Ensemble of transfer learnt classifiers for recognition of cardiovascular tissues from histological images

Recognition of tissues and organs is a recurrent step performed by experts during analyses of histological images. With advancement in the field of machine learning, such steps can be automated using computer vision methods. This paper presents an ensemble-based approach for improved classification of non-pathological tissues and organs in histological images using convolutional neural networks (CNNs). With limited dataset size, we relied upon transfer learning where pre-trained CNNs are re-used for new classification problems. The transfer learning was done using eleven CNN architectures upon 6000 image patches constituting training and validation subsets of a public dataset containing six cardiovascular categories. The CNN models were fine-tuned upon a much larger dataset obtained by augmenting training subset to obtain agreeable performance on validation subset. Lastly, we created various ensembles of trained classifiers and evaluate them on testing subset of 7500 patches. The best ensemble classifier gives, precision, recall, and accuracy of 0.876, 0.869 and 0.869, respectively upon test images. With an overall F₁-score of 0.870, our ensemble-based approach outperforms previous approaches with single fine-tuned CNN, CNN trained from scratch, and traditional machine learning by 0.019, 0.064 and 0.183, respectively. Ensemble approach can perform better than individual classifier-based ones, provided the constituent classifiers are chosen wisely. The empirical choice of classifiers reinforces the intuition that models which are newer and outperformed in their native domain are more likely to outperform in transferred-domain, since the best ensemble dominantly consists of more lately proposed and better architectures.