Extending pretrained segmentation networks with additional anatomical structures.

Purpose: For comprehensive surgical planning with sophisticated patient-specific models, all relevant anatomical structures need to be segmented. This could be achieved using deep neural networks given sufficiently many annotated samples; however, datasets of multiple annotated structures are often unavailable in practice and costly to procure. Therefore, being able to build segmentation models with datasets from different studies and centers in an incremental fashion is highly desirable.

Methods: We propose a class-incremental framework for extending a deep segmentation network to new anatomical structures using a minimal incremental annotation set. Through distilling knowledge from the current network state, we overcome the need for a full retraining.

Results: We evaluate our methods on 100 MR volumes from SKI10 challenge with varying incremental annotation ratios. For 50% incremental annotations, our proposed method suffers less than 1% Dice score loss in retaining old-class performance, as opposed to 25% loss of conventional finetuning. Our framework inherently exploits transferable knowledge from previously trained structures to incremental tasks, demonstrated by results superior even to non-incremental training: In a single volume one-shot incremental learning setting, our method outperforms vanilla network performance by>11% in Dice.

Conclusions: With the presented method, new anatomical structures can be learned while retaining performance for older structures, without a major increase in complexity and memory footprint, hence suitable for lifelong class-incremental learning. By leveraging information from older examples, a fraction of annotations can be sufficient for incrementally building comprehensive segmentation models. With our meta-method, a deep segmentation network is extended with only a minor addition per structure, thus can be applicable also for future network architectures.