Functional Electrical Impedance Tomography Research Articles

Fraction of reverse impedance change (FRIC): a quantitative electrical impedance tomography measure of intrapulmonary pendelluft

Objective. Pendelluft is the movement of air between lung regions, and electrical impedance tomography (EIT) has shown an ability to detect and monitor it.Approach.In this note, we propose a functional EIT measure which quantifies the reverse airflow seen in pendelluft: theFraction of Reverse Impedance Change(FRIC).MainResults. FRIC measures the fraction of reverse flow in each pixel waveform (as an image) or globally (as a single parameter).Significance. Such a measure is designed to be a more specific measure than previous approaches, to enable comparative studies of the pendelluft, and to help clarify the effect of ventilation strategies.

Electrical impedance tomography in anaesthetised chickens (Gallus domesticus).

The applicability of electrical impedance tomography (EIT) in birds is unknown. This study aimed to evaluate the use of EIT in anaesthetised chickens in four recumbency positions. Four adult Hyline chickens were anaesthetised with isoflurane in oxygen, and intubated endotracheally for computed tomography (CT). A rubber belt was placed around the coelom caudal to the shoulder joint. A chicken-specific finite element (FE) model, which is essential to generate anatomically accurate functional EIT images for analysis, was constructed based on the CT images obtained at the belt level. Ten additional chickens were anaesthetised with the same protocol. An EIT electrode belt was placed at the same location. The chickens were breathing spontaneously and positioned in dorsal, ventral, right and left lateral recumbency in a randomised order. For each recumbency, raw EIT data were collected over 2 min after 13 min of stabilisation. The data were reconstructed into functional EIT images. EIT variables including tidal impedance variation (TIV), centre of ventilation right to left (CoVRL) and ventral to dorsal (CoVVD), right to left (RL) ratio, impedance change (ΔZ) and eight regional impedance changes including the dorsal, central-dorsal, central-ventral and ventral regions of the right and left regions were analysed. Four breathing patterns (BrP) were observed and categorised based on the expiratory curve. A linear mixed model was used to compare EIT variables between recumbencies. Fisher's exact test was used to compare the frequencies of breathing patterns for each recumbency. The ΔZ observed was synchronous to ventilation, and represented tidal volume of the cranial air sacs as confirmed by CT. Significant differences were found in CoVVD and regional impedance changes between dorsal and ventral recumbencies (P < 0.05), and in CoVRL, RL ratio and regional impedance changes between right and left recumbencies (P < 0.05), which suggested a tendency for the distribution of ventilation to shift towards non-dependent air sacs. No differences were found for TIV and respiratory rate between recumbencies. Recumbency had a significant effect on the frequencies of each of the four BrPs (P = 0.001). EIT can monitor the magnitude and distribution of ventilation of the cranial air sacs in different recumbencies in anaesthetised chickens.

Structural priors represented by discrete cosine transform improve EIT functional imaging.

Structural prior information can improve electrical impedance tomography (EIT) reconstruction. In this contribution, we introduce a discrete cosine transformation-based (DCT-based) EIT reconstruction algorithm to demonstrate a way to incorporate the structural prior with the EIT reconstruction process. Structural prior information is obtained from other available imaging methods, e.g., thorax-CT. The DCT-based approach creates a functional EIT image of regional lung ventilation while preserving the introduced structural information. This leads to an easier interpretation in clinical settings while maintaining the advantages of EIT in terms of bedside monitoring during mechanical ventilation. Structural priors introduced in the DCT-based approach are of two categories in terms of different levels of information included: a contour prior only differentiates lung and non-lung region, while a detail prior includes information, such as atelectasis, within the lung area. To demonstrate the increased interpretability of the EIT image through structural prior in the DCT-based approach, the DCT-based reconstructions were compared with reconstructions from a widely applied one-step Gauss-Newton solver with background prior and from the advanced GREIT algorithm. The comparisons were conducted both on simulation data and retrospective patient data. In the simulation, we used two sets of forward models to simulate different lung conditions. A contour prior and a detail prior were derived from simulation ground truth. With these two structural priors, the reconstructions from the DCT-based approach were compared with the reconstructions from both the one-step Gauss-Newton solver and the GREIT. The difference between the reconstructions and the simulation ground truth is calculated by the ℓ2-norm image difference. In retrospective patient data analysis, datasets from six lung disease patients were included. For each patient, a detail prior was derived from the patient's CT, respectively. The detail prior was used for the reconstructions using the DCT-based approach, which was compared with the reconstructions from the GREIT. The reconstructions from the DCT-based approach are more comprehensive and interpretable in terms of preserving the structure specified by the priors, both in simulation and retrospective patient data analysis. In simulation analysis, the ℓ2-norm image difference of the DCT-based approach with a contour prior decreased on average by 34% from GREIT and 49% from the Gauss-Newton solver with background prior; for reconstructions of the DCT-based approach with detail prior, on average the ℓ2-norm image difference is 53% less than GREIT and 63% less than the reconstruction with background prior. In retrospective patient data analysis, the reconstructions from both the DCT-based approach and GREIT can indicate the current patient status, but the DCT-based approach yields more interpretable results. However, it is worth noting that the preserved structure in the DCT-based approach is derived from another imaging method, not from the EIT measurement. If the structural prior is outdated or wrong, the result might be misleadingly interpreted, which induces false clinical conclusions. Further research in terms of evaluating the validity of the structural prior and detecting the outdated prior is necessary.

