissue impedance yields the expected impedance and boundary voltage changes, therefore setting the measurement instrument specifications. The instrument errors produced by different sources are analysed, and, from this analysis it is possible to determine the feasibility of building the instrument, the limit values for some parameters (or components) and indications on the most suitable design of critical parts. This analysis also shows what kinds of error can be expected in the reconstructed images. It is concluded that it is possible to build an instrument with limited errors, allowing static images to be obtained. An instrument has been built that meets some of the design requirements and fails in others because of technological problems. In vivo images obtained with this instrument will be presented in Part 2 of this work.
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Impedancia Eléctrica , Tomografía/instrumentación , Electrodos , Electrónica Médica , Humanos , Matemática , Fenómenos Fisiológicos de la PielAsunto(s)
Impedancia Eléctrica , Electrodos , Electrónica Médica , Humanos , Matemática , Tomografía/instrumentaciónRESUMEN
A widely accepted method for static imaging in electrical impedance tomography (EIT) is to measure at two frequencies. The choice of measurement frequencies is application-dependent because some different tissues cannot be distinguished when using two fixed frequencies. We have developed a system that generates signals from 8-10(3) kHz and applies two of these signals simultaneously to the body through a broadband current mirror. Great care has been taken in the design of the current injection multiplexer in order to keep the current source output capacitance as low as possible. Furthermore design of the layout of the patient interface board, in order to reduce feedthrough capacitances, also needs great care. Other parameters for driving and detection sections have been designed according to our results from FEM and circuit simulations including skin and electrode effects. Simulations using FEM with available tissue impedance data and preliminary measurements in a discrete phantom show that static imaging is possible for both the real and imaginary parts of the impedance.