Anti-Confounding Hashing: Enhancing Radiological Image Retrieval via Debiased Weighting and Counterfactual Reasoning

Content-based medical image retrieval (CBMIR) enables physicians to make evidence-based diagnoses by retrieving similar medical images and recalling previous cases stored in databases. However, existing CBMIR models are prone to capturing superficial correlations due to confounding factors such as complex host organs and lesions, imaging discrepancies, artifacts, and inconsistent protocols. To address this issue, we propose a plug-and-play anti-confounding hashing (ACH) method, which uses debiased sample weighting and lesion counterfactual reasoning (LCR) to directly capture the natural direct effect (NDE) of lesions on query medical images without bias. The devised debiased weighting (DBW) loss adopts a backdoor adjustment to separate lesions from confounders. To effectively locate salient areas of lesions, we present a coarse-to-fine lesion positioning (C2F-LP) module by counterfactual reasoning. On two real-world radiological image datasets, ACH achieves 0.2%–9% improvement in mean average precision (mAP) over the six state-of-the-art methods, when using code lengths ranging from 8-bit to 32-bit. Its robustness to confounding factors is demonstrated through explainable visual analysis.