Keypoint-Augmented Self-Supervised Learning for Medical Image Segmentation with Limited Annotation

TL;DR

This paper introduces a keypoint-augmented self-supervised learning framework for medical image segmentation that captures both short- and long-range spatial dependencies, improving performance with limited annotations.

Contribution

It proposes a novel fusion layer incorporating keypoints for enhanced self-attention and a dual-scale pretraining strategy, advancing medical image segmentation accuracy.

Findings

Outperforms CNN and Transformer-based UNets on MRI and CT segmentation.

Achieves state-of-the-art results with limited annotations.

Provides more robust self-attention mechanisms.

Abstract

Pretraining CNN models (i.e., UNet) through self-supervision has become a powerful approach to facilitate medical image segmentation under low annotation regimes. Recent contrastive learning methods encourage similar global representations when the same image undergoes different transformations, or enforce invariance across different image/patch features that are intrinsically correlated. However, CNN-extracted global and local features are limited in capturing long-range spatial dependencies that are essential in biological anatomy. To this end, we present a keypoint-augmented fusion layer that extracts representations preserving both short- and long-range self-attention. In particular, we augment the CNN feature map at multiple scales by incorporating an additional input that learns long-range spatial self-attention among localized keypoint features. Further, we introduce both global and local self-supervised pretraining for the framework. At the global scale, we obtain global representations from both the bottleneck of the UNet, and by aggregating multiscale keypoint features. These global features are subsequently regularized through image-level contrastive objectives. At the local scale, we define a distance-based criterion to first establish correspondences among keypoints and encourage similarity between their features. Through extensive experiments on both MRI and CT segmentation tasks, we demonstrate the architectural advantages of our proposed method in comparison to both CNN and Transformer-based UNets, when all architectures are trained with randomly initialized weights. With our proposed pretraining strategy, our method further outperforms existing SSL methods by producing more robust self-attention and achieving state-of-the-art segmentation results. The code is available at https://github.com/zshyang/kaf.git.