Constructing and Exploring Intermediate Domains in Mixed Domain Semi-supervised Medical Image Segmentation
Authors: Qinghe Ma, Jian Zhang, Lei Qi, Qian Yu, Yinghuan Shi, Yang Gao

Both limited annotation and domain shift are prevalent challenges in medical image segmentation. Traditional semi-supervised segmentation and unsupervised domain adaptation methods address one of these issues separately. However the coexistence of limited annotation and domain shift is quite common which motivates us to introduce a novel and challenging scenario: Mixed Domain Semi-supervised medical image Segmentation (MiDSS). In this scenario we handle data from multiple medical centers with limited annotations available for a single domain and a large amount of unlabeled data from multiple domains. We found that the key to solving the problem lies in how to generate reliable pseudo labels for the unlabeled data in the presence of domain shift with labeled data. To tackle this issue we employ Unified Copy-Paste (UCP) between images to construct intermediate domains facilitating the knowledge transfer from the domain of labeled data to the domains of unlabeled data. To fully utilize the information within the intermediate domain we propose a symmetric Guidance training strategy (SymGD) which additionally offers direct guidance to unlabeled data by merging pseudo labels from intermediate samples. Subsequently we introduce a Training Process aware Random Amplitude MixUp (TP-RAM) to progressively incorporate style-transition components into intermediate samples. Compared with existing state-of-the-art approaches our method achieves a notable 13.57% improvement in Dice score on Prostate dataset as demonstrated on three public datasets. Our code is available at https://github.com/MQinghe/MiDSS