Comparative reasoning in radiology artificial intelligence: longitudinal and reference-case approaches

Artificial intelligence (AI) in radiology remains predominantly examination-centric, whereas clinical interpretation frequently depends on comparison across time or with relevant prior cases. This narrative review examines comparative reasoning in radiology AI through two related but clinically distinct forms: longitudinal reasoning, which compares examinations from the same patient over time, and reference-case reasoning, which uses clinically analogous cases from other patients to support interpretation. We propose a capability hierarchy that distinguishes simple access to comparative information from higher-level capabilities, including selection of relevant historical context, anatomical and lesion-level correspondence, characterization of interval change, temporal-state interpretation, multi-interval trajectory reasoning, and entity-specific assessment of clinical similarity. This distinction matters because providing prior examinations or retrieving visually similar cases does not itself demonstrate meaningful comparative reasoning. We review developments in prior-examination selection, registration, lesion tracking, temporal vision-language modeling, treatment-response assessment, analogous-case retrieval, and longitudinal report generation. Particular emphasis is placed on evaluation strategies that determine whether model performance depends appropriately on clinically relevant comparative information rather than on additional input volume, global image similarity, acquisition characteristics, or other shortcuts. Key challenges include incorrect prior selection, lesion mismatching, temporal inconsistency, clinically irrelevant reference-case retrieval, data leakage, misleading comparative language, and propagation of historical errors. Future progress will require models that preserve entity identity, reason coherently across multi-visit imaging histories, identify clinically informative reference cases, and demonstrate benefit under realistic clinical conditions. Comparative reasoning is therefore a necessary step in the transition from examination-level toward patient-level radiology AI.