Wearable pulmonary monitoring system with integrated functional lung imaging and chest sound recording: a clinical investigation in healthy subjects

Objective. Current wearable respiratory monitoring devices provide a basic assessment of the breathing pattern of the examined subjects. More complex monitoring is needed for healthcare applications in patients with lung diseases. A multi-sensor vest allowing continuous lung imaging by electrical impedance tomography (EIT) and auscultation at six chest locations was developed for such advanced application. The aims of our study were to determine the vest’s capacity to record the intended bio-signals, its safety and the comfort of wearing in a first clinical investigation in healthy adult subjects. Approach. Twenty subjects (age range: 23–65 years) were studied while wearing the vests during a 14-step study protocol comprising phases of quiet and deep breathing, slow and forced full expiration manoeuvres, coughing, breath-holding in seated and three horizontal postures. EIT, chest sound and accelerometer signals were streamed to a tablet using a dedicated application and uploaded to a back-end server. The subjects filled in a questionnaire on the vest properties using a Likert scale. Main results. All subjects completed the full protocol. Good to excellent EIT waveforms and functional EIT images were obtained in 89% of the subjects. Breathing pattern and posture dependent changes in ventilation distribution were properly detected by EIT. Chest sounds were recorded in all subjects. Detection of audible heart sounds was feasible in 44%–67% of the subjects, depending on the sensor location. Accelerometry correctly identified the posture in all subjects. The vests were safe and their properties positively rated, thermal and tactile properties achieved the highest scores. Significance. The functionality and safety of the studied wearable multi-sensor vest and the high level of its acceptance by the study participants were confirmed. Availability of personalized vests might further advance its performance by improving the sensor-skin contact.

Preliminary evaluation of the effects of a 1:1 inspiratory-to-expiratory ratio in anesthetized and ventilated horses

ObjectiveTo describe some cardiorespiratory effects of an inspiratory-to-expiratory (IE) ratio of 1:1 compared with 1:3 in ventilated horses in dorsal recumbency. Study designRandomized crossover experimental study. AnimalsA total of eight anesthetized horses, with 444 (330–485) kg body weight [median (range)]. MethodsHorses were ventilated in dorsal recumbency with a tidal volume of 15 mL kg–1 and a respiratory rate of 8 breaths minute–1, and IE ratios of 1:1 (IE1:1) and 1:3 (IE1:3) in random order, each for 25 minutes after applying a recruitment maneuver. Spirometry, arterial blood gases and dobutamine requirements were recorded in all horses during each treatment. Electrical impedance tomography (EIT) data were recorded in four horses and used to generate functional EIT variables including regional ventilation delay index (RVD), a measure of speed of lung inflation, and end-expiratory lung impedance (EELI), an indicator of functional residual capacity (FRC). Results were assessed with linear and generalized linear mixed models. ResultsCompared with treatment IE1:3, horses ventilated with treatment IE1:1 had higher mean airway pressures and respiratory system compliance (p < 0.014), while peak, end-inspiratory and driving airway pressures were lower (p < 0.001). No differences in arterial oxygenation or dobutamine requirements were observed. PaCO2 was lower in treatment IE1:1 (p = 0.039). Treatment IE1:1 resulted in lower RVD (p < 0.002) and higher EELI (p = 0.023) than treatment IE1:3. Conclusions and clinical relevanceThese results suggest that IE1:1 improved respiratory system mechanics and alveolar ventilation compared with IE1:3, whereas oxygenation and dobutamine requirements were unchanged, although differences were small. In the four horses where EIT was evaluated, IE1:1 led to a faster inflation rate of the lung, possibly the result of increased FRC. The clinical relevance of these findings needs to be further investigated.

