Model for the dielectric properties of human lung tissue against frequency and air content

Regional Pulmonary Ventilation Assessment Method and System Based on Impedance Sensing Information from the Pentapulmonary Lobes

Regional lung ventilation assessment is a critical tool for the early detection of lung diseases and postoperative evaluation. Biosensor-based impedance measurements, known for their non-invasive nature, among other benefits, have garnered significant attention compared to traditional detection methods that utilize pressure sensors. However, solely utilizing overall thoracic impedance fails to accurately capture changes in regional lung air volume. This study introduces an assessment method for lung ventilation that utilizes impedance data from the five lobes, develops a nonlinear model correlating regional impedance with lung air volume, and formulates an approach to identify regional ventilation obstructions based on impedance variations in affected areas. The electrode configuration for the five lung lobes was established through numerical simulations, revealing a power-function nonlinear relationship between regional impedance and air volume changes. An analysis of 389 pulmonary function tests refined the equations for calculating pulmonary function parameters, taking into account individual differences. Validation tests on 30 cases indicated maximum relative errors of 0.82% for FVC and 0.98% for FEV1, all within the 95% confidence intervals. The index for assessing regional ventilation impairment was corroborated by CT scans in 50 critical care cases, with 10 validation trials showing agreement with CT lesion localization results.

Histology-validated electromagnetic characterization of ex-vivo ovine lung tissue for microwave-based medical applications

Microwave thermal ablation is an established therapeutic technique for treating malignant tissue in various organs. Its success greatly depends on the knowledge of dielectric properties of the targeted tissue and on how they change during the treatment. Innovation in lung navigation has recently increased the clinical interest in the transbronchial microwave ablation treatment of lung cancer. However, lung tissue is not largely characterized, thus its dielectric properties investigation prior and post ablation is key. In this work, dielectric properties of ex-vivo ovine lung parenchyma untreated and ablated at 2.45 GHz were recorded in the 0.5-8 GHz frequency range. The measured dielectric properties were fitted to 2-pole Cole-Cole relaxation model and the obtained model parameters were compared. Based on observed changes in the model parameters, the physical changes of the tissue post-ablation were discussed and validated through histology analysis. Additionally, to investigate the link of achieved results with the rate of heating, another two sets of samples, originating from both ovine and porcine tissues, were heated with a microwave oven for different times and at different powers. Dielectric properties were measured in the same frequency range. It was found that lung tissue experiences a different behavior according to heating rates: its dielectric properties increase post-ablation while a decrease is found for low rates of heating. It is hypothesized, and validated by histology, that during ablation, although the tissue is losing water, the air cavities deform, lowering air content and increasing the resulting tissue properties.

Histology-Validated Dielectric Characterisation of Lung Carcinoma Tissue for Microwave Thermal Ablation Applications

Microwave thermal ablation is a promising emerging treatment for early-stage lung cancer. Applicator design optimisation and treatment planning rely on accurate knowledge of dielectric tissue properties. Limited dielectric data are available in the literature for human lung tissue and pulmonary tumours. In this work, neoplastic and non-neoplastic lung dielectric properties are characterised and correlated with gross and histological morphology. Fifty-six surgical specimens were obtained from twelve patients undergoing lung resection for lung cancer in University Hospital of Galway, Ireland. Dielectric spectroscopy in the microwave frequency range (500 MHz-8.5 GHz) was performed on the ex vivo lung specimens with the open-ended coaxial probe technique (in the Department of Pathology). Dielectric data were analysed and correlated with the tissue histology. The dielectric properties of twelve lung tumours (67% non-small cell carcinoma (NSCC)) and uninvolved lung parenchyma were obtained. The values obtained from the neoplastic lung specimens (relative permittivity: 52.0 ± 5.4, effective conductivity: 1.9 ± 0.2 S/m, at 2.45 GHz) were on average twice the value of the non-neoplastic lung specimens (relative permittivity: 28.3 ± 6.7, effective conductivity: 1.0 ± 0.3 S/m, at 2.45 GHz). Dense fibrosis was comparable with tumour tissue (relative permittivity 49.3 ± 4.6, effective conductivity: 1.8 ± 0.1 S/m, at 2.45 GHz).

