Lin Yang1, Meng Dai2, Knut Möller3, Inéz Frerichs4, Andy Adler5, Feng Fu2, Zhanqi Zhao2,3. 1. Department of Aerospace Medicine, Fourth Military Medical University, Xi'an, China. 2. Department of Biomedical Engineering, Fourth Military Medical University, Xi'an, China. 3. Institute of Technical Medicine, Furtwangen University, Villingen-Schwenningen, Germany. 4. Department of Anaesthesiology and Intensive Care Medicine, University Medical Centre of Schleswig-Holstein Campus Kiel, Germany. 5. Department of Systems and Computer Engineering, Carleton University, Ottawa, Canada.
Abstract
BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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. 2021 Quantitative Imaging in Medicine and Surgery. All rights reserved.
BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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. 2021 Quantitative Imaging in Medicine and Surgery. All rights reserved.
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