Evaluation of Pulmonary Structure and Function in Patients with Cystic Fibrosis from Electrical Impedance Tomography Data.

Electrical impedance tomography (EIT) is a medical imaging technique in which low frequency, low amplitude electromagnetic fields applied through electrodes on the skin are used to compute the conductivity and/or permittivity inside the body and form functional images from the reconstructed values. This work describes methods of computing EIT-derived surrogate measures of pulmonary function and identifying regions of air trapping and consolidation from functional EIT images. These methods were developed for pediatric patients with cystic fibrosis, for whom a real-time non-ionizing imaging modality can be of great benefit for monitoring disease progression or a pulmonary exacerbation.

Lung regions identified with CT improve the value of global inhomogeneity index measured with electrical impedance tomography.

The global inhomogeneity (GI) index is a functional electrical impedance tomography (EIT) parameter which is used clinically to assess ventilation distribution. However, GI may underestimate the actual heterogeneity when the size of lung regions is underestimated. We propose a novel method to use anatomical information to correct the GI index calculation. EIT measurements were performed at the level of the fifth intercostal space in six patients with acute respiratory distress syndrome. The thorax and lungs were segmented automatically from serial individual CT scans. The anatomically derived lung regions were calculated in EIT images from simulating a homogeneous ventilation distribution in a finite element model. The conventional approach (GImeas,func ), analyzes images in functionally-defined lung regions, while our proposed measure (GImeas,anat ) is based on analysis in anatomically-defined regions. We additionally define a simulated comparison (GIsim,anat ) to determine the lower limit of the GI measure for a homogenous distribution of ventilation. As expected, the conventional GImeas,func [0.382 (0.088), median (interquartile range)] were significantly lower than the proposed GImeas,anat [0.823 (0.152), P<0.05], and were much closer to the lower limit GIsim,anat [0.343 (0.039)]. Both GImeas,anat and GImeas,func were strongly correlated with arterial oxygen partial pressure to fractional inspired oxygen ratio (R=-0.88, P<0.05), whereas GIsim,anat (R=0.23) was not. GImeas,anat had a linear-regression slope 3.2 times that of GImeas,func suggesting a higher sensitivity to the changes in lung condition. The proposed GImeas,anat (or shortened as GIanat ) is an improved measure of ventilation inhomogeneity over GI, and better reflects portion of non-ventilated regions due to alveolar collapse or overdistension.

Regional pulmonary effects of bronchoalveolar lavage procedure determined by electrical impedance tomography

BackgroundThe provision of guidance in ventilator therapy by continuous monitoring of regional lung ventilation, aeration and respiratory system mechanics is the main clinical benefit of electrical impedance tomography (EIT). A new application was recently described in critically ill patients undergoing diagnostic bronchoalveolar lavage (BAL) with the intention of using EIT to identify the region where sampling was performed. Increased electrical bioimpedance was reported after fluid instillation. To verify the accuracy of these findings, contradicting the current EIT knowledge, we have systematically analysed chest EIT data acquired under controlled experimental conditions in animals undergoing a large number of BAL procedures.MethodsOne hundred thirteen BAL procedures were performed in 13 newborn piglets positioned both supine and prone. EIT data was obtained at 13 images before, during and after each BAL. The data was analysed at three time points: (1) after disconnection from the ventilator before the fluid instillation and by the ends of fluid (2) instillation and (3) recovery by suction and compared with the baseline measurements before the procedure. Functional EIT images were generated, and changes in pixel electrical bioimpedance were calculated relative to baseline. The data was examined in the whole image and in three (ventral, middle, dorsal) regions-of-interest per lung.ResultsCompared with the baseline phase, chest electrical bioimpedance fell after the disconnection from the ventilator in all animals in both postures during all procedures. The fluid instillation further decreased electrical bioimpedance. During fluid recovery, electrical bioimpedance increased, but not to baseline values. All effects were highly significant (p < 0.001). The fractional changes in individual regions-of-interest were posture-dependent. The regional fall in electrical bioimpedance was smaller in the ventral and larger in the dorsal regions after the fluid instillation than after the initial disconnection to ambient pressure in supine animals (p < 0.001) whereas these changes were of comparable amplitude in prone position.ConclusionsThe results of this study show a regionally dissimilar initial fall in electrical bioimpedance caused by non-uniform aeration loss at the beginning of the BAL procedure. They also confirm a further pronounced fall in bioimpedance during fluid instillation, incomplete recovery after suction and a posture-dependent distribution pattern of these effects.

