Swallow tail sign (substantia nigra)
The swallow tail sign describes the normal axial imaging appearance of nigrosome1 within the substantia nigra on highresolution T2*/SWI weighted MRI.
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Terminology
Yes, this is one of those annoying signs where its presence is normal. A normalappearing substantia nigra is reminiscent of a swallow's tail. If nigrosome1 loses its high signal on SWI, the splittail appearance is lost and this should, therefore, be reported as an "absent swallow tail sign". Better yet, one can simply report the fact that nigrosome1 cannot be visualized.
Pathology
Nigrosome1, 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 nigrosome1 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}.
Radiographic features
MRI
It is important to note that adequate visualization of nigrosome1 requires targeted 3D highresolution 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,
Interpretation
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 nigrosome1 bilaterally predicts the absence of Parkinson disease) was 100% ^{1} and for dementia with Lewy bodies 89% ^{2}.
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