A simple normalization technique using window statistics to improve the out-of-distribution generalization in medical images

Since data scarcity and data heterogeneity are prevailing for medical images, well-trained Convolutional Neural Networks (CNNs) using previous normalization methods may perform poorly when deployed to a new site. However, a reliable model for real-world applications should be able to generalize well both on in-distribution (IND) and out-of-distribution (OOD) data (e.g., the new site data). In this study, we present a novel normalization technique called window normalization (WIN), which is a simple yet effective alternative to existing normalization methods. Specifically, WIN perturbs the normalizing statistics with the local statistics computed on a window of features. This feature-level augmentation technique regularizes the models well and improves their OOD generalization significantly. Taking its advantage, we propose a novel self-distillation method called WIN-WIN to further improve the OOD generalization in classification. WIN-WIN is easily implemented with twice forward passes and a consistency constraint, which can be a simple extension for existing methods. Extensive experimental results on various tasks (such as glaucoma detection, breast cancer detection, chromosome classification, optic disc and cup segmentation, etc.) and datasets (26 datasets) demonstrate the generality and effectiveness of our methods. The code is available at https://github.com/joe1chief/windowNormalizaion.