Monitoring changes in distribution of pulmonary ventilation by functional electrical impedance tomography in anaesthetized ponies

ObjectiveTo assess changes in the distribution in pulmonary ventilation in anaesthetized ponies using electrical impedance tomography (EIT). Study designProspective experimental study. AnimalsA group of eight adult pony geldings. MethodsGeneral anaesthesia was induced and maintained using isoflurane in oxygen with volume-controlled ventilation [tidal volume (VT) (10 mL kg−1), respiratory rate (8–12 breaths minute−1)] to maintain end-tidal carbon dioxide between 35 and 40 mmHg (4.66–5.32 kPa). Regional distribution of ventilation was assessed with EIT (plane of fifth to sixth intercostal space) at 30, 60 and 90 minutes after intubation. The resulting functional images were divided into four regions of interest (ROI, A–D) to determine: 1) the ratio of VT distribution between dependent to nondependent lung (D/ND) and 2) changes in distribution within the lungs in the latero-lateral direction. The centre of ventilation (COV) was calculated. The dimension of the chest in the latero-lateral (W) and dorso-ventral (H) direction was measured at the height of the withers and the W/H ratio was calculated. The influence of time on the D/ND ratio, COV and ROI A–D were analysed with anova followed by post hoc Bonferroni tests. Pearson correlation coefficient and the coefficient of determination (r2) were calculated to evaluate the relationship between the W/H ratio and D/ND ratio, and COV. Statistical significance was set at p < 0.05. ResultsThe median D/ND ratio (T30–T90) was 0.67 (0.40–1.01) and for COV 31.15% (11.20–45.20), confirming the expected uneven distribution of ventilation. The D/ND ratio had a moderate negative correlation with the W/H ratio (r = −0.68, r2 = 0.46, p < 0.001), while the COV did not correlate with the W/H ratio (r = −0.04). Conclusions and clinical relevanceUneven ventilation in mechanically ventilated right laterally recumbent anaesthetized ponies occurs within 30 minutes and changed little over the following 60 minutes.

Impact of four different recumbencies on the distribution of ventilation in conscious or anaesthetized spontaneously breathing beagle dogs: An electrical impedance tomography study.

The aim was to examine the effects of recumbency and anaesthesia on distribution of ventilation in beagle dogs using Electrical Impedance Tomography (EIT). Nine healthy beagle dogs, aging 3.7±1.7 (mean±SD) years and weighing 16.3±1.6 kg, received a series of treatments in a fixed order on a single occasion. Conscious dogs were positioned in right lateral recumbency (RLR) and equipped with 32 EIT electrodes around the thorax. Following five minutes of equilibration, two minutes of EIT recordings were made in each recumbency in the following order: RLR, dorsal (DR), left (LLR) and sternal (SR). The dogs were then positioned in RLR, premedicated (medetomidine 0.01, midazolam 0.1, butorphanol 0.1 mg kg-1 iv) and pre-oxygenated. Fifteen minutes later anaesthesia was induced with 1 mg kg-1 propofol iv and maintained with propofol infusion (0.1–0.2 mg kg-1 minute-1 iv). After induction, the animals were intubated and allowed to breathe spontaneously (FIO2 = 1). Recordings of EIT were performed again in four recumbencies similarly to conscious state. Centre of ventilation (COV) and global inhomogeneity (GI) index were calculated from the functional EIT images. Repeated-measures ANOVA and Bonferroni tests were used for statistical analysis (p < 0.05). None of the variables changed in the conscious state. During anaesthesia left-to-right COV increased from 46.8±2.8% in DR to 49.8±2.9% in SR indicating a right shift, and ventral-to-dorsal COV increased from 49.8±1.7% in DR to 51.8±1.1% in LLR indicating a dorsal shift in distribution of ventilation. Recumbency affected distribution of ventilation in anaesthetized but not in conscious dogs. This can be related to loss of respiratory muscle tone (e.g. diaphragm) and changes in thoracic shape. Changing position of thoraco-abdominal organs under the EIT belt should be considered as alternative explanation of these findings.

