Phrase-grounded APO for Improving Chest X-ray Report Generation

The deployment of automatic radiology report generator (RRG) models in clinical workflows is being hampered by the lack of factual correctness in the produced reports. Existing methods to improve the report generators use alignment approaches that require pairs of ground truth preferred and dis-preferred responses. As these are not available at inference time in clinical workflows, new alignment methods are needed to improve report quality at inference time. In this paper, we present a new phrase-grounded automatic preference optimization (APO) alignment method which offers such improvement during inference without needing additional ground truth. Specifically, the method generates surrogate ground truth preference data for alignment automatically from the RRG model response itself though fact-checking and LLM-prompted correction. We also develop a novel APO loss function that combines preference response alignment loss with phrasal grounding loss paying attention to both the description of the finding and its image location. We show that this method of alignment, on the average, improves the report quality at inference time by 30-40\% across various SOTA report generators as tested on multi-institutional chest X-ray datasets.