Masked strategies for images with small objects

The hematology analytics used for detection and classification of small blood components is a significant challenge. In particular, when objects exists as small pixel-sized entities in a large context of similar objects. Deep learning approaches using supervised models with pre-trained weights (e.g., ImageNet), such as residual networks (ResNets) and vision transformers (ViTs) have demonstrated success for many applications. Unfortunately, when applied to images outside the domain of learned representations, these methods often result with less than acceptable performance. A strategy to overcome this can be achieved by using self-supervised models, where representations are learned from in-domain images and weights are then applied for downstream applications, such as semantic segmentation. Recently, masked autoencoders (MAEs) have proven to be effective in pre-training ViTs to obtain representations that captures global context information. By masking regions of an image and having the model learn to reconstruct both the masked and non-masked regions, the resulting weights can be used for various applications. However, if the sizes of the objects in images are less than the size of the mask (i.e., patch size), the global context information is lost, making it almost impossible to reconstruct the image. In this study, we investigated the effect of mask ratios and patch sizes for blood components using a "small-scale" MAE to obtain learned ViT encoder representations. We then applied the encoder weights to train a U-Net Transformer (UNETR) for semantic segmentation to obtain both local and global contextual information. Our experimental results demonstrates that both smaller mask ratios and patch sizes improve the reconstruction of images using a MAE. We also show the results of semantic segmentation with and without pre-trained weights, where smaller-sized blood components benefited with pre-training. Overall, our proposed method offers an efficient and effective strategy for the segmentation and classification of small objects.

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