Swallow tail sign (substantia nigra)

The swallow tail sign describes the normal axial imaging appearance of nigrosome-1 within 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 disease and dementia with Lewy bodies ^1,2. 

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. In Parkinson disease the high SWI signal within nigrosome-1 is absent and hence the swallow tail sign is absent. There are two proposed mechanisms for this: increased iron content, or decreased neuromelanin content with decreased iron storage capacity leading to more free iron with paramagnetic properties ³ .