Spatial patterns and frequency distributions of regional deformation in the healthy human lung

Abstract

Understanding regional deformation in the lung has long attracted the medical community, as parenchymal deformation plays a key role in respiratory physiology. Recent advances in image registration make it possible to noninvasively study regional deformation, showing that volumetric deformation in healthy lungs follows complex spatial patterns not necessarily shared by all subjects, and that deformation can be highly anisotropic. In this work, we systematically study the regional deformation in the lungs of eleven human subjects by means of in vivo image-based biomechanical analysis. Regional deformation is quantified in terms of 3D maps of the invariants of the right stretch tensor, which are related to regional changes in length, surface and volume. Based on the histograms of individual lungs, we show that log-normal distributions adequately represent the frequency distribution of deformation invariants in the lung, which naturally motivates the normalization of the invariant fields in terms of the log-normal score. Normalized maps of deformation invariants allow for a direct intersubject comparison, as they display spatial patterns of deformation in a range that is common to all subjects. For the population studied, we find that lungs in supine position display a marked gradient along the gravitational direction not only for volumetric but also for length and surface regional deformation, highlighting the role of gravity in the regional deformation of normal lungs under spontaneous breathing.

Mapping regional strain in anesthetised healthy subjects during spontaneous ventilation

“…The image-based biomechanical analysis was performed as previously described 5 6. Basically, tetrahedral finite element meshes were used in the segmented masks of the lungs (left and right), the meshing process was carried out via the usage of the Computational Geometry Algorithms Library 22.…”

“…Recently, three-dimensional (3D) biomechanical analysis based on finite-element techniques has been introduced to quantify regional lung strain from CT images 5 6. Using this method, our group quantified regional strain in 11 human healthy subjects during a single full inspiration (total lung capacity) in supine decubitus, finding a significant effect of gravity on volumetric regional deformation 6.…”

“…Recently, three-dimensional (3D) biomechanical analysis based on finite-element techniques has been introduced to quantify regional lung strain from CT images 5 6. Using this method, our group quantified regional strain in 11 human healthy subjects during a single full inspiration (total lung capacity) in supine decubitus, finding a significant effect of gravity on volumetric regional deformation 6. Accurate measure of regional lung strain is relevant from a clinical standpoint, because strategies to reduce supraphysiological levels of global lung tidal deformation, that is, low V, driving pressure limitation, among others, are associated with better clinical outcomes in patients with and without lung injury 7–10.…”

“…Regional lung strain has been studied with the finite element method in subjects with healthy and acutely injured lungs. These studies have in common that measurements were during a single respiratory cycle and controlled ventilation, whether under mechanical ventilation or voluntary full inspiration 5–7 11. Regional strain and heterogeneity are key concepts for studying the biomechanical characteristics of the lung in many diseases 12–14.…”

Primal and Mixed Finite Element Methods for Deformable Image Registration Problems

“…From a continuum-mechanics perspective, the optimal transformation u obtained from the solution of the DIR problem can be considered as a displacement field, from which a strain tensor field can be computed using the gradient of the displacement field ∇u. The study of not only motion but also deformation at a regional level in the lung has revealed that deformation in the lung tissue can be highly heterogenous and anisotropic [3,25], thus providing new deformation-based markers to understand lung physiology [13], showcasing the potential of DIR strain analysis as a tool in the detection and diagnosis of pulmonary disease. These advances notwithstanding, it has been recently shown that state-of-the-art strain analysis techniques based on direct differentiation of DIR solutions can be highly inaccurate as they are very sensitive to noise, discretization level and embedded anatomical boundaries [26].…”