• CARING (Centre for Advanced Research in Imaging, Neuroscience and Genomics), Mahajan Imaging, E19, Main Ring Road, Defence Colony, New Delhi 110024, India. Electronic address: drvasanth@mahajanimaging.com.
• Predible Health, IKP Eden, Bengaluru, Karnataka India.
• CARING (Centre for Advanced Research in Imaging, Neuroscience and Genomics), Mahajan Imaging, E19, Main Ring Road, Defence Colony, New Delhi 110024, India.

RATIONALE AND OBJECTIVES: To explain predictions of a deep residual convolutional network for characterization of lung nodule by analyzing heat maps.

MATERIALS AND METHODS: A 20-layer deep residual CNN was trained on 1245 Chest CTs from National Lung Screening Trial (NLST) trial to predict the malignancy risk of a nodule. We used occlusion to systematically block regions of a nodule and map drops in malignancy risk score to generate clinical attribution heatmaps on 103 nodules from Lung Image Database Consortium image collection and Image Database Resource Initiative (LIDC-IDRI) dataset, which were analyzed by a thoracic radiologist. The features were described as heat inside nodule -bright areas inside nodule, peripheral heat continuous/interrupted bright areas along nodule contours, heat in adjacent plane -brightness in scan planes juxtaposed with the nodule, satellite heat - a smaller bright spot in proximity to nodule in the same scan plane, heat map larger than nodule bright areas corresponding to the shape of the nodule seen outside the nodule margins and heat in calcification.

RESULTS: These six features were assigned binary values. This feature vector was fedinto a standard J48 decision tree with 10-fold cross-validation, which gave an 85 % weighted classification accuracy with a 77.8% True Positive (TP) rate, 8% False Positive (FP) rate for benign cases and 91.8% TP and 22.2% FP rates for malignant cases. Heat Inside nodule was more frequently observed in nodules classified as malignant whereas peripheral heat, heat in adjacent plane, and satellite heat were more commonly seen in nodules classified as benign.

CONCLUSION: We discuss the potential ability of a radiologist to visually parse the deep learning algorithm generated "heat map" to identify features aiding classification.