Continual Self-supervised Learning: Towards Universal Multi-modal Medical Data Representation Learning

Self-supervised learning (SSL) is an efficient pre-training method for medical image analysis. However current research is mostly confined to certain modalities consuming considerable time and resources without achieving universality across different modalities. A straightforward solution is combining all modality data for joint SSL which poses practical challenges. Firstly our experiments reveal conflicts in representation learning as the number of modalities increases. Secondly multi-modal data collected in advance cannot cover all real-world scenarios. In this paper we reconsider versatile SSL from the perspective of continual learning and propose MedCoSS a continuous SSL approach for multi-modal medical data. Different from joint representation learning MedCoSS assigns varying data modalities to separate training stages creating a multi-stage pre-training process. We propose a rehearsal-based continual learning approach to manage modal conflicts and prevent catastrophic forgetting. Specifically we use the k-means sampling to retain and rehearse previous modality data during new modality learning. Moreover we apply feature distillation and intra-modal mixup on buffer data for knowledge retention bypassing pretext tasks. We conduct experiments on a large-scale multi-modal unlabeled dataset including clinical reports X-rays CT MRI and pathological images. Experimental results demonstrate MedCoSS's exceptional generalization ability across 9 downstream datasets and its significant scalability in integrating new modality data. The code and pre-trained model are available at https://github.com/yeerwen/MedCoSS.