Estimating Total Lung Volume from Pixel-Level Thickness Maps of Chest Radiographs Using Deep Learning

Purpose To estimate the total lung volume (TLV) from real and synthetic frontal chest radiographs on a pixel level using lung thickness maps generated by a U-Net deep learning model. Materials and Methods This retrospective study included 5959 chest CT scans from two public datasets, the Lung Nodule Analysis 2016 (Luna16) (<i>n</i> = 656) and the Radiological Society of North America Pulmonary Embolism Detection Challenge 2020 (<i>n</i> = 5303). Additionally, 72 participants were selected from the Klinikum Rechts der Isar dataset (October 2018 through December 2019), each with a corresponding chest radiograph obtained within 7 days. Synthetic radiographs and lung thickness maps were generated using forward projection of CT scans and their lung segmentations. A U-Net model was trained on synthetic radiographs to predict lung thickness maps and estimate TLV. Model performance was assessed using mean squared error (MSE), Pearson correlation coefficient, and two-sided Student <i>t</i> distribution. Results The study included 72 participants (45 male and 27 female participants; 33 healthy participants: mean age, 62 years [range, 34-80 years]; 39 with chronic obstructive pulmonary disease: mean age, 69 years [range, 47-91 years]). TLV predictions showed low error rates (MSE_{Public-Synthetic}, 0.16 L²; MSE_{KRI-Synthetic}, 0.20 L²; MSE_KRI-Real, 0.35 L²) and strong correlations with CT-derived reference standard TLV (<i>n</i>_{Public-Synthetic}, 1191; <i>r</i> = 0.99; <i>P</i> < .001) (<i>n</i>_{KRI-Synthetic}, 72; <i>r</i> = 0.97; <i>P</i> < .001) (<i>n</i>_KRI-Real, 72; <i>r</i> = 0.91; <i>P</i> < .001). When evaluated on different datasets, the U-Net model achieved the highest performance for TLV estimation on the Luna16 test dataset, with the lowest MSE (0.09 L²) and strongest correlation (<i>r</i> = 0.99; <i>P</i> < .001) compared with CT-derived TLV. Conclusion The U-Net-generated pixel-level lung thickness maps successfully estimated TLV for both synthetic and real radiographs. <b>Keywords:</b> Frontal Chest Radiographs, Lung Thickness Map, Pixel-Level, Total Lung Volume, U-Net <i>Supplemental material is available for this article.</i> © RSNA, 2025.