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.

Yes, this is one of those annoying signs where its presence is normal. A normal-appearing substantia nigra is reminiscent of a swallow's tail. If nigrosome-1 loses its high signal on SWI, the split-tail appearance is lost and this should, therefore, be reported as an "absent swallow tail sign". Better yet, one can simply report the fact that nigrosome-1 cannot be visualized. 

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. This is believed to be the result of abnormal iron metabolism, and 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.

It is important to note that adequate visualization of nigrosome-1 requires targeted 3D high-resolution SWI. In the original publication the parameters were:

  • FEEPI, TR/TE 60/30, echo train length 5, flip angle 19°
  • number of slices: 70
  • voxel size 0.55 x 0.55 x 0.7 mm,

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 1,2,4. In the original description, the negative predictive value (in other words visualizing nigrosome-1 bilaterally predicts the absence of Parkinson disease) was 100% 1 and for dementia with Lewy bodies 89% 2

Inspired signs

Article information

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

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Cases and figures

  • Figure 1: swallow tail
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  • Case 1: swallow tail sign - normal
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  • Figure 2: swallow tail sign - normal
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  • Case 2: absent swallow tail sign - Parkinson disease
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  • Figure 3: absent swallow tail sign - Parkinson disease
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  • Case 3: absent swallow tail sign - Parkinson disease
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