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Determination of regional lung function in cystic fibrosis using electrical impedance tomography

  • Sabine Krueger-Ziolek EMAIL logo , Benjamin Schullcke , Zhanqi Zhao , Bo Gong and Knut Moeller
Published/Copyright: September 30, 2016
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Current Directions in Biomedical Engineering
From the journal Current Directions in Biomedical Engineering Volume 2 Issue 1

Abstract

Electrical impedance tomography (EIT) can be used to monitor regional lung ventilation. Due to its relatively high temporal resolution, EIT has already been applied during lung function tests in spontaneously breathing subjects with obstructive lung diseases like chronic obstructive pulmonary disease (COPD) or cystic fibrosis (CF). In our study, ratios of the maximal volume exhaled in 1 s during forced expiration and forced vital capacity (FEV1/FVC) were calculated in predefined lung regions for five CF patients and five lung healthy subjects. The degree of FEV1/FVC homogeneity was assessed by using a slightly modified version of the global inhomogeneity index (GIFEV1/FVC). CF patients showed a higher degree of inhomogeneity in pixel FEV1/FVC than lung healthy subjects. Since EIT is able to deliver regional information to assess airway obstruction in CF patients, it might represent a promising supplement to existing methods like spirometry providing global lung parameters.

Keywords: cystic fibrosis; electrical impedance tomography; obstructive lung disease; regional lung function

1 Introduction

Electrical impedance tomography (EIT) is a non-invasive, radiation-free imaging technique which can be applied to visualize regional lung dynamics. Variations in blood and gas volumes due to breathing result in impedance changes of the lung tissue. Changes in electrical potentials at the skin during breathing, measured by surface electrodes attached at the human thorax, are used to reconstruct a 2-dimesional image of impedance distribution within the thorax. Up to now, EIT is mainly used at the bedside e.g. to trace ventilation distribution during mechanical ventilation [1], [2] or to optimize ventilator settings [3], [4].

However, it was already demonstrated that EIT is also able to measure volume changes during lung function testing in spontaneously breathing COPD (chronic obstructive pulmonary disease) and CF (cystic fibrosis) patients due to its relatively high temporal resolution [5], [6]. In 2013, Zhao et al. showed that regional airway obstructions determined in CF patients by EIT correlated with obstructions identified by CT [7]. Since EIT works without radiation and provides regional information of the lung, it might depict a promising supplementary tool to well-established methods like CT or spirometry for the diagnosis of obstructive lung diseases.

A typical parameter to evaluate airway obstruction is the ratio of the maximal volume exhaled in 1 s during a forced expiration and the forced vital capacity (FEV1/FVC) determined during lung function testing. In the present study, we investigated ratios of regional impedance changes corresponding to FEV1/FVC in CF patients and lung healthy volunteers using EIT. Identifying differences in impedance ratios of FEV1 and FVC between CF patients and lung healthy subjects might improve the diagnosis and assessment of obstructive lung diseases.

2 Methods

2.1 Data acquisition

EIT and spirometry measurements were carried out simultaneously on five CF patients [two male and three female, age 34.0 ± 5.5 years (mean ± SD)] and five lung healthy subjects [five male, age 24.2 ± 2.7 years (mean ± SD)], respectively. EIT measurements were conducted in the 5th intercostal space using an electrode belt comprising 16 electrodes (PulmoVista 500, Dräger, Germany). EIT data were gathered with a frame rate of 30–40 Hz. Subjects were performing normal tidal breathing followed by an inspiratory vital capacity (IVC) manoeuvre and a FVC manoeuvre according to the guideline [8] to determine FEV1 and FVC (SpiroScout, Ganshorn Medizin Electronic, Germany). Spirometry and EIT measurements were repeated up to three times per subject depending on their health status.

2.2 EIT data processing

EIT data were reconstructed with a linearized Newton-Raphson algorithm provided by the EIT manufacturer (Dräger EIT Data Analysis Tool 6.1, Dräger, Germany) obtaining EIT images with a resolution of 32 × 32 pixels. To filter out the cardiac activity, a low-pass filter with a cut-off frequency of 50 min−1 was applied. Lung regions included in further analyses were defined by generating functional EIT images using a linear regression fit [9]. Pixel values smaller than 20% of the maximum value of the functional image [10] were not assigned to respiration and therefore excluded from analyses.

2.3 Data analysis

EIT data measured during the FVC manoeuvre leading to the highest FVC value were analysed for each subject. Relative impedance changes corresponding to FEV1 and FVC were determined for each pixel of the predefined lung regions. FEV1/FVC images were generated by calculating the ratio of FEV1 and FVC in each pixel. A slightly modified version of the inhomogeneity index (GI) introduced by Zhao et al. in 2009 [11], was applied to evaluate the distribution of FEV1/FVC within the predefined lung regions (see eq. 1).

