Swallow tail sign on MRI: biology, imaging, and future directions in parkinsonism

Loss of the swallow tail sign—hyperintensity in the posterolateral substantia nigra on susceptibility-weighted MRI—can aid Parkinson's disease differentiation but requires cautious adjunctive use.

• The swallow tail sign is a hyperintensity in the posterolateral substantia nigra pars compacta on susceptibility-sensitive MRI; its attenuation or loss is characteristic of Parkinson's disease.
• This non-invasive biomarker helps distinguish PD from other parkinsonian disorders, though it requires careful interpretation and should not be used in isolation.
• Ultra-high-field MRI (≥7 T) and artificial intelligence are poised to further refine the sign's diagnostic value when combined with other MRI biomarkers.

Automated summary

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