Low Power Contactless Bioimpedance Sensor for Monitoring Breathing Activity

An electronic circuit for contactless detection of impedance changes in a tissue is presented. It operates on the principle of resonant frequency change of the resonator having the observed tissue as a dielectric. The operating frequency reflects the tissue dielectric properties (i.e., the tissue composition and on the tissue physiological changes). The sensor operation was tested within a medical application by measuring the breathing of a patient, which was an easy detectable physiological process. The advantage over conventional contact bioimpedance measurement methods is that no direct contact between the resonator and the body is required. Furthermore, the sensor's wide operating range, ability to adapt to a broad range of measured materials, fast response, low power consumption, and small outline dimensions enables applications not only in the medical sector, but also in other domains. This can be extended, for example, to food industry or production maintenance, where the observed phenomena are reflected in dynamic dielectric properties of the observed object or material.

Understanding the physical relations governing the noise navigator

Purpose

The noise navigator is a passive way to detect physiological motion occurring in a patient through thermal noise modulations measured by standard clinical radiofrequency receive coils. The aim is to gain a deeper understanding of the potential and applications of physiologically induced thermal noise modulations.

Methods

Numerical electromagnetic simulations and MR measurements were performed to investigate the relative contribution of tissue displacement versus modulation of the dielectric lung properties over the respiratory cycle, the impact of coil diameter and position with respect to the body. Furthermore, the spatial motion sensitivity of specific noise covariance matrix elements of a receive array was investigated.

Results

The influence of dielectric lung property variations on the noise variance is negligible compared to tissue displacement. Coil size affected the thermal noise variance modulation, but the location of the coil with respect to the body had a larger impact. The modulation depth of a 15 cm diameter stationary coil approximately 3 cm away from the chest (i.e. radiotherapy setup) was 39.7% compared to 4.2% for a coil of the same size on the chest, moving along with respiratory motion. A combination of particular noise covariance matrix elements creates a specific spatial sensitivity for motion.

Conclusions

The insight gained on the physical relations governing the noise navigator will allow for optimized use and development of new applications. An optimized combination of elements from the noise covariance matrix offer new ways of performing, e.g. motion tracking.

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.

Modelling approach to obtain regional respiratory mechanics using electrical impedance tomography and volume-dependent elastance model

OBJECTIVE

This paper presents a method for breath-by-breath estimation of regional respiratory mechanics without the need for special manoeuvres (such as inspiratory pause or low-flow inflation) using electrical impedance tomography (EIT) associated with pressure/airflow waveforms.

APPROACH

We developed a method to estimate regional parameters using the regional impedance fraction, by multiplying it by global flow and volume waveforms. A volume-dependent elastance model was used to obtain compliance, resistance, volume-independent (E ₁), and volume-dependent (E ₂) components. Three swine under invasive mechanical ventilation were used to assess internal consistency and illustrate potential applications of our method. One animal (case 1) was ventilated with a broad range of tidal volumes to compare the consistency between regional and global resistances and compliances. Two other animals (cases 2 and 3) had respiratory compliance decreased, respectively, by overdistension and collapse as quantified by x-ray computed tomography.

MAIN RESULTS

In case 1, derived global estimates obtained from the independent regional estimates were strongly associated with direct measurements of global mechanics (correlation coefficients of 0.9976 and 0.9981 for compliances and resistances, respectively), suggesting consistency of our modelling. In cases 2 and 3, the development of lung overdistension and collapse over time was captured by regional estimates.

CONCLUSIONS

Using EIT and pressure/airflow waveforms, regional respiratory parameters can be obtained cycle-by-cycle, refining lung function monitoring.

SIGNIFICANCE

The method allows real-time monitoring of regional parameters and their trends over time, which might be helpful to differentiate deterioration in lung compliance due to overdistension or collapse.

Pleural electrical impedance is a sensitive, real-time indicator of pneumothorax

BACKGROUND

Chest tube management protocols, particularly in patients with alveolar-pleural air leak due to recent surgery or trauma, are limited by concerns over safety, especially concerns about rapid and occult development of pneumothorax. A continuous, real-time monitor of pneumothorax could improve the quality and safety of chest tube management. We developed a rat model of pneumothorax to test a novel approach of measuring electrical impedance within the pleural space as a monitor of lung expansion.

