Application of electrical impedance tomography in diagnosis of emphysema--a clinical study

Sex differences in chest electrical impedance tomography findings

Objective.Electrical impedance tomography (EIT) has been used to determine regional lung ventilation distribution in humans for decades, however, the effect of biological sex on the findings has hardly ever been examined. The aim of our study was to determine if the spatial distribution of ventilation assessed by EIT during quiet breathing was influenced by biological sex.Approach.219 adults with no known acute or chronic lung disease were examined in sitting position with the EIT electrodes placed around the lower chest (6th intercostal space). EIT data were recorded at 33 images/s during quiet breathing for 60 s. Regional tidal impedance variation was calculated in all EIT image pixels and the spatial distribution of the values was determined using the established EIT measures of centre of ventilation in ventrodorsal (CoVvd) and right-to-left direction (CoVrl), the dorsal and right fraction of ventilation, and ventilation defect score.Main results.After exclusion of one subject due to insufficient electrode contact, 218 data sets were analysed (120 men, 98 women) (age: 53 ± 18 vs 50 ± 16 yr (p= 0.2607), body mass index: 26.4 ± 4.0 vs 26.4 ± 6.6 kg m-2(p= 0.9158), mean ± SD). Highly significant differences in ventilation distribution were identified between men and women between the right and left chest sides (CoVrl: 47.0 ± 2.9 vs 48.8 ± 3.3% of chest diameter (p< 0.0001), right fraction of ventilation: 0.573 ± 0.067 vs 0.539 ± 0.071 (p= 0.0004)) and less significant in the ventrodorsal direction (CoVvd: 55.6 ± 4.2 vs 54.5 ± 3.6% of chest diameter (p= 0.0364), dorsal fraction of ventilation: 0.650 ± 0.121 vs 0.625 ± 0.104 (p= 0.1155)). Ventilation defect score higher than one was found in 42.5% of men but only in 16.6% of women.Significance.Biological sex needs to be considered when EIT findings acquired in upright subjects in a rather caudal examination plane are interpreted. Sex differences in chest anatomy and thoracoabdominal mechanics may explain the results.

Lung area estimation using functional tidal electrical impedance variation images and active contouring

OBJECTIVE

Electrical impedance tomography is a valuable tool for monitoring global and regional lung mechanics. To evaluate the recorded data, an accurate estimate of the lung area is crucial.

APPROACH

We present two novel methods for estimating the lung area using functional tidal images or active contouring methods. A convolutional neural network was trained to determine, whether or not the heart region was visible within tidal images. In addition, the effects of lung area mirroring were investigated. The performance of the methods and the effects of mirroring were evaluated via a score based on the impedance magnitudes in functional tidal images.

MAIN RESULTS

Our analyses showed that the method based on functional tidal images provided the best estimate of the lung area. Mirroring of the lung area had an impact on the accuracy of area estimation for both methods. The achieved accuracy of the neural network's classification was 94%. For images without a visible heart area, the subtraction of a heart template proved to be a pragmatic approach with good results.

SIGNIFICANCE

In summary, we developed a routine for estimation of the lung area combined with estimation of the heart area in electrical impedance tomography images.

A narrative review of electrical impedance tomography in lung diseases with flow limitation and hyperinflation: methodologies and applications

Electrical impedance tomography (EIT) is a functional radiation-free imaging technique that measures regional lung ventilation distribution by calculating the impedance changes in the corresponding regions. The aim of the present review was to summarize the current literature concerning the methodologies and applications of EIT in lung diseases with flow limitation and hyperinflation. PubMed was searched up to May 2020 to identify studies investigating the use of EIT in patients with asthma, bronchiectasis, bronchitis, bronchiolitis, chronic obstructive pulmonary disease, and cystic fibrosis. The extracted data included study design, EIT methodologies, interventions, validation and comparators, population characteristics, and key findings. Of the 44 included studies, seven were related to simulation, animal experimentation, or reconstruction algorithm development with evaluation on patients; 27 studies had the primary objective of validating EIT technique and measures including regional ventilation distribution, regional EIT-spirometry parameters, end-expiratory lung impedance, and regional time constants; and 10 studies had the primary objective of applying EIT to monitor the response to therapeutic interventions, including various ventilation supports, patient repositioning, and airway suctioning. In pediatric and adult patients, EIT has been successfully validated for assessing spatial and temporal ventilation distribution, measuring changes in lung volume and flow, and studying regional respiratory mechanics. EIT has also demonstrated potential as an alternative or supplement to well-established measurement modalities (e.g., conventional pulmonary function testing) to monitor the progression of obstructive lung diseases, although the existing literature lacks prediction values as references and lacks clinical outcome evidence.