(1)GIFEV1FVC=∑x,y∈lung|FEV1FVCxy−Median⁢(FEV1FVClung)|∑x,y∈lungFEV1FVCxy

where FEV1/FVC depicts the ratio of the relative impedance changes corresponding to FEV1 and FVC, FEV1/FVCxy represents a pixel in the predefined lung region and FEV1/FVClung comprises all pixels belonging to the lung region.

GIFEV1/FVC was calculated for the CF patients and the lung healthy volunteers, respectively.

Ratios of FEV1 and FVC determined by spirometry were stated as mean ± standard deviation.

3 Results

Averaged FEV1/FVC ascertained by spirometry was 83.7 ± 2.5% for the healthy subjects and 63.8 ± 6.8% for the CF patients.

EIT measurements revealed that FEV1/FVC images of the CF patients were more inhomogeneous compared to the FEV1/FVC images of the healthy subjects. Figure 1 exemplarily shows one FEV1/FVC image of a healthy subject and one FEV1/FVC image of a CF patient.

Figure 1 Regional FEV1/FVC images of (A) one lung healthy subject and (B) one CF patient. EIT measurements were performed in the 5th intercostal space. The colour bars indicate the ratio values in each pixel.
Figure 1

Regional FEV1/FVC images of (A) one lung healthy subject and (B) one CF patient. EIT measurements were performed in the 5th intercostal space. The colour bars indicate the ratio values in each pixel.

The boxplot in Figure 2 represents a comparison of the GIFEV1/FVC obtained from the lung healthy subjects and the CF patients. All CF patients exhibited a higher GIFEV1/FVC compared to the lung healthy subjects [CF: 0.103 (0.045), lung healthy subjects: 0.049 (0.017), median (interquartile range)]. Furthermore, the CF patients showed a higher variation in GIFEV1/FVC than the healthy subjects.

Figure 2 Modified global inhomogeneity index (GIFEV1/FVC) of the lung healthy subjects (HL) and the cystic fibrosis patients (CF).Box: 25% percentile, median and 75% percentile. Whiskers: minimum and maximum values.
Figure 2

Modified global inhomogeneity index (GIFEV1/FVC) of the lung healthy subjects (HL) and the cystic fibrosis patients (CF).Box: 25% percentile, median and 75% percentile. Whiskers: minimum and maximum values.

4 Discussion

Global ratios of FEV1 and FVC measured by spirometry were distinctly higher in the lung healthy subjects compared to the CF patients confirming airway obstructions in the CF patients.

Differences in the FEV1/FVC images between the CF patients and the lung healthy subjects could be visualized. In 2012 Zhao et al. showed that the ratio of maximum expiratory flow at 25% and 75% of vital capacity (MEF25/MEF75) could be used to identify regional airway obstruction in CF [6]. Since the CF patients in our study showed a more inhomogenous distribution of FEV1/FVC compared to the lung healthy subjects, FEV1/FVC images might also be applicable to identify regional airway obstructions. Differences within a FEV1/FVC image might indicate that some lung regions are more affected than others. In 2012 Vogt et al. presented that COPD patients exhibited a higher degree of heterogeneity and a flatter frequency distribution of pixel FEV1/FVC ratios compared to lung healthy subjects [5]. These findings coincide with the results of our study. To characterize the heterogeneity of the spatial distribution of regional FEV1 and FVC within a chest cross section, Vogt et al. calculated the coefficient of variation (CV) of all pixel values.

In our study, we utilized a modified global inhomogeneity index GIFEV1/FVC to assess the inhomogeneity degree of the regional FEV1/FVC. GIFEV1/FVC indicated a more homogenous distribution of FEV1/FVC in the lung healthy subjects than in the CF patients. Determining GIFEV1/FVC might benefit the diagnosis but also the follow-up of obstructive lung diseases. GIFEV1/FVC could possibly help the physicians to evaluate the progress of the diseases or the effectiveness of therapy.

One limitation of the study is the limited number of subjects. To confirm results statistically, additional trials with more subjects are necessary. Additionally, the regional information obtained by EIT was restricted to a certain thorax plane. Further measurements in different thorax planes are needed to investigate the influence of the belt position on our results. To verify the results obtained by EIT, a comparison with structural imaging modalities such as CT is required.

5 Conclusion

Results of our study revealed that CF patients showed a higher degree of inhomogeneity in pixel FEV1/FCV ratios compared to lung healthy subjects. FEV1/FVC images might depict a promising tool to identify regional airway obstructions in CF patients benefitting diagnosis and therapy.

Author’s Statement

Research funding: This work was partially supported by the Federal Ministry of Education and Research (BMBF) under grant no. 03FH038I3 (MOSES). Conflict of interest: Z. Zhao receives a consulting fee from Dräger Medical. The remaining authors declare that they have no conflicts of interest. Material and methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance with the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.