MATERIALS AND METHODS

Anesthetized Sprague-Dawley rats underwent right thoracotomy. A novel impedance sensor and a thoracostomy tube were introduced into the right pleural space. Pneumothorax of varying volumes ranging from 0.2 to 20 mL was created by syringe injection of air via the thoracostomy tube. Electrical resistance measurements from the pleural sensor and fluoroscopic images were obtained at baseline and after the creation of pneumothorax and results compared.

RESULTS

A statistically significant, dose-dependent increase in electrical resistance was observed with increasing volume of pneumothorax. Resistance measurement allowed for continuous, real-time monitoring of pneumothorax development and the ability to track pneumothorax resolution by aspiration of air via the thoracostomy tube. Pleural resistance measurement demonstrated 100% sensitivity and specificity for all volumes of pneumothorax tested and was significantly more sensitive for pneumothorax detection than fluoroscopy.

CONCLUSIONS

The electrical impedance-based pleural space sensor described in this study provided sensitive and specific pneumothorax detection, which was superior to radiographic analysis. Real-time, continuous monitoring for pneumothorax has the potential to improve the safety, quality, and efficiency of postoperative chest tube management.

Coupling of EIT with computational lung modeling for predicting patient-specific ventilatory responses

Providing optimal personalized mechanical ventilation for patients with acute or chronic respiratory failure is still a challenge within a clinical setting for each case anew. In this article, we integrate electrical impedance tomography (EIT) monitoring into a powerful patient-specific computational lung model to create an approach for personalizing protective ventilatory treatment. The underlying computational lung model is based on a single computed tomography scan and able to predict global airflow quantities, as well as local tissue aeration and strains for any ventilation maneuver. For validation, a novel “virtual EIT” module is added to our computational lung model, allowing to simulate EIT images based on the patient's thorax geometry and the results of our numerically predicted tissue aeration. Clinically measured EIT images are not used to calibrate the computational model. Thus they provide an independent method to validate the computational predictions at high temporal resolution. The performance of this coupling approach has been tested in an example patient with acute respiratory distress syndrome. The method shows good agreement between computationally predicted and clinically measured airflow data and EIT images. These results imply that the proposed framework can be used for numerical prediction of patient-specific responses to certain therapeutic measures before applying them to an actual patient. In the long run, definition of patient-specific optimal ventilation protocols might be assisted by computational modeling.

NEW & NOTEWORTHY In this work, we present a patient-specific computational lung model that is able to predict global and local ventilatory quantities for a given patient and any selected ventilation protocol. For the first time, such a predictive lung model is equipped with a virtual electrical impedance tomography module allowing real-time validation of the computed results with the patient measurements. First promising results obtained in an acute respiratory distress syndrome patient show the potential of this approach for personalized computationally guided optimization of mechanical ventilation in future.

Tidal breathing flow-volume curves with impedance pneumography during expiratory loading

Diagnosis of asthma in the preschoold children is difficult due to lack of objective lung function tests suitable for this age group. Impedance pneumography (IP) is a mode of measurement that may potentially enable ambulatory 24h recording of tidal breathing indices and respiratory dynamics that are known to relate to small airway obstruction. The aim of this research was to induce changes in breathing control and mechanics and study the ability of IP to reproduce TBFVC and track its changes under potentially difficult conditions. This was achieved by a comparison of direct mouth pneumotachograph (PNT) and IP tidal breathing flow-volume curves (TBFVC) during free breathing and expiratory loading obtained from 17 young lung-healthy subjects. The expiratory loading produced strong and significant changes in the respiratory pattern and mouth pressure. The agreement of PNT and IP normalized TBFVCs was found excellent having the highest distance between the normalized TBFVCs of (mean ± SD) 7.4 % ± 3.6 % and 6.2 % ± 3.0 % during free and loaded breathing, respectively. The agreement was not affected by the presence of the expiratory load despite it poses multiple potential hazards for the IP measurements. We conclude that by using correct electrode placement and cardiac filtering, IP was able to accurately reproduce and track changes in normalized TBFVCs under normal and abnormal respiratory conditions in healthy adult subjects.

