The swallow tail sign: mechanisms, neuroimaging advances and future perspectives

Parkinson's disease (PD), the second most common neurodegenerative disorder, poses a significant challenge to elderly health. Its diagnosis currently relies primarily on clinical symptoms, which often proves inadequate for differentiating PD from other forms of parkinsonism, particularly in early disease stages. Although positron emission tomography (PET) has offered compelling evidence supporting the differential diagnosis of PD, its high cost and radiation exposure limit its widespread use. Consequently, there is a pressing need for more accessible, convenient, non-invasive, and safe diagnostic approaches. The "swallow tail sign" (STS)-an MRI-based biomarker characterized by a hyperintensity in the posterolateral substantia nigra pars compacta (SNc), surrounded by hypointense areas-has emerged as a highly promising adjunct tool. This biomarker holds potential not only to enhance diagnostic certainty in distinguishing PD from similar neurodegenerative disorders but also to improve the understanding of its pathophysiology. In this review, we outline the anatomical basis of the STS and the pathological changes in PD leading to its disappearance. We further summarize its applications across various magnetic resonance imaging (MRI) techniques and evaluate its value in identifying PD and differentiating it from other parkinsonian syndromes. With significant advancements in ultra-high-field MRI (≥ 7 T) and artificial intelligence (AI), combined with other MRI biomarkers, the value of STS will be further explored in the future. KEY POINTS: Question Despite its diagnostic potential, the biological basis, imaging interpretation, and clinical utility of the STS remain incompletely understood and warrant further investigation in parkinsonism. Findings The normal STS appears as dorsolateral nigral hyperintensity on susceptibility-sensitive MRI, while its attenuation or loss in PD supports cautious adjunctive diagnostic use. Critical Relevance Statement This review critically evaluates the biological basis, MRI interpretation, diagnostic limitations, and evolving multimodal and AI-based assessment of the STS, guiding radiologists toward its standardized and cautious adjunct use in the clinical imaging assessment of parkinsonism.