Continual and Incremental Learning in Computer-aided Diagnosis Systems

Prathyusha Akundi

Abstract

Deep Neural Networks (DNNs) have shown remarkable performance in a broad range of computer vision tasks, including in the medical domain. With the advent of DNNs, the medical community has witnessed significant developments in segmentation, classification, and detection. But this success comes with a cost of heavy reliance on the abundance of data. Medical data, however, is often highly limited in volume and quality due to sparsity of patient contact, variability in medical care, and privacy concerns. Hence, to train large networks we seek data from different sources. In such a scenario, it is of interest to design a model that learns continuously and adapts to datasets or tasks as and when they are available. However, one of the important steps to achieve such a never-ending learning process is to overcome Catastrophic Forgetting (CF) of previously seen data or tasks. CF refers to the significant degradation in performance on the old task/dataset. To avoid confusion, we call a training regime Continual Learning (CL) when CAD systems have to handle a sequence of datasets collected over time from different sites with different imaging parameters/populations. Similarly, Incremental Learning (IL) is when CAD systems have to learn new classes as and when new annotations are made available. The work described in this thesis address core aspects of both CL & IL and has been compared against the state-of-the-art methods. In this thesis, we assume that access to the data belonging to previously trained datasets or tasks is not available which makes both CL and IL processes even more challenging. We start with developing a CL system that learns sequentially on different datasets and handles CF using the Uncertainty mechanism. The system consists of an ensemble of models which are trained or finetuned on each dataset and considers the prediction from the model which has the least uncertainty. We then investigate a new way to tackle CF in CL by manifold learning, inspired by the defense mechanisms against adversarial attacks. Our method uses a ‘Reformer’ which is essentially a denoising autoencoder that ‘reforms’ or brings the data from all the datasets together towards a common manifold. These reformed samples are then passed to the network to learn the desired task. Towards IL, we propose a novel approach that ensures that a model remembers the causal factor behind the decisions on the old classes, while incrementally learning new classes. We introduce a common auxiliary task during the course of incremental training, whose hidden representations are shared across all the classification heads. All the experiments for both CL and IL are conducted on multiple datasets and have shown significant performance over the state-of-the-art methods

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