A Review Study of the Deep Learning Techniques used for the Classification of Chest Radiological Images for COVID-19 Diagnosis

In the fight against COVID-19, the immediate and accurate screening of infected patients is of great significance. Chest X-Ray (CXR) and Computed Tomography (CT) screening play an important role in the diagnosis of COVID-19. Studies showed that changes occur in Chest Radiological images before the beginning of COVID-19 symptoms for some patients, and the symptoms of COVID-19 and other lung diseases can be similar in their very early stages. Further, it is crucial to effectively distinguish COVID-19 patients from healthy people, and patients with other lung diseases as soon as possible, otherwise inaccurate diagnosis may expose more people to the coronavirus. Many researchers have developed end-to-end deep learning techniques for the classification of COVID-19 patients without manual feature engineering. In this paper, we review the different deep learning techniques that have been used to analyze Chest X-Ray and Computed Tomography scans to classify patients with Covid-19. In addition, we also summarize the common public datasets, challenges, limitations, and possible future work. This review paper is extremely valuable as it confirms that (1) Deep Learning models are effective in classifying chest X-Ray images provided the training dataset is sufficiently large. (2) Data augmentation and generative adversarial networks (GANs) solve the small training dataset problem. (3) Transfer learning methods greatly enhanced the extraction and selection of features that were important for chest image classification. (4) Hyperparameter tuning was valuable for increasing the deep learning model accuracies to generally more than 97%. Our review study helps new researchers identify the gaps and opportunities for further or new research.