Redistribution of pulmonary ventilation after lung surgery detected with electrical impedance tomography

BACKGROUND

Regional ventilation of the lung can be visualized by pulmonary electrical impedance tomography (EIT). The aim of this study was to examine the post-operative redistribution of regional ventilation after lung surgery dependent on the side of surgery and its association with forced vital capacity.

METHODS

In this prospective, observational cohort study 13 patients undergoing right and 13 patients undergoing left-sided open or video-thoracoscopic procedures have been investigated. Pre-operative measurements with EIT and spirometry were compared with data obtained 3 days post-operation. The center of ventilation (COV) within a 32 × 32 pixel matrix was calculated from EIT data. The transverse axis coordinate of COV, COVx (left/right), was modified to COVx' (ipsilateral/contralateral). Thus, COVx' shows a negative change if ventilation shifts contralateral independent of the side of surgery. This enabled testing with two-way ANOVA for repeated measurements (side, time).

RESULTS

The perioperative shift of COVx' was dependent on the side of surgery (P = .007). Ventilation shifted away from the side of surgery after the right-sided surgery (COVx'-1.97 pixel matrix points, P < .001), but not after the left-sided surgery (COVx'-0.61, P = .425). The forced vital capacity (%predicted) decreased from 94 (83-109)% (median [quartiles]; [left-sided]) and 89 (80-97)% (right-sided surgery) to 61 (59-66)% and 62 (40-72)% (P < .05), respectively. The perioperative changes in forced vital capacity (%predicted) were weakly associated with the shift of COVx'.

CONCLUSION

Only after right-sided lung surgery, EIT showed reduced ventilation on the side of surgery while vital capacity was markedly reduced in both groups.

Time-based pulmonary features from electrical impedance tomography demonstrate ventilation heterogeneity in chronic obstructive pulmonary disease

Pulmonary electrical impedance tomography (EIT) is a functional imaging technique that allows real-time monitoring of ventilation distribution. Ventilation heterogeneity (VH) is a characteristic feature of chronic obstructive pulmonary disease (COPD) and has previously been quantified using features derived from tidal variations in the amplitude of the EIT signal. However, VH may be better described by time-based metrics, the measurement of which is made possible by the high temporal resolution of EIT. We aimed 1) to quantify VH using novel time-based EIT metrics and 2) to determine the physiological relevance of these metrics by exploring their relationships with complex lung mechanics measured by the forced oscillation technique (FOT). We performed FOT, spirometry, and tidal-breathing EIT measurements in 11 healthy controls and 9 volunteers with COPD. Through offline signal processing, we derived 3 features from the impedance-time (Z-t) curve for each image pixel: 1) t_E, mean expiratory time; 2) PHASE, mean time difference between pixel and global Z-t curves; and 3) AMP, mean amplitude of Z-t curve tidal variation. Distribution was quantified by the coefficient of variation (CV) and the heterogeneity index (HI). Both CV and HI of the t_E and PHASE features were significantly increased in COPD compared with controls, and both related to spirometry and FOT resistance and reactance measurements. In contrast, distribution of the AMP feature showed no relationships with lung mechanics. These novel time-based EIT metrics of VH reflect complex lung mechanics in COPD and have the potential to allow real-time visualization of pulmonary physiology in spontaneously breathing subjects.NEW & NOTEWORTHY Pulmonary electrical impedance tomography (EIT) is a real-time imaging technique capable of monitoring ventilation with exquisite temporal resolution. We report novel, time-based EIT measurements that not only demonstrate ventilation heterogeneity in chronic obstructive pulmonary disease (COPD), but also reflect oscillatory lung mechanics. These EIT measurements are noninvasive, radiation-free, easy to obtain, and provide real-time visualization of the complex pathophysiology of COPD.

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.