References

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Published Online: 2016-9-30
Published in Print: 2016-9-1

©2016 Sabine Krueger-Ziolek et al., licensee De Gruyter.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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https://doi.org/10.1515/cdbme-2016-0139
Keywords for this article
cystic fibrosis; electrical impedance tomography; obstructive lung disease; regional lung function
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  96. 3D Local in vivo Environment (LivE) imaging for single cell protein analysis of bone tissue
  97. Inside-Out access strategy using new trans-vascular catheter approach
  98. US/MRI fusion with new optical tracking and marker approach for interventional procedures inside the MRI suite
  99. Impact of different registration methods in MEG source analysis
  100. 3D segmentation of thyroid ultrasound images using active contours
  101. Designing a compact MRI motion phantom
  102. Cerebral cortex classification by conditional random fields applied to intraoperative thermal imaging
  103. Classification of indirect immunofluorescence images using thresholded local binary count features
  104. Analysis of muscle fatigue conditions using time-frequency images and GLCM features
  105. Numerical evaluation of image parameters of ETR-1
  106. Fabrication of a compliant phantom of the human aortic arch for use in Particle Image Velocimetry (PIV) experimentation
  107. Effect of the number of electrodes on the reconstructed lung shape in electrical impedance tomography
  108. Hardware dependencies of GPU-accelerated beamformer performances for microwave breast cancer detection
  109. Computer assisted assessment of progressing osteoradionecrosis of the jaw for clinical diagnosis and treatment
  110. Evaluation of reconstruction parameters of electrical impedance tomography on aorta detection during saline bolus injection
  111. Evaluation of open-source software for the lung segmentation
  112. Automatic determination of lung features of CF patients in CT scans
  113. Image analysis of self-organized multicellular patterns
  114. Effect of key parameters on synthesis of superparamagnetic nanoparticles (SPIONs)
  115. Radiopacity assessment of neurovascular implants
  116. Development of a desiccant based dielectric for monitoring humidity conditions in miniaturized hermetic implantable packages
  117. Development of an artifact-free aneurysm clip
  118. Enhancing the regeneration of bone defects by alkalizing the peri-implant zone – an in vitro approach
  119. Rapid prototyping of replica knee implants for in vitro testing
  120. Protecting ultra- and hyperhydrophilic implant surfaces in dry state from loss of wettability
  121. Advanced wettability analysis of implant surfaces
  122. Patient-specific hip prostheses designed by surgeons
  123. Plasma treatment on novel carbon fiber reinforced PEEK cages to enhance bioactivity
  124. Wear of a total intervertebral disc prosthesis
  125. Digital health and digital biomarkers – enabling value chains on health data
  126. Usability in the lifecycle of medical software development
  127. Influence of different test gases in a non-destructive 100% quality control system for medical devices
  128. Device development guided by user satisfaction survey on auricular vagus nerve stimulation
  129. Empirical assessment of the time course of innovation in biomedical engineering: first results of a comparative approach
  130. Effect of left atrial hypertrophy on P-wave morphology in a computational model
  131. Simulation of intracardiac electrograms around acute ablation lesions
  132. Parametrization of activation based cardiac electrophysiology models using bidomain model simulations
  133. Assessment of nasal resistance using computational fluid dynamics
  134. Resistance in a non-linear autoregressive model of pulmonary mechanics
  135. Inspiratory and expiratory elastance in a non-linear autoregressive model of pulmonary mechanics
  136. Determination of regional lung function in cystic fibrosis using electrical impedance tomography
  137. Development of parietal bone surrogates for parietal graft lift training
  138. Numerical simulation of mechanically stimulated bone remodelling
  139. Conversion of engineering stresses to Cauchy stresses in tensile and compression tests of thermoplastic polymers
  140. Numerical examinations of simplified spondylodesis models concerning energy absorption in magnetic resonance imaging
  141. Principle study on the signal connection at transabdominal fetal pulse oximetry
  142. Influence of Siluron® insertion on model drug distribution in the simulated vitreous body
  143. Evaluating different approaches to identify a three parameter gas exchange model
  144. Effects of fibrosis on the extracellular potential based on 3D reconstructions from histological sections of heart tissue
  145. From imaging to hemodynamics – how reconstruction kernels influence the blood flow predictions in intracranial aneurysms
  146. Flow optimised design of a novel point-of-care diagnostic device for the detection of disease specific biomarkers
  147. Improved FPGA controlled artificial vascular system for plethysmographic measurements
  148. Minimally spaced electrode positions for multi-functional chest sensors: ECG and respiratory signal estimation
  149. Automated detection of alveolar arches for nasoalveolar molding in cleft lip and palate treatment
  150. Control scheme selection in human-machine- interfaces by analysis of activity signals
  151. Event-based sampling for reducing communication load in realtime human motion analysis by wireless inertial sensor networks
  152. Automatic pairing of inertial sensors to lower limb segments – a plug-and-play approach
  153. Contactless respiratory monitoring system for magnetic resonance imaging applications using a laser range sensor
  154. Interactive monitoring system for visual respiratory biofeedback
  155. Development of a low-cost senor based aid for visually impaired people
  156. Patient assistive system for the shoulder joint
  157. A passive beating heart setup for interventional cardiology training
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