​​Self-supervised Learning for Medical images

Due to fixed imaging procedures, medical images like X-ray or CT scans are usually well aligned geometrically.
This gives an opportunity to utilize such an alignment to automatically mine similar pairs of image patches for self-supervised training.

The basic idea is to fix K random locations in the unlabeled medical images (K locations are the same for every image) and crop image patches across different images (which correspond to scans of different patients).
Now we create a surrogate classification task by assigning a unique pseudo-label to every location 1...K.
Authors combine the surrogate classification task with image restoration using a denoising autoencoder: they randomly perturb the cropped patches (color jittering, random noise, random cut-outs) and train a decoder to restore the original view.

However, sometimes the alignment between medical images is not perfect by default and images may depict different body parts. To make sure that the images are aligned, we train an autoencoder on full images (before cropping) and select only similar images by comparing the distances between them in the learned autoencoder latent space.

Authors show that their method is significantly better than other self-supervised learning approaches on medical data and can even be combined with existing self-supervised methods like RotNet (predicting image rotations). But unfortunately, the comparison is rather limited, and they didn't compare to Jigsaw Puzzle, SwaV, or recent contrastive self-supervised methods like MoCO, BYOL, and SimCLR.

📝 Paper
🛠 Code & Models

#paper_tldr #cv #self_supervised