Correlating Local Volumetric Tissue Strains with Global Lung Mechanics Measurements

Abstract

The mechanics of breathing is a fascinating and vital process. The lung has complexities and subtle heterogeneities in structure across length scales that influence mechanics and function. This study establishes an experimental pipeline for capturing alveolar deformations during a respiratory cycle using synchrotron radiation micro-computed tomography (SR-micro-CT). Rodent lungs were mechanically ventilated and imaged at various time points during the respiratory cycle. Pressure-Volume (P-V) characteristics were recorded to capture any changes in overall lung mechanical behaviour during the experiment. A sequence of tomograms was collected from the lungs within the intact thoracic cavity. Digital volume correlation (DVC) was used to compute the three-dimensional strain field at the alveolar level from the time sequence of reconstructed tomograms. Regional differences in ventilation were highlighted during the respiratory cycle, relating the local strains within the lung tissue to the global ventilation measurements. Strains locally reached approximately 150% compared to the averaged regional deformations of approximately 80–100%. Redistribution of air within the lungs was observed during cycling. Regions which were relatively poorly ventilated (low deformations compared to its neighbouring region) were deforming more uniformly at later stages of the experiment (consistent with its neighbouring region). Such heterogenous phenomena are common in everyday breathing. In pathological lungs, some of these non-uniformities in deformation behaviour can become exaggerated, leading to poor function or further damage. The technique presented can help characterize the multiscale biomechanical nature of a given pathology to improve patient management strategies, considering both the local and global lung mechanics.

Examining lung mechanical strains as influenced by breathing volumes and rates using experimental digital image correlation

“…Building upon previously introduced methods, we interface two systems to simultaneously assess local strain and global pulmonary measurements throughout the lung expansion cycle [ 26 , 27 ]. Existing strain measurement studies, such as digital volume correlation (DVC), optical coherence tomography (OCT) [ 28 – 30 ], computerized tomography (CT) scans [ 31 , 32 ] and doppler elastography and speckle tracking for biological tissues [ 33 , 34 ] are limited because they are at discrete time points, retrieve 2D contour images with projected strains, or are time intensive, which affects the resulting measured lung behavior.…”

Developing a Lung Model in the Age of COVID-19: A Digital Image Correlation and Inverse Finite Element Analysis Framework

“…Second, while the optimization algorithms minimized the error, the final error was still not completely vanished; this indicates lung-specific constitutive models are needed to better replicate the DIC measurements, as concluded by earlier works ( Eskandari et al, 2019 ). Third, while DIC allows continuous and evolutionary behaviors of the lung to be examined whereas digital volume correlation techniques are at discrete snapshots ( Arora et al, 2021 ), DIC can only access the lung surface and the internal structure of the lung and the volumetric strain distributions are not represented; a potential enhancement to this technique could be the use of mirrors and prisms to collect multi-angled views. Fourth, the framework is built on ex-vivo setting, hence the predicted material properties are likely to be influenced by the deformation and kinetics of the ribcage and diaphragm.…”

“…There has been notable progress to characterize the lung at the organ scale through classical pressure-volume curves and at the tissue level using indentation and uniaxial tensile tests ( Lai-Fook et al, 1976 ; Zeng et al, 1987 ; Fung, 1988 ; Eskandari et al, 2018 ); however these investigations remain siloed at disconnected scales. Amalgamating these multiphysics and multiscale behaviors is central to understanding lung disease mechanisms, predicting disease progression, and mitigating ventilator-induced-lung-injuries (VILI) to eliminate tissue over stretching (volutrauma) and stressing (barotrauma) ( Dreyfuss and Saumon, 1998 ; Vlahakis et al, 1999 ; Arora et al, 2017 ; Arora et al, 2021 ). Unless an atlas for pulmonary kinetics and kinematics can be established, current ventilation protocols will continue to be subject to trial and error approaches and hindered from advancements despite exigent demands instilled by a worldwide pandemic ( The Acute Respiratory Distress Syndrome Network, 2000 ; Amato et al, 2015 ).…”

Mechanics of lung cancer: A finite element model shows strain amplification during early tumorigenesis

“…Plane strain boundary conditions fixed the deformation in the z-direction. We applied equibiaxial strains of 5%, 20%, and 50%, which represented the range of normal respiration (5-20%, [20]), and maximum elongation (50%, [47]). We assessed the relationship between applied global strain and local strain by calculating the ratio.…”