The swallow tail sign describes the normal axial imaging appearance of nigrosome-1within the substantia nigra on high resolution T2*/SWI weighted MRI. Absence of the sign (absent swallow tail sign) is reported to have a diagnostic accuracy of greater than 90% for Parkinson's disease 1.
Nigrosome-1 within the posterior third of the substantia nigra returns high signal on axial SWI in a linear or comma shape. It is surrounded anteriorly, laterally and medially by low SWI signal intensity which resembles the distinctive split tail of a swallow. InParkinson's disease the high SWI signal within nigrosome-1 is absent and hence the swallow tail sign is absent.