M2SNet: Multi-scale in Multi-scale Subtraction Network for Medical Image Segmentation
Existing U-shaped medical image segmentation networks typically fuse multi-level features via element-wise addition or concatenation, which introduces redundancy and weakens cross-level complementarity, leading to inaccurate lesion localization and blurred boundaries. To address this, we propose the Multi-scale Intra–Multi-scale Subtraction Network (M2S-Net), featuring two key innovations: (1) a novel Multi-scale Intra-subtraction Unit (SU) and a pyramid-style cross-level multi-scale subtraction architecture that explicitly models complementary information through hierarchical differencing; and (2) a training-free LossNet that enables bottom-up, task-aware feature supervision. Evaluated across 11 cross-modality datasets—including colonoscopy, ultrasound, CT, and OCT—M2S-Net consistently outperforms state-of-the-art methods, achieving significant improvements in segmentation accuracy and boundary sharpness. This work establishes an efficient and interpretable paradigm for feature fusion in medical image segmentation.