Deep neural hashing for content-based medical image retrieval: A survey

The ever-growing digital repositories of medical data provide opportunities for advanced healthcare by forming a foundation for a digital healthcare ecosystem. Such an ecosystem facilitates digitized solutions to aspects like early diagnosis, evidence-based treatments, precision medicine, etc. Content-based medical image retrieval (CBMIR) plays a pivotal role in delivering advanced diagnostic healthcare within such an ecosystem. The concept of deep neural hashing (DNH) is introduced with CBMIR systems to aid in faster and more relevant retrievals from such large repositories. The fusion of DNH with CBMIR is an interesting and blooming area whose potential, impact, and methods have not been summarized so far. This survey attempts to summarize this blooming area through an in-depth exploration of the methods of DNH for CBMIR. This survey portrays an end-to-end pipeline for DNH within a CBMIR system. As part of this, concepts like the design of the DNH network, utilizing diverse learning strategies, different loss functions, and evaluation metrics for retrieval performance are discussed in detail. The learning strategies for DNH are further explored by categorizing them based on the loss function into pointwise, pairwise, and triplet-wise. Centered around this categorization, various existing methods are discussed in-depth, mainly focusing on the key contributing aspects of each method. Finally, the future vision for this field is shared in detail by emphasizing three key aspects: current and immediate areas of research, realizing the current and near-future research into practical applications, and finally, some unexplored research topics for the future. In summary, this survey depicts the current state of research and the future vision of the field of CBMIR systems with DNH.