First-time imaging of effects of inspired oxygen concentration on regional lung volumes and breathing pattern during hypergravity

PURPOSE

Aeroatelectasis can develop in aircrew flying the latest generation high-performance aircraft. Causes alleged are relative hyperoxia, increased gravity in the head-to-foot direction (+Gz), and compression of legs and stomach by anti-G trousers (AGT). We aimed to assess, in real time, the effects of hyperoxia, +Gz accelerations and AGT inflation on changes in regional lung volumes and breathing pattern evaluated in an axial plane by electrical impedance tomography (EIT).

METHODS

The protocol mimicked a routine peacetime flight in combat aircraft. Eight subjects wearing AGT were studied in a human centrifuge during 1 h 15 min exposure of +1 to +3.5Gz. They performed this sequence three times, breathing AIR, 44.5 % O2 or 100 % O2. Continuous recording of functional EIT enabled uninterrupted assessment of regional lung volumes at the 5th intercostal level. Breathing pattern was also monitored.

RESULTS

EIT data showed that +3.5Gz, compared with any moment without hypergravity, caused an abrupt decrease in regional tidal volume (VT) and regional end-expiratory lung volume (EELV) measured in the EIT slice, independently of inspired oxygen concentration. Breathing AIR or 44.5 % O2, sub-regional EELV measured in the EIT slice decreased similarly in dorsal and ventral regions, but sub-regional VT measured in the EIT slice decreased significantly more dorsally than ventrally. Breathing 100 % O2, EELV and VT decreased similarly in both regions. Inspired tidal volume increased in hyperoxia, whereas breathing frequency increased in hypergravity and hyperoxia.

CONCLUSIONS

Our findings suggest that hypergravity and AGT inflation cause airway closure and air trapping in gravity-dependent lung regions, facilitating absorption atelectasis formation, in particular during hyperoxia.

Choice of reconstructed tissue properties affects interpretation of lung EIT images

Electrical impedance tomography (EIT) estimates an image of change in electrical properties within a body from stimulations and measurements at surface electrodes. There is significant interest in EIT as a tool to monitor and guide ventilation therapy in mechanically ventilated patients. In lung EIT, the EIT inverse problem is commonly linearized and only changes in electrical properties are reconstructed. Early algorithms reconstructed changes in resistivity, while most recent work using the finite element method reconstructs conductivity. Recently, we demonstrated that EIT images of ventilation can be misleading if the electrical contrasts within the thorax are not taken into account during the image reconstruction process. In this paper, we explore the effect of the choice of the reconstructed electrical properties (resistivity or conductivity) on the resulting EIT images. We show in simulation and experimental data that EIT images reconstructed with the same algorithm but with different parametrizations lead to large and clinically significant differences in the resulting images, which persist even after attempts to eliminate the impact of the parameter choice by recovering volume changes from the EIT images. Since there is no consensus among the most popular reconstruction algorithms and devices regarding the parametrization, this finding has implications for potential clinical use of EIT. We propose a program of research to develop reconstruction techniques that account for both the relationship between air volume and electrical properties of the lung and artefacts introduced by the linearization.

Development and evaluation of an improved technique for pulmonary function testing using electrical impedance pneumography intended for the diagnosis of chronic obstructive pulmonary disease patients

Spirometry is regarded as the only effective method for detecting pulmonary function test (PFT) indices. In this study, a novel impedance pulmonary function measurement system (IPFS) is developed for directly assessing PFT indices. IPFS can obtain high resolution values and remove motion artifacts through real-time base impedance feedback. Feedback enables the detection of PFT indices using only both hands for convenience. IPFS showed no differences in the sitting, supine, and standing postures during the measurements, indicating that patient posture has no effect on IPFS. Mean distance analysis showed good agreement between the volume and flow signal of IPFS (p < 0.05). PFT indices were detected in subjects to differentiate a chronic obstructive pulmonary disease (COPD) patient group from a normal group. The forced vital capacity (FVC), forced expiratory volume in the first second (FEV₁), FEV_1/FVC, and peak expiratory flow (PEF) in the COPD group were lower than those in the normal group by IPFS (p < 0.05). IPFS is therefore suitable for evaluating pulmonary function in normal and COPD patients. Moreover, IPFS could be useful for periodic monitoring of existing patients diagnosed with obstructive lung disease.

