Contextual Two-Stage U-Nets for Robust Pulmonary Lobe Segmentation in CT Scans of COVID-19 and COPD Patients
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Pulmonary lobe segmentation in computed tomography scans is essential for regional assessment of pulmonary diseases. Automated segmentation is still an open problem, especially for scans with substantial abnormalities, such as in COVID-19 infection. Recent works used Convolutional Neural Networks for automatic pulmonary lobe segmentation. Convolution kernels in these networks only respond to local information within the scope of their effective receptive field, and this may be insufficient to capture all necessary contextual information. We argue that contextual information is critically important for accurate delineation of pulmonary lobes, especially when the lungs are severely affected by diseases such as COVID-19 or COPD. In this paper, we propose a contextual two-stage U-net (CTSU-Net) that leverages global context by introducing a first stage in which the receptive field encompasses the entire scan and by using a novel non-local neural network module. The proposed module computes the filter response at one position as a weighted sum of feature responses at all positions, where geometric and visual correlations between features determine weights. With a limited amount of training data available from COVID-19 subjects, we initially train and validate CTSU-Net on a cohort of 5000 subjects from the COPDGene study (4000 for training and 1000 for evaluation). Using models pretrained COPDGene, we apply transfer learning to retrain and evaluate CTSU-Net on 204 COVID-19 subjects (104 for retraining and 100 for evaluation). Experimental results show that CTSU-Net outperforms state-of-the-art baselines and performs robustly on cases with incomplete fissures and severe lung infection due to COVID-19.
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