Candidate-Heuristic In-Context Learning: A new framework for enhancing medical visual question answering with LLMs

Medical Visual Question Answering (MedVQA) is designed to answer natural language questions related to medical images. Existing methods largely adopting the cross-modal pre-training and fine-tuning paradigm, face limitations in accuracy due to data scarcity and insufficient incorporation of extensive medical knowledge. Drawing inspiration from the Knowledge-Based Visual Question Answering (KB-VQA) domain, which leverages Large Language Models (LLMs) and external knowledge bases, we introduce the Candidate-Heuristic In-Context Learning (CH-ICL) framework, a novel approach that leverages LLMs augmented with external knowledge to directly enhance existing MedVQA models. Specifically, we collect a pathology terminology dictionary from a public digital pathology library as an external knowledge base and use it to train a knowledge scope discriminator, which helps identify the knowledge scope required to answer a question. Then, we employ existing MedVQA models to provide reliable answer candidates along with their confidence scores. Finally, the knowledge scope and candidates, combined with retrieved in-context exemplars, are aggregated into prompts for heuristically guiding LLMs in answer generation. Experimental results on the PathVQA, VQA-RAD, and SLAKE public benchmarks show state-of-the-art performance, with improvements of 1.91%, 1.88%, and 2.17% respectively over the baseline. Code and dataset are available at https://github.com/ecoxial2007/CH-ICL.