MASS: Modality-collaborative semi-supervised segmentation by exploiting cross-modal consistency from unpaired CT and MRI images

Training deep segmentation models for medical images often requires a large amount of labeled data. To tackle this issue, semi-supervised segmentation has been employed to produce satisfactory delineation results with affordable labeling cost. However, traditional semi-supervised segmentation methods fail to exploit unpaired multi-modal data, which are widely adopted in today's clinical routine. In this paper, we address this point by proposing Modality-collAborative Semi-Supervised segmentation (i.e., MASS), which utilizes the modality-independent knowledge learned from unpaired CT and MRI scans. To exploit such knowledge, MASS uses cross-modal consistency to regularize deep segmentation models in aspects of both semantic and anatomical spaces, from which MASS learns intra- and inter-modal correspondences to warp atlases' labels for making predictions. For better capturing inter-modal correspondence, from a perspective of feature alignment, we propose a contrastive similarity loss to regularize the latent space of both modalities in order to learn generalized and robust modality-independent representations. Compared to semi-supervised and multi-modal segmentation counterparts, the proposed MASS brings nearly 6% improvements under extremely limited supervision.