Recently, a growing interest has been seen in deep learning-based semantic segmentation. UNet, which is one of deep learning networks with an encoder-decoder architecture, is widely used in medical image segmentation. Combining multi-scale features is one of important factors for accurate segmentation. UNet++ was developed as a modified Unet by designing an architecture with nested and dense skip connections. However, it does not explore sufficient information from full scales and there is still a large room for improvement. In this paper, we propose a novel UNet 3+, which takes advantage of full-scale skip connections and deep supervisions. The full-scale skip connections incorporate low-level details with high-level semantics from feature maps in different scales; while the deep supervision learns hierarchical representations from the full-scale aggregated feature maps. The proposed method is especially benefiting for organs that appear at varying scales. In addition to accuracy improvements, the proposed UNet 3+ can reduce the network parameters to improve the computation efficiency. We further propose a hybrid loss function and devise a classification-guided module to enhance the organ boundary and reduce the over-segmentation in a non-organ image, yielding more accurate segmentation results. The effectiveness of the proposed method is demonstrated on two datasets. The code is available at: github.com/ZJUGiveLab/UNet-Version

本文提出了基于UNet的新型深度学习网络UNet 3+进行医学图像语义分割任务，此方法在多尺度特征融合、深度监督等方面优化UNet，通过增加全尺度skip连接和采用深度监督的方式获得更准确的分割结果，特别适用于器官在不同尺度下出现的情况，并且该方法能在减少参数的同时提高计算效率，此外还提出了混合损失函数和分类引导模块来增强分割效果。

UNet 3+: 医学影像分割的全尺度联通UNet