In this study, we explored the significance of texture information for lesion detection using counterfactuals, a framework that can explain causal situation by exploring different actions or conditions and its associated results. Specifically, we investigated counterfactuals on mammogram texture; one example could be “what would happen on classifiers if there is no texture (or grayed-out) inside/outside the lesion.” We used the dataset of 10,415 2D screening mammograms, including 4,942 recalled lesions (BI-RADS 0) with lesion masks and 5,473 normal cases (BI-RADS 1). For identifying foreground and background textures, we applied lesion masks directly on lesion cases, while artificially imposed selected lesion masks on normal controls. Using this, four counterfactual cases were examined: 1) replacing lesion foreground (LF) with its mean intensity (MI) vs. normal (N), 2) replacing normal foreground (NF) with its MI vs. lesion (L), 3) replacing lesion background (LB) with its MI vs. N, and 4) replacing normal background (NB) with its MI vs. L. We trained three convolutional neural networks, which were ResNet50v2, ResNet50, and MobileNet, to classify lesion and normal cases (non-counterfactual, baseline). The classifiers on the first three counterfactual cases (LF vs. N, L vs. NF, and LB vs. N) performed similar to the baseline, while their performances on the cases of NB vs. L was significant dropped (p-value < 0.0001). Our findings indicate that lesion shape plays a key role for lesion image classification, while background texture is important for classifying normal cases.
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