Tidal breathing flow measurement in awake young children by using impedance pneumography

Characteristics of tidal breathing (TB) relate to lung function and may be assessed even in young children. Thus far, the accuracy of impedance pneumography (IP) in recording TB flows in young children with or without bronchial obstruction has not been evaluated. The aim of this study was to evaluate the agreement between IP and direct flow measurement with pneumotachograph (PNT) in assessing TB flow and flow-derived indices relating to airway obstruction in young children. Tidal flow was recorded for 1 min simultaneously with IP and PNT during different phases of a bronchial challenge test with methacholine in 21 wheezy children aged 3 to 7 years. The agreement of IP with PNT was found to be excellent in direct flow signal comparison, the mean deviation from linearity ranging from 2.4 to 3.1% of tidal peak inspiratory flow. Methacholine-induced bronchoconstriction or consecutive bronchodilation induced only minor changes in the agreement. Between IP and PNT, the obstruction-related tidal flow indices were equally repeatable, and agreement was found to be high, with intraclass correlation coefficients for T PTEF/T E, V PTEF/V E, and parameter S being 0.94, 0.91, and 0.68, respectively. Methacholine-induced changes in tidal flow indices showed significant associations with changes in mechanical impedance of the respiratory system assessed by the oscillometric technique, with the highest correlation found in V PTEF/V E (r = -0.54; P < 0.005 and r = -0.55; P < 0.005 by using IP or PNT, respectively). The results indicate that IP can be considered as a valid method for recording tidal airflow profiles in young children with wheezing disorders.

Regional lung perfusion estimated by electrical impedance tomography in a piglet model of lung collapse

The assessment of the regional match between alveolar ventilation and perfusion in critically ill patients requires simultaneous measurements of both parameters. Ideally, assessment of lung perfusion should be performed in real-time with an imaging technology that provides, through fast acquisition of sequential images, information about the regional dynamics or regional kinetics of an appropriate tracer. We present a novel electrical impedance tomography (EIT)-based method that quantitatively estimates regional lung perfusion based on first-pass kinetics of a bolus of hypertonic saline contrast. Pulmonary blood flow was measured in six piglets during control and unilateral or bilateral lung collapse conditions. The first-pass kinetics method showed good agreement with the estimates obtained by single-photon-emission computerized tomography (SPECT). The mean difference (SPECT minus EIT) between fractional blood flow to lung areas suffering atelectasis was -0.6%, with a SD of 2.9%. This method outperformed the estimates of lung perfusion based on impedance pulsatility. In conclusion, we describe a novel method based on EIT for estimating regional lung perfusion at the bedside. In both healthy and injured lung conditions, the distribution of pulmonary blood flow as assessed by EIT agreed well with the one obtained by SPECT. The method proposed in this study has the potential to contribute to a better understanding of the behavior of regional perfusion under different lung and therapeutic conditions.

Absolute electrical impedance tomography (aEIT) guided ventilation therapy in critical care patients: simulations and future trends

Thoracic electrical impedance tomography (EIT) is a noninvasive, radiation-free monitoring technique whose aim is to reconstruct a cross-sectional image of the internal spatial distribution of conductivity from electrical measurements made by injecting small alternating currents via an electrode array placed on the surface of the thorax. The purpose of this paper is to discuss the fundamentals of EIT and demonstrate the principles of mechanical ventilation, lung recruitment, and EIT imaging on a comprehensive physiological model, which combines a model of respiratory mechanics, a model of the human lung absolute resistivity as a function of air content, and a 2-D finite-element mesh of the thorax to simulate EIT image reconstruction during mechanical ventilation. The overall model gives a good understanding of respiratory physiology and EIT monitoring techniques in mechanically ventilated patients. The model proposed here was able to reproduce consistent images of ventilation distribution in simulated acutely injured and collapsed lung conditions. A new advisory system architecture integrating a previously developed data-driven physiological model for continuous and noninvasive predictions of blood gas parameters with the regional lung function data/information generated from absolute EIT (aEIT) is proposed for monitoring and ventilator therapy management of critical care patients.