Variability in EIT Images of Lung Ventilation as a Function of Electrode Planes and Body Positions

This study is aimed at investigating the variability in resistivity changes in the lung region as a function of air volume, electrode plane and body position. Six normal subjects (33.8 ± 4.7 years, range from 26 to 37 years) were studied using the Sheffield Electrical Impedance Tomography (EIT) portable system. Three transverse planes at the level of second intercostal space, the level of the xiphisternal joint, and midway between upper and lower locations were chosen for measurements. For each plane, sixteen electrodes were uniformly positioned around the thorax. Data were collected with the breath held at end expiration and after inspiring 0.5, 1.0, or 1.5 liters of air from end expiration, with the subject in both the supine and sitting position. The average resistivity change in five regions, two 8x8 pixel local regions in the right lung, entire right, entire left and total lung regions, were calculated. The results show the resistivity change averaged over electrode positions and subject positions was 7-9% per liter of air, with a slightly larger resistivity change of 10 % per liter air in the lower electrode plane. There was no significant difference (p>0.05) between supine and sitting. The two 8x8 regions show a larger inter individual variability (coefficient of variation, CV, is from 30% to 382%) compared to the entire left, entire right and total lung (CV is from 11% to 51%). The results for the global regions are more consistent. The large inter individual variability appears to be a problem for clinical applications of EIT, such as regional ventilation. The variability may be mitigated by choosing appropriate electrode plane, body position and region of interest for the analysis.

The EIT-based global inhomogeneity index is highly correlated with regional lung opening in patients with acute respiratory distress syndrome

BACKGROUND

The electrical impedance tomography (EIT)-based global inhomogeneity (GI) index was introduced to quantify tidal volume distribution within the lung. Up to now, the GI index was evaluated for plausibility but the analysis of how it is influenced by various physiological factors is still missing. The aim of our study was to evaluate the influence of proportion of open lung regions measured by EIT on the GI index.

METHODS

A constant low-flow inflation maneuver was performed in 18 acute respiratory distress syndrome (ARDS) patients (58 ± 14 years, mean age ± SD) and 8 lung-healthy patients (41 ± 12 years) under controlled mechanical ventilation. EIT raw data were acquired at 25 scans/s and reconstructed offline. Recruited lung regions were identified as those image pixels of the lung regions within the EIT scans where local impedance amplitudes exceeded 10% of the maximum amplitude during the maneuver. A series of GI indices was calculated during mechanical lung inflation, based on the differential images obtained between different time points. Respiratory system elastance (Ers) values were calculated at 10 lung volume levels during low-flow maneuver.

RESULTS

The GI index decreased during low-flow inflation, while the percentage of open lung regions increased. The values correlated highly in both ARDS (r2 = 0.88 ± 0.08, p < 0.01) and lung-healthy patients (r2 = 0.92 ± 0.05, p < 0.01). Ers and GI index were also significantly correlated in 16 out of 18 ARDS (r2 = 0.84 ± 0.13, p < 0.01) and in 6 out of 8 lung-healthy patients (r2 = 0.84 ± 0.07, p < 0.01). Significant differences were found in GI values between two groups (0.52 ± 0.21 for ARDS and 0.41 ± 0.04 for lung-healthy patients, p < 0.05) as well in Ers values (0.017 ± 0.008 cmH2O/ml for ARDS and 0.009 ± 0.001 cmH2O/ml for lung-healthy patients, p < 0.01).

CONCLUSIONS

We conclude that the GI index is a reliable measure of ventilation heterogeneity highly correlated with lung recruitability measured with EIT. The GI index may prove to be a useful EIT-based index to guide ventilation therapy.

Analysis on the influence of tissues/organs' movements in EIT images of lung ventilation using finite difference thorax models

Monitoring lung air and fluid volume is a promising EIT application. Respiratory activity causes both chest expansion and position changes of tissues/organs in the thorax, which contributes measurement error. In EIT application, the electrodes are placed around chest in a two-dimensional ring, and image is reconstructed on the assumption that the object is two-dimensional and the images are due to resistivity change. EIT reconstruction algorithms generally do not consider organ and tissue positional change. Finite difference models (FDM) were developed based on MR images with different air volumes and used to study the effect of tissues/organs' motion. Results show that the tissues/organs' motion influence the region around the heart, which introduce the artifacts in the middle anterior region. The tissues/organs' motion also increases the resistivity change in the lung regions, which cannot be neglected. We conclude that tissues/organs' motion can cause serious error in the interpretation of EFT ventilation images.

