4DCT-derived lung kinematics in a rodent model of radiation-induced lung injury

Radiation-induced lung injuries (RILI) are major complications in thoracic radiotherapy, which is used to treat lung and breast cancers. RILI initially manifests as pneumonitis in the short term and as lung fibrosis in the long term. RILI affects the lungs heterogeneously, leading to spatial variations in the mechanical behavior of the lung parenchyma. This heterogeneous alteration of lung biomechanics can lead to altered parenchymal kinematics during respiration. In this study, we present an investigation into the evolution of kinematic biomarkers in a rat model of RILI before and after radiation. We performed dynamic imaging in a healthy Sprague-Dawley rat before and after irradiation with a single 20 Gy dose. We used image registration to estimate the displacement at the voxel level, which was used to estimate the deformation gradient. The deformation gradient was used to estimate the volumetric strain as the third invariant. In addition, we estimated regional distortion, defined as the ratio of maximum to eigenvalues of the isochoric portion of the deformation gradient. The results indicated an increase in volumetric strain and increased heterogeneity post-radiation. We also observed an increase in the median distortion post-radiation. Kinematic biomarkers such as volumetric strain and distortion can help improve our understanding of lung injuries, their evolution, and their effect on lung health.