Real-time detection of pneumothorax using electrical impedance tomography

OBJECTIVES

Pneumothorax is a frequent complication during mechanical ventilation. Electrical impedance tomography (EIT) is a noninvasive tool that allows real-time imaging of regional ventilation. The purpose of this study was to 1) identify characteristic changes in the EIT signals associated with pneumothoraces; 2) develop and fine-tune an algorithm for their automatic detection; and 3) prospectively evaluate this algorithm for its sensitivity and specificity in detecting pneumothoraces in real time.

DESIGN

Prospective controlled laboratory animal investigation.

SETTING

Experimental Pulmonology Laboratory of the University of São Paulo.

SUBJECTS

Thirty-nine anesthetized mechanically ventilated supine pigs (31.0 +/- 3.2 kg, mean +/- SD).

INTERVENTIONS

In a first group of 18 animals monitored by EIT, we either injected progressive amounts of air (from 20 to 500 mL) through chest tubes or applied large positive end-expiratory pressure (PEEP) increments to simulate extreme lung overdistension. This first data set was used to calibrate an EIT-based pneumothorax detection algorithm. Subsequently, we evaluated the real-time performance of the detection algorithm in 21 additional animals (with normal or preinjured lungs), submitted to multiple ventilatory interventions or traumatic punctures of the lung.

MEASUREMENTS AND MAIN RESULTS

Primary EIT relative images were acquired online (50 images/sec) and processed according to a few imaging-analysis routines running automatically and in parallel. Pneumothoraces as small as 20 mL could be detected with a sensitivity of 100% and specificity 95% and could be easily distinguished from parenchymal overdistension induced by PEEP or recruiting maneuvers. Their location was correctly identified in all cases, with a total delay of only three respiratory cycles.

CONCLUSIONS

We created an EIT-based algorithm capable of detecting early signs of pneumothoraces in high-risk situations, which also identifies its location. It requires that the pneumothorax occurs or enlarges at least minimally during the monitoring period. Such detection was operator-free and in quasi real-time, opening opportunities for improving patient safety during mechanical ventilation.

Quantification of lung injury using ventilation and perfusion distributions obtained from gamma scintigraphy

This paper explores the potential of isotope V/Q lung scans to quantify lung disease. Areas of restricted perfusion in subjects with a pulmonary embolus (PE) were identified in 3D reconstructions of V/Q images achieved using anatomical data from the Visible Human Project. From these, the extent of lung damage was quantified. Significant differences in the values of both LogSD V and LogSD Q (p > 0.05) obtained from plots of V and Q against Log(V/Q) were found between normal subjects and subjects with a PE, but no correlation was found between either of these parameters and the degree of lung damage in subjects with a PE (p > 0.05). Whilst V/Q values were log normally distributed, the V/Q distributions from the subjects with a PE failed to show the bimodal distribution predicted from theoretical considerations and MIGET measurements previously reported. There was a statistically significant difference in the mean and standard deviation values of the V/Q distributions between normal subject and subjects with a PE (p < 0.05) but not in the median values (p > 0.05). There was no correlation between the mean, median and standard deviation of the distributions from the subjects with a PE and the percentage of damage present (p > 0.05).

Direct reconstruction of tissue parameters from differential multifrequency EITin vivo

The basic purpose of electrical impedance tomography (EIT) is the reconstruction of conductivity distributions. While multifrequency measurements are of common use, the majority of reconstructed images are still conductivity distributions from one single frequency. More interesting than conductivities at each frequency are electrical tissue parameters, which describe the frequency-dependent conductivity changes of tissue. These parameters give information about physiological or electrical properties of tissues. By using this spectral information, a classification of different tissue types is possible. To get a distribution of tissue parameters, usually a posterior fitting of a tissue model to the conductivity spectra obtained with classical reconstruction algorithms at various frequencies is used. In this work, a single-step reconstruction algorithm for differential imaging was developed for the direct estimation of Cole parameters. This method is termed differential parametric reconstruction. The Cole model was used as the underlying tissue model, where only the relative changes of the two conductivity parameters sigma(0) and sigma(infinity) were reconstructed and the other two parameters of the model which are less identifiable were set to constant values. The reconstruction algorithm was tested with simulated noisy datasets and real measurement data from EIT measurements on the human thorax. These measurements were taken from healthy subjects and from patients with a serious lung injury. The new method yields a good image quality and higher robustness against noise compared to conventional reconstruction methods.