Neural network based approach for anomaly detection in the lungs region by electrical impedance tomography

In this paper, we have shown a simple procedure to detect anomalies in the lungs region by electrical impedance tomography. The main aim of the present study is to investigate the possibility of anomaly detection by using neural networks. Radial basis function neural networks are used as classifiers to classify the anomaly as belonging to the anterior or posterior region of the left lung or the right lung. The neural networks are trained and tested with the simulated data obtained by solving the mathematical model equation governing current flow through the simulated thoracic region. The equation solution and model simulation are done with FEMLAB. The effect of adding a higher number of neurons to the hidden layer can be clearly seen by the reduction in classification error. The study shows that there is interaction between the size (radius) and conductivity of anomalies and for some combination of these two factors the classification error of neural networks will be very small.

A parametric model of the relationship between EIT and total lung volume

Spirometry and electrical impedance tomography (EIT) data from 26 healthy subjects (14 males, 12 females) were used to develop a model linking contrast variations in EIT difference images to lung volume changes. Eight recordings, each 64 s long, were made for each subject in four postures (standing, sitting, reclining at 45 degrees, supine) and two breathing modes (quiet tidal and deep breathing). Age, gender and five anthropometric variables were recorded. The database was divided into four subsets. The first subset, data from 22 subjects (12 males, 10 females) recorded in deep breathing mode, was used to create the model. Validation was done with the other subsets: data recorded during quiet tidal breathing in the same 22 subjects, and data recorded in both breathing modes for the other four subjects. A quadratic equation in DeltaV(P) (lung volume changes recorded by the spirometer) provided a very good fit to total contrast changes in the EIT images. The model coefficients were found to depend on posture, gender, thoracic circumference and scapular skin fold. To validate the model, the quadratic equation was inverted to estimate lung volume changes from the EIT images. The estimated changes were then compared to the measured volume changes. Validations with each data subset yielded mean standard errors ranging from 9.3% to 12.4%. The proposed model is a first step in enabling inter individual comparisons of EIT images since: (1) it provides a framework for incorporating the effects of anthropometric variables, gender and posture, and (2) it references the images to a physical quantity (volume) verifiable by spirometry.

EIT images of ventilation: what contributes to the resistivity changes?

One promising application of electrical impedance tomography (EIT) is the monitoring of pulmonary ventilation and edema. Using three-dimensional (3D) finite difference human models as virtual phantoms, the factors that contribute to the observed lung resistivity changes in the EIT images were investigated. The results showed that the factors included not only tissue resistivity or vessel volume changes, but also chest expansion and tissue/organ movement. The chest expansion introduced artifacts in the center of the EIT images, ranging from -2% to 31% of the image magnitude. With the increase of simulated chest expansion, the percentage contribution of chest expansion relative to lung resistivity change in the EIT image remained relatively constant. The averaged resistivity changes in the lung regions caused by chest expansion ranged from 0.65% to 18.31%. Tissue/organ movement resulted in an increased resistivity in the lung region and in the center anterior region of EIT images. The increased resistivity with inspiration observed in the heart region was caused mainly by a drop in the heart position, which reduced the heart area at the electrode level and was replaced by the lung tissue with higher resistivity. This study indicates that for the analysis of EIT, data errors caused by chest expansion and tissue/organ movement need to be considered.

Evaluation of an EIT reconstruction algorithm using finite difference human thorax models as phantoms