Assessment of distribution of ventilation and regional lung compliance by electrical impedance tomography in anaesthetized horses undergoing alveolar recruitment manoeuvres

ObjectiveTo examine changes in the distribution of ventilation and regional lung compliances in anaesthetized horses during the alveolar recruitment manoeuvre (ARM). Study designExperimental study in which a series of treatments were administered in a fixed order on one occasion. AnimalsFive adult Warmblood horses. MethodsAnimals were anaesthetized (xylazine, midazolam–ketamine, isoflurane), placed in dorsal recumbency and ventilated with 100% oxygen using peak inspiratory pressure (PIP) and positive end-expiratory pressure (PEEP) of 20 cmH2O and 0 cmH2O, respectively. Thoracic electrical impedance tomography (EIT), spirometry and routine anaesthesia monitoring were performed. At 90 minutes after induction of anaesthesia, PIP and PEEP were increased in steps of 5 cmH2O to 50 cmH2O and 30 cmH2O, respectively, and then decreased to baseline values. Each step lasted 10 minutes. Data were recorded and functional EIT images were created using three breaths at the end of each step. Arterial blood samples were analysed. Values for left-to-right and sternal-to-dorsal centre of ventilation (COV), lung compliances and Bohr dead space were calculated. ResultsDistribution of ventilation drifted leftward and dorsally during recruitment. Mean±standard deviation (SD) values at baseline and highest airway pressures, respectively, were 49.9±0.7% and 48.0±0.6% for left-to-right COV (p=0.009), and 46.3±2.0% and 54.6±2.0% for sternal-to-dorsal COV (p=0.0001). Compliance of dependent lung regions and PaO2 increased, whereas compliance of non-dependent lung regions decreased during ARM and then returned to baseline (p<0.001). Bohr dead space decreased after ARM (p=0.007). Interestingly, PaO2 correlated to the compliance of the dependent lung (r2=0.71, p<0.001). Conclusions and clinical relevanceThe proportion of tidal volume distributed to dependent and left lung regions increased during ARM, presumably as a result of opening atelectasis. Monitoring compliance of the dependent lung with EIT may substitute PaO2 measurements during ARM to identify an optimal PEEP.

Electrical impedance tomography in 3D using two electrode planes: characterization and evaluation

Electrical impedance tomography (EIT) uses body surface electrical stimulation and measurements to create conductivity images; it shows promise as a non-invasive technology to monitor the distribution of lung ventilation. Most applications of EIT have placed electrodes in a 2D ring around the thorax, and thus produced 2D cross-sectional images. These images are unable to distinguish out-of-plane contributions, or to image volumetric effects. Volumetric EIT can be calculated using multiple electrode planes and a 3D reconstruction algorithm. However, while 3D reconstruction algorithms are available, little has been done to understand the performance of 3D EIT in terms of the measurement configurations available. The goal of this paper is to characterize the phantom and in vivo performance of 3D EIT with two electrode planes. First, phantom measurements are used to measure the reconstruction characteristics of seven stimulation and measurement configurations. Measurements were then performed on eight healthy volunteers as a function of body posture, postures, and with various electrode configurations. Phantom results indicate that 3D EIT using two rings of electrodes provides reasonable resolution in the electrode plane but low vertical resolution. For volunteers, functional EIT images are created from inhalation curve features to analyze the effect of posture (standing, sitting, supine and decline) on regional lung behaviour. An ability to detect vertical changes in lung volume distribution was shown for two electrode configurations. Based on tank and volunteer results, we recommend the use of the ‘square’ stimulation and measurement pattern for two electrode plane EIT.

