RESUMO
The limited availability of medical images is a major limitation when using deep learning, which requires large amounts of data to improve performance. To address this problem, transfer learning has become the de facto standard, using convolutional neural networks (CNNs) previously trained on natural images, such as ImageNet, and fine-tuned with medical images. Recently, vision transformers (ViT), which require large annotated medical images, have been studied from various perspectives. In this study, we investigated an effective pre-training method, especially for ViT. Specifically, an evaluation of the binary classification of COVID-19 and normal chest X-ray images was conducted. The following conclusions were drawn from the evaluation: (1) the fine-tuning method was more effective than the feature extraction method; (2) pre-trained natural images as a fine-tuning method are more effective than task-specific images, namely medical images; (3) the pre-trained natural images learned more Position Embeddings (PEs) with long-range dependencies than medical images; (4) ViT is more effective than CNNs when there are a large number of pre-training natural images, and vice versa when the number of pre-training natural images is limited. These results suggest that the fine-tuning method with a large number of natural images as pre-training data using ViT had the best discrimination performance for the binary classification in this study.