A finite difference model of the human thorax with 113,400 control volumes (nodes) based on ECG gated MRI images was used to evaluate the Sheffield DAS-01P EIT system. Sixteen simulated electrode positions equally spaced around the thorax model at approximately the fourth intercostals space level were selected. Pairs of adjacent positions were excited sequentially by injecting current in a manner similar to that used by the Sheffield DAS-01P EIT system. The resulting voltages on the non-excited electrode positions were calculated and used to reconstruct the image using the Sheffield filtered back projection algorithm. By changing the resistivities of the lungs, the ventricles and the atria over a range of 1% to 40%, the resulting changes in the images were quantified by measuring the average resistivity change over a region defined automatically by two thresholds, 40% or 80% of the average of the first four pixels with the largest change. The results show that the changes observed in the images are consistently less than the changes in the model, but changed in a nearly linear manner as a function of resistivity in the model. For 40% resistivity changes in the model for right lung, right ventricle and right atrium, the observed resistivity changes in the region of interest (ROI, defined by the 80% threshold) of the images are 32% for the right lung, 11% for the right ventricle and 5.5% for the right atrium, which suggests strong volume dependence of EIT imaging. The effect of structural (size) change between end diastole and end systole was also studied, which showed large resistivity changes caused in the heart region of the constructed image. The study demonstrates that the Sheffield DAS-01P EIT reconstruction algorithm tracks the change occurring in the lungs most closely and with proper scaling may be used to observe physiological changes.

Propagation of measurement noise through backprojection reconstruction in electrical impedance tomography

A framework to analyze the propagation of measurement noise through backprojection reconstruction algorithms in electrical impedance tomography (EIT) is presented. Two measurement noise sources were considered: noise in the current drivers and in the voltage detectors. The influence of the acquisition system architecture (serial/semi-parallel) is also discussed. Three variants of backprojection reconstruction are studied: basic (unweighted), weighted and exponential backprojection. The results of error propagation theory have been compared with those obtained from simulated and experimental data. This comparison shows that the approach provides a good estimate of the reconstruction error variance. It is argued that the reconstruction error in EIT images obtained via backprojection can be approximately modeled as a spatially nonstationary Gaussian distribution. This methodology allows us to develop a spatial characterization of the reconstruction error in EIT images.

Preliminary static EIT images of the thorax in health and disease

The results of a preliminary clinical evaluation of a one-frequency electrical impedance tomography (EIT) system enabling static in vivo imaging are presented. The design of the measuring system and image reconstruction software are described. Thirty-one subjects were examined and divided into four clinical groups. The first group consisted of 22 patients with clinical diagnosis of lung cancer with tumour localization in one lung. The second group consisted of seven healthy subjects. A patient after a one-sided pneumectomy and another with one-sided emphysema diagnosis were also examined. Static EIT images of a healthy human chest and a chest with various abnormalities are given and discussed. The evaluated system distinguishably visualizes various states of lungs and thorax including lung cancer. The average static conductivity of an affected lung in the first clinical group statistically differs from the average conductivity of a healthy lung. In spite of low spatial resolution, according to preliminary results, the method can be sensitive to cancer and other lung diseases in screening investigations.

Electrical impedance tomography (EIT) in applications related to lung and ventilation: a review of experimental and clinical activities

This review article is a summary of the publications dealing with the pulmonary applications of electrical impedance tomography (EIT). Original papers on EIT lung imaging published over 15 years are analysed and several aspects of the performed EIT measurements summarized. Information on the type of the EIT device and electrodes used, the studied transverse thoracic planes, the data acquisition rate, the number of studied animals, normal subjects or patients, the kind of lung pathology, the performed ventilatory manoeuvres and other interventions, as well as the applied reference techniques, is given. The type of the generated pulmonary EIT images and the quantitative analysis of the EIT data are described. Finally, the major results achieved are presented, followed by an analysis of the perspectives of EIT in clinical applications. A comparative analysis of the EIT hardware and the quality of the evaluation tools was not performed.

Electrical impedance tomography in monitoring experimental lung injury

OBJECTIVE

To apply electrical impedance tomography (EIT) and the new evaluation approach (the functional EIT) in monitoring the development of artificial lung injury.

DESIGN

Acute experimental trial.

SETTING

Operating room for animal experimental studies at a university hospital.

SUBJECTS

Five pigs (41.3 +/- 4.1 kg, mean body weight +/- SD).

INTERVENTIONS

The animals were anaesthetised and mechanically ventilated. Sixteen electrodes were attached on the thoracic circumference and used for electrical current injection and surface voltage measurement. Oleic acid was applied sequentially (total dose 0.05 ml/kg body weight) into the left pulmonary artery to produce selective unilateral lung injury.

