Multiple representation contrastive self-supervised learning for pulmonary nodule detection

Self-supervised learning aims to create semantic-enriched representation from unannotated data. A prevalent strategy in this field involves training a unified representation space that is invariant to various transformation combinations. However, creating a single invariant representation to multiple transformations poses several challenges. The efficacy of such a representation space depends on factors such as the intensity, sequence, and various combination scenarios of transformations. As a result, features generated in single representation space may exhibit limited adaptability for subsequent tasks. In contrast to the conventional SSL training approach, we introduce a novel method that involves constructing multiple atomic transformation-invariant representation subspaces. Each subspace in the proposed method is invariant to a specific atomic transformation from a predefined reference set. Our method offers increased flexibility by enabling the downstream task to weigh every atomic transformation-invariant subspace based on the desired feature space. A series of experiments were conducted to compare our approach to traditional self-supervised learning methods in order to assess its effectiveness. This evaluation encompassed diverse data regimes, datasets, evaluation protocols, and perspectives on source-destination data distribution. Our results highlight the superiority of our method compared to training strategies based on single transformation-invariant representation spaces. Additionally, our proposed method demonstrated superior performance in reducing false positives in the context of pulmonary nodule detection when compared to several recent supervised and self-supervised approaches.