Entropy-guided partial annotation for cross-domain rib segmentation

Deep learning methods have been widely used in medical imaging, including rib segmentation. Nevertheless, their dependence on large annotated datasets poses a significant challenge, as expert-annotated rib Computed Tomography (CT) scans are notably scarce. An additional complication arises from domain shift, which often limits the direct applicability of models trained on public datasets to specific clinical tasks, thus requiring further resource-intensive annotations on target domains for adaptation. Although semi-supervised methods have been developed to mitigate annotation costs, the prevailing strategies largely remain at the sample level. This results in unavoidable redundancy within each annotated sample, making the process of labeling an entire CT scan exceedingly tedious and costly. To address these issues, we propose a semi-supervised approach named the Entropy-Guided Partial Annotation (EGPA) method for rib segmentation. This method actively identifies the most informative regions in images for annotation based on entropy metrics, thereby substantially reducing the workload for experts during both model training and cross-domain adaptation. By integrating contrastive learning, active learning, and self-training strategies, EGPA not only significantly saves annotation cost and time when training from scratch but also effectively addresses the challenges of migrating from source to target domains. On the public RibSegV2 dataset (source domain) and a private chest CT rib segmentation dataset (target domain), EGPA achieved Dice scores of 89.5 and 90.7, respectively, nearly matching the performance of fully supervised models (89.9 and 91.2) with only 19 % and 18 % of the full annotation workload. This remarkable reduction in annotation effort shortens the development timeline for reliable segmentation tools and enhances their clinical feasibility. By simplifying the creation of high-quality annotated datasets, our approach facilitates the broad deployment of rib analysis tools in varied clinical settings, promoting standardized and efficient diagnostic practices.