Swallow tail sign (substantia nigra)

Dr Andrew Dixon et al.

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 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. In Parkinson disease the high SWI signal within nigrosome-1 is absent and hence the swallow tail sign is absent. There are two main possible mechanisms for this, increased iron content or decreased neuromelanin content with decreased iron storage capacity leading to more free iron with paramagnetic properties 2 .


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Article Information

rID: 31109
Section: Signs
Synonyms or Alternate Spellings:
  • Swallow tail appearance
  • Absent swallow tail sign

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Cases and Figures

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    The tail of the s...
    Figure 1: swallow tail
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    Substantia nigra ...
    Case 1: swallow tail sign - normal
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    Clinical high res...
    Case 2: absent swallow tail sign - Parkinson's disease
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