Lung function tests in neonates and infants with chronic lung disease: global and regional ventilation inhomogeneity

This review considers measurement of global and regional ventilation inhomogeneity (VI) in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). We focus primarily on multiple-breath inert gas washout (MBW) and electrical impedance tomography (EIT). The literature is critically reviewed and the relevant methods, equipment, and studies are summarized, including the limitations and strengths of individual techniques, together with the availability and appropriateness of any reference data. There has been a recent resurgence of interest in using MBW to monitor lung function within individuals and between different groups. In the mechanically ventilated, sedated, and paralyzed patient, VI indices can identify serial changes occurring following exogenous surfactant. Similarly, global VI indices appear to be increased in infants with CLDI and to differentiate between infants without lung disease and those with mild, moderate, and severe lung disease following preterm birth. While EIT is a relatively new technique, recent studies suggest that it is feasible in newborn infants, and can quantitatively identify changes in regional lung ventilation following alterations to ventilator settings, positive end expiratory pressure (PEEP), and administration of treatments such as surfactant. As such, EIT represents one of the more exciting prospects for continuous bedside pulmonary monitoring. For both techniques, there is an urgent need to establish guidelines regarding data collection, analysis, and interpretation in infants both with and without CLDI.

Dielectric behavior of pulmonary edema induced in the rat lung

The dielectric properties (conductivity, kappa and relative permittivity, epsilon) of excised rat lung are modified by lung air and water content. The measurements of these quantities were made over the frequency range of 10 kHz to 100 MHz with an open-ended coaxial probe. The following relationships were analyzed in an oleic acid-induced pulmonary edema model using 18 animals: the spectra of kappa, epsilon and the loss tangent as a function of lung air and water content. Secondly, an isolated-perfused lung system was produced to induce a gradual increase in lung water. The time course of kappa, epsilon and the loss tangent for one excised lung was analyzed. The principal findings were: (i) a decrease in kappa and epsilon with increasing air content, (ii) an increase in kappa and epsilon with increasing water content, and (iii) a good correlation between lung water content and maximum loss tangent that was insensitive to changes in air content. We conclude that this technique could provide a quantitative assessment of lung water during pulmonary edema formation.

Monitoring of lung edema using focused impedance spectroscopy: a feasibility study

Currently only ionizing or invasive methods are used in clinical applications for the monitoring of extracellular lung water. Alternatively a method called focused conductivity spectroscopy (FCS) is suggested, which aims at reconstructing a pulmonary edema index (PEIX) by measuring the electrical conductivity of the region of interest (ROI) at several frequencies. In contrast to electrical impedance tomography (EIT) a minimum number of strategically placed electrodes is used. The goals of this study were the analysis of the sensitivity for the PEIX, an estimate of the optimal electrode configuration and the determination of the required frequencies. In order to calculate the solution of the FCS forward problem a realistic 3D model of a human torso was developed containing both lungs, the heart, the liver and the thorax musculature. The bioelectrical properties for each compartment were described with appropriate tissue models which relate the conductivity spectra to physiological parameters. The PEIX was defined as the interstitial volume fraction of the alveolar septa. Furthermore the model includes 48 electrodes subdivided into three layers. The optimal electrode configuration was selected by minimizing the number of electrodes, among certain subsets of these electrodes. The analysis shows that eight to ten electrodes and six frequencies are theoretically sufficient to obtain a coefficient of variation.