Assessment of distribution of ventilation by electrical impedance tomography in standing horses

The aim was to evaluate the feasibility of using electrical impedance tomography (EIT) in horses. Thoracic EIT was used in nine horses. Thoracic and abdominal circumference changes were also measured with respiratory ultrasound plethysmography (RUP). Data were recorded during baseline, rebreathing of CO2 and sedation. Three breaths were selected for analysis from each recording. During baseline breathing, horses regularly took single large breaths (sighs), which were also analysed. Functional EIT images were created using standard deviations (SD) of pixel signals and correlation coefficients (R) of each pixel signal with a reference respiratory signal. Left-to-right ratio, centre-of-ventilation and global-inhomogeneity-index were calculated. RM-ANOVA and Bonferroni tests were used (P < 0.05). Distribution of ventilation shifted towards right during sighs and towards dependent regions during sighs, rebreathing and sedation. Global-inhomogeneity-index did not change for SD but increased for R images during sedation. The sum of SDs for the respiratory EIT signals correlated well with thoracic (r2 = 0.78) and abdominal (r2 = 0.82) tidal circumferential changes. Inverse respiratory signals were identified on the images at sternal location and based on reviewing CT images, seemed to correspond to location of gas filled intestines. Application of EIT in standing non-sedated horses is feasible. EIT images may provide physiologically useful information even in situations, such as sighs, that cannot easily be tested by other methods.

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

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). 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. 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. 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.

Electrical impedance tomography during major open upper abdominal surgery: a pilot-study.

BackgroundElectrical impedance tomography (EIT) of the lungs facilitates visualization of ventilation distribution during mechanical ventilation. Its intraoperative use could provide the basis for individual optimization of ventilator settings, especially in patients at risk for ventilation-perfusion mismatch and impaired gas exchange, such as patients undergoing major open upper abdominal surgery. EIT throughout major open upper abdominal surgery could encounter difficulties in belt positioning and signal quality. Thus, we conducted a pilot-study and tested whether EIT is feasible in patients undergoing major open upper abdominal surgery.MethodsFollowing institutional review board’s approval and written informed consent, we included patients scheduled for major open upper abdominal surgery of at least 3 hours duration. EIT measurements were conducted prior to intubation, at the time of skin incision, then hourly during surgery until shortly prior to extubation and after extubation. Number of successful intraoperative EIT measurements and reasons for failures were documented. From the valid measurements, a functional EIT image of changes in tidal impedance was generated for every time point. Regions of interest were defined as horizontal halves of the picture. Monitoring of ventilation distribution was assessed using the center of ventilation index, and also using the total and dorsal ventilated lung area. All parameter values prior to and post intubation as well as extubation were compared. A p < 0.05 was considered statistically significant.ResultsA total of 120 intraoperative EIT measurements during major abdominal surgery lasting 4-13 hours were planned in 14 patients. The electrode belt was attached between the 2nd and 4th intercostal space. Consecutive valid measurements could be acquired in 13 patients (93%). 111 intraoperative measurements could be retrieved as planned (93%). Main obstacle was the contact of skin electrodes. Despite the high belt position, distribution of tidal volume showed a significant shift of ventilation towards ventral lung regions after intubation. This was reversed after weaning from mechanical ventilation.ConclusionsDespite a high belt position, monitoring of ventilation distribution is feasible in patients undergoing major open upper abdominal surgery lasting from 4 to 13 hours. Therefore, further interventional trials in order to optimize ventilatory management should be initiated.

SU‐E‐I‐77: A Phantom to Assess EIT/CT Imaging System

Purpose: Fusion of Electrical Impedance Tomography (EIT) and Computed Tomography (CT) can potentially provide functional EIT information along with high resolution anatomical information from CT. In this study, we have developed a phantom for evaluating an EIT/CT imaging system. Methods: An electrically conductive phantom was prepared by pouring a molten mixture of gelatin solution and glycerol into a cylindrical container. The mixture was then allowed to solidify. A cylindrical cavity was then created at the center of the medium. Sixteen electrodes were attached equi‐distantly around the phantom. A fixed current was injected through a pair of electrodes, and the induced boundary voltages across all pairs of neighboring electrodes were measured sequentially. The entire voltage measurement process was repeated for all combinations of electrodes through which the current was injected. EIT images were obtained from the boundary voltage data using EIDORS software. The CT data was acquired using a Phillips CT Scanner. EIT and CT data were also collected when the central cavity of the medium was filled with de‐ionized water and 0.9% saline separately. The two images were combined using the boundary and statistical information, extracted using the morphological image processing modules of Matlab. Image quality was evaluated by measuring the entropy of the CT, EIT and the fused images. Results: Phantom regions of saline and distilled water show distinct contrast as compared to CT which fails to differentiate the regions. The location of the contrast objects was found to match their expected locations with respect to CT. Conclusion: We have developed a phantom to assess EIT/CT image quality. The phantom can be extended to include measures such as signal to noise, contrast to noise, relative information content and geometric accuracy of an EIT/CT imaging system.