MEASUREMENTS AND RESULTS

The presence of lung injury was documented by significant changes of PaCO2 (40.1 mmHg vs control 37.1 mmHg), PaO2 (112.3 mmHg vs 187.5 mmHg), pH (7.35 vs 7.42), mean pulmonary arterial pressure (29.2 mmHg vs 20.8 mmHg) and chest radiography. EIT detected 1) a regional decrease in mean impedance variation over the affected left lung (-41.4% vs control) and an increase over the intact right lung (+ 20.4% vs control) indicating reduced ventilation of the affected, and a compensatory augmented ventilation of the unaffected lung and 2) a pronounced fall in local baseline electrical impedance over the injured lung (-20.6% vs control) with a moderate fall over the intact lung (-10.0% vs control) indicating the development of lung oedema in the injured lung with a probable atelectasis formation in the contralateral one.

CONCLUSION

The development of the local impairment of pulmonary ventilation and the formation of lung oedema could be followed by EIT in an experimental model of lung injury. This technique may become a useful tool for monitoring local pulmonary ventilation in intensive care patients suffering from pulmonary disorders associated with regionally reduced ventilation, fluid accumulation and/or cell membrane changes.

Influences of lung parenchyma density and thoracic fluid on ventilatory EIT measurements

Ventilatory impedance changes can be measured by electrical impedance tomography (EIT). Several studies have pointed out that the ventilatory-induced impedance change measured over the lungs shows a linear relationship with tidal volume. However, EIT measures the ventilatory impedance changes relative to a reference. Therefore, changes in the reference due to lung parenchyma destruction (increase of thoracic impedance) or lung water (decrease of thoracic impedance) might influence ventilatory EIT measurements. A study was designed to evaluate the influence of the density of lung parenchyma and the thoracic fluid content on ventilatory EIT measurements. Eleven emphysema patients with a variable degree of lung parenchyma destruction, nine haemodialysis patients with general fluid overload and ten healthy subjects were measured. The impedance changes were measured with the subject in the supine position breathing a constant tidal volume of 1 litre starting at the maximum end-expiratory level. In the emphysema group a significantly lower impedance change between ins- and expiration was found in comparison with the healthy subjects (11.6 +/- 6.4 AU l-1 versus 18.6 +/- 4.2 AU l-1, p < 0.05), whereas the haemodialysis group showed a significantly larger impedance change between ins- and expiration before haemodialysis (30.5 +/- 13.1 AU l-1, p < 0.05). A significant decrease in ventilation-induced impedance change during dialysis was found (30.5 +/- 13.1 AU l-1 versus 21.4 +/- 8.6 AU l-1, p < 0.01). Furthermore, a significant correlation between lung function parameters, which indicate the severity of lung parenchyma destruction, and the measured impedance change was found in emphysema patients. From these results it can be concluded that the density of lung parenchyma and the thoracic fluid content have a serious impact on the ventilation-induced impedance change.

Electrical impedance tomography

Electrical impedance tomography (EIT) is a technique which allows cross-sectional images related to the local electrical impedance within an object to be reconstructed from sets of measurements made on its surface. The main drive behind the development of EIT has been its possible application in medical imaging, as biological tissues are known to exhibit a wide range of electrical impedance and many physiological events are accompanied by electrical impedance changes. This article reviews the technical aspects of EIT as a medical imaging modality, and considers the range of applications over which it might be employed. Existing technical limitations and future developments are discussed. It is concluded that the future of EIT as a clinical diagnostic tool is likely to lie in the area of functional monitoring, where the capability of performing image-guided localized electrical impedance measurements with high acquisition speed, good sensitivity and no hazard can be exploited.

Gravity-dependent phenomena in lung ventilation determined by functional EIT

Gravity exerts an effect on the distribution of intrapulmonary ventilation. A study on the detection of gravity-dependent inhomogeneity of ventilation by a functional EIT technique is presented. The study was performed on five human subjects, whose ventilation distribution was modified by changes in body position. The subjects were studied during spontaneous tidal breathing. The qualitative and quantitative analysis of the functional EIT images revealed that the ventilation is higher in the dependent lung regions when compared with the non-dependent ones. These EIT findings correspond to current knowledge of the physiological behaviour of the lungs as derived from the radioactive-gas methods and raise the possibility of applying the less complicated functional EIT in future studies on ventilation distribution in the lungs. This may be of major interest in the monitoring of intensive care patients with severe pulmonary disorders.