Entropy Teacher: Entropy-Guided Pseudo Label Mining for Semi-Supervised Small Object Detection in Panoramic Dental X-Rays

Small-scale object detection remains a significant challenge in semi-supervised object detection (SSOD), particularly in panoramic dental X-rays. Due to the small lesion size, low contrast, and complex anatomical background, conventional teacher models often fail to extract accurate lesion features, leading to noisy pseudo labels and suboptimal detection performance. Additionally, most existing SSOD methods rely on high-confidence thresholds to select pseudo labels, which may mistakenly discard valuable predictions with low scores but accurate localization—especially for small-scale targets. To address these challenges, we propose Entropy Teacher, a novel SSOD framework specifically designed for small-scale dental disease detection. Our method introduces an Entropy-Guided Feature Pyramid that integrates entropy-guided representations to enhance fine-grained structural learning. Moreover, we develop a low-confidence pseudo-label mining (LCPLM) strategy with a class-adaptive thresholding mechanism to effectively recover high-quality pseudo labels below conventional confidence thresholds. Extensive experiments on the Dental Disease Dataset and ChestX-Det demonstrate that Entropy Teacher achieves state-of-the-art performance, surpassing the baseline Unbiased Teacher by +3.8 AP50 and +4.5 APS. These results confirm the effectiveness of entropy-guided representations and low-confidence mining in improving small-scale lesion detection under limited supervision.