Elastic Registration Algorithm Based on Three-dimensional Pulmonary MRI in Quantitative Assessment of Severity of Idiopathic Pulmonary Fibrosis

Purpose: To quantitatively analyze lung elasticity in idiopathic pulmonary fibrosis (IPF) using elastic registration based on 3-dimensional pulmonary magnetic resonance imaging (3D-PMRI) and to assess its' correlations with the severity of IPF patients. Material and Methods: Thirty male patients with IPF (mean age: 62 +/- 6 y) and 30 age-matched male healthy controls (mean age: 62 +/- 6 y) were prospectively enrolled. 3D-PMRI was acquired with a 3-dimensional ultrashort echo time sequence in end-inspiration and end-expiration. MR images were registered from end-inspiration to end-expiration with the elastic registration algorithm. Jacobian determinants were calculated from deformation fields on color maps. The log means of the Jacobian determinants (Jac-mean) and Dice similarity coefficient were used to describe lung elasticity between 2 groups. Then, the correlation of lung elasticity with dyspnea Medical Research Council (MRC) score, exercise tolerance, health-related quality of life, lung function, and the extent of pulmonary fibrosis on chest computed tomography were analyzed. Results: The Jac-mean of IPF patients (-0.19, [IQR: -0.22, -0.15]) decreased (absolute value), compared with healthy controls (-0.28, [IQR: -0.31, -0.24], P< 0.001). The lung elasticity in IPF patients with dyspnea MRC >= 3 (Jac-mean: -0.15; Dice: 0.06) was significantly lower than MRC 1 (Jac-mean: -0.22, P= 0.001; Dice: 0.10, P= 0.001) and MRC 2 (Jac-mean: -0.21, P= 0.007; Dice: 0.09, P< 0.001). In addition, the Jac-mean negatively correlated with forced vital capacity % (r=-0.487, P< 0.001), forced expiratory volume 1% (r=-0.413, P= 0.004), TLC% (r=-0.488, P< 0.001), diffusing capacity of the lungs for carbon monoxide % predicted (r=-0.555, P< 0.001), 6-minute walk distance (r=-0.441, P= 0.030) and positively correlated with respiratory symptoms (r= 0.430, P= 0.042). Meanwhile, the Dice similarity coefficient positively correlated with forced vital capacity % (r= 0.577, P= 0.004), forced expiratory volume 1% (r= 0.526, P= 0.012), diffusing capacity of the lungs for carbon monoxide % predicted (r= 0.435, P= 0.048), 6-minute walk distance (r= 0.473, P= 0.016), final peripheral oxygen saturation (r= 0.534, P= 0.004), the extent of fibrosis on chest computed tomography (r=-0.421, P= 0.021) and negatively correlated with activity (r=-0.431, P= 0.048). Conclusion: Lung elasticity decreased in IPF patients and correlated with dyspnea, exercise tolerance, health-related quality of life, lung function, and the extent of pulmonary fibrosis. The lung elasticity based on elastic registration of 3D-PMRI may be a new non-radiation imaging biomarker for quantitative evaluation of the severity of IPF.