Imbalances in Regional Lung Ventilation

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right-left ventilation imbalances (bias = 0% and limits of agreement = -10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r(2) > or = 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Serial changes in nasal potential difference and lung electrical impedance tomography at high altitude

Recent work suggests that treatment with inhaled beta(2)-agonists reduces the incidence of high-altitude pulmonary edema in susceptible subjects by increasing respiratory epithelial sodium transport. We estimated respiratory epithelial ion transport by transepithelial nasal potential difference (NPD) measurements in 20 normal male subjects before, during, and after a stay at 3,800 m. NPD hyperpolarized on ascent to 3,800 m (P < 0.05), but the change in potential difference with superperfusion of amiloride or isoprenaline was unaffected. Vital capacity (VC) fell on ascent to 3,800 m (P < 0.05), as did the normalized change in electrical impedance (NCI) measured over the right lung parenchyma (P < 0.05) suggestive of an increase in extravascular lung water. Echo-Doppler-estimated pulmonary artery pressure increases were insufficient to cause clinical pulmonary edema. There was a positive correlation between VC and NCI (R(2) = 0.633) and between NPD and both VC and NCI (R(2) = 0.267 and 0.418). These changes suggest that altered respiratory epithelial ion transport might play a role in the development of subclinical pulmonary edema at high altitude in normal subjects.

Neonatal lungs: maturational changes in lung resistivity spectra

The electrical resistivity of lung tissue can be related to the structure and composition of the tissue and also to the air content. Electrical impedance tomographic measurements have been used on 155 normal children over the first three years of life and 25 pre-term infants, to determine the absolute resistivity of lung tissue as a function of frequency. The results show consistent changes with increasing age in both lung tissue resistivity (5.8 ohm m at birth to 20.9 ohm m at 3 years of age) and in the changes of resistivity with frequency (Cole parameter ratio R/S=0.41 at birth and 0.84 at 3 years of age). Comparison with a lung model showed that the measurements are consistent with maturational changes in the number and size of alveoli, the extracapillary blood volume and the size of the extracapillary vessels. However, the results show that the process of maturation is not complete at the age of three years.

Neonatal lungs--can absolute lung resistivity be determined non-invasively?

The electrical resistivity of lung tissue can be related to the structure and composition of the tissue and also to the air content. Conditions such as pulmonary oedema and emphysema have been shown to change lung resistivity. However, direct access to the lungs to enable resistivity to be measured is very difficult. We have developed a new method of using electrical impedance tomographic (EIT) measurements on a group of 142 normal neonates to determine the absolute resistivity of lung tissue. The methodology involves comparing the measured EIT data with that from a finite difference model of the thorax in which lung tissue resistivity can be changed. A mean value of 5.7 +/- 1.7 omega(m) was found over the frequency range 4 kHz to 813 kHz. This value is lower than that usually given for adult lung tissue but consistent with the literature on the composition of the neonatal lung and with structural modelling.

Mk3.5: a modular, multi-frequency successor to the Mk3a EIS/EIT system

This paper describes the Sheffield Mk3.5 EIT/EIS system which measures both the real and imaginary part of impedance at 30 frequencies between 2 kHz and 1.6 MHz. The system uses eight electrodes with an adjacent drive/receive electrode data acquisition protocol. The system is modular, containing eight identical data acquisition boards, which contain DSPs to generate the drive frequencies and to perform the FFT used for demodulation. The current drive is in three sequentially applied packets, where each packet contains ten summed sine waves. The data acquisition system is interfaced to a host PC through an optically isolated high speed serial link (RS485) running at 2 Mbaud (2 Mbits s(-1)). Measurements on a saline filled tank show that the average signal to noise performance of the system is 40 dB measured across all frequencies and that this figure is independent of frequency of measurement. These results suggest that the current system is 10 dB better in absolute terms than the previous Sheffield (Mk3a) system.

A comparison of neonatal and adult lung impedances derived from EIT images

An objective method of extracting respiratory data from lung images is presented, together with a technique for automatically generating regions of interest delineating the anterior and posterior regions of the lungs. The method is used to extract data on the change in lung impedance with frequency, and on calculated Cole parameters, from 19 normal neonates (gestational age 32 to 42 weeks) and 8 normal adults (age 21 to 82 years). A comparison of the impedance properties of neonatal and adult lungs was made. The variation of lung impedance with frequency in neonates, as derived from EIT images, is significantly different from that found for adults. The implications for a model of the electrical impedance of lung tissue are discussed.