Functional electrical impedance tomography by evoked response (fEITER): Sub-second changes in brain function during induction of anaesthesia with propofol

Pollard, B. J.; Pomfrett, C. J.; Bryan, A.; Quraishi, T.; Davidson, J. L.; McCann, H. Author Information

A method for removing artefacts from continuous EEG recordings during functional electrical impedance tomography for the detection of epileptic seizures

Electrical impedance tomography (EIT) is a portable, non-invasive medical imaging method, which could be employed to image the seizure onset in subjects undergoing assessment prior to epilepsy surgery. Each image is obtained from impedance measurements conducted with imperceptible current at tens of kHz. For concurrent imaging with video electroencephalogram (EEG), the EIT introduces a substantial artefact into the EEG due to current switching at frequencies in the EEG band. We present here a method for its removal, so that EIT and the EEG could be acquired simultaneously. A low-pass analogue filter for EEG channels (−6 dB at 48 Hz) and a high-pass filter (−3 dB at 72 Hz) for EIT channels reduced the artefact from 2–3 mV to 50–300 µV, but still left a periodic artefact at about 3 Hz. This was reduced to less than 10 µV with a software filter, which subtracted an artefact template from the EEG raw traces. The EEG was made clinically acceptable at four times its acquisition speed. This method could enable EIT to become a technique for imaging on telemetry units alongside EEG, without interfering with routine EEG reporting.

FEITER – a new EIT instrument for functional brain imaging

We report on human tests of the new EIT-based system fEITER (functional Electrical Impedance Tomography of Evoked Responses), targeted principally at functional brain imaging. It is designed and built to medical standard BS EN 60601-1:2006 and clinical trials have been approved by the MHRA in the UK. fEITER integrates an EIT sub-system with an evoked response sub-system capable of providing visual, auditory or other stimuli, and the timing of each stimulus is recorded within the EIT data to a resolution of 500 microseconds. The EIT sub-system operates at 100 frames per second using 20 polar/near-polar current patterns distributed among 32 scalp electrodes that are arranged in a 3-dimensional array on the subject. Presently, current injection is fixed in firmware at 1 mA pk-pk and 10 kHz. Performance testing on inanimate subjects has shown voltage measurement SNR better than 75 dB, at 100 frames per second. We describe the fEITER system and give example topographic results for a human subject under no-stimulus (i.e. reference) conditions and on application of auditory stimuli. The system's excellent noise properties and temporal resolution show clearly the influence of basic physiological phenomena on the EIT voltages. In response to stimulus presentation, the voltage data contain fast components (~100 ms) and components that persist for many seconds.

A lung area estimation method for analysis of ventilation inhomogeneity based on electrical impedance tomography

To evaluate a novel method for lung area estimation (LAE method) in electrical impedance tomography (EIT) images as a prerequisite of quantitative analysis of ventilation inhomogeneity. The LAE method mirrors the lung regions in the functional EIT (fEIT) image and subsequently subtracts the cardiac related areas. In this preliminary study, 51 mechanically ventilated patients were investigated, including 39~patients scheduled for thoracic surgery (test group); 10 patients scheduled for orthopedic surgery without pulmonary disease (control group) and 2 ICU patients undergoing chest computed tomography (CT) examination. EIT data was recorded in all groups. The results of the LAE method were compared to those obtained with the fEIT method and to CT images. The lung area size determined with fEIT in control group is S(C,fEIT) = 361 +/- 35 (mean +/- SD) and in test group S(T,fEIT) = 299 +/- 61 (p< 0.01). The sizes estimated with the LAE method in control group S(C,LAE) = 353 +/- 27 and in test group S(T,LAE) = 353 +/- 61 (p=0.41). The result demonstrates that the novel LAE method improves the identification of lung region in EIT images, from which the analysis of ventilation distribution will benefit. The preliminary comparison with CT images exemplary indicates a closer match of the lung area shapes after the LAE than after the fEIT-based analysis. The LAE method is a robust lung area determination method, suitable for patients with healthy or seriously injured lungs.