Nigrosomes are small clusters of dopaminergic cells within the substantia nigra that exhibit calbindin D28K negativity on immunohistochemistry. Five nigrosomes measuring up to a few millimetres in size have been described with the largest labelled as nigrosome-1 ¹.

Nigrosome-1

Nigrosome-1 within the posterior third of the substantia nigra is able to be visualised on high resolution T2*/SWI weighted MRI, although its exact radiographic appearance is debated in the literature. This It was initially thought that nigrosome-1 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 and hence is referredleading to as the swallow tail sign in the substantia nigra, however, this is not shown to be correct in more recent autopsy studies where it was instead found that nigrosome-1 either only partially accounted for this swallow tail appearance or actually demonstrated low (rather than high) signal on axial SWI ³. 

Nigrosome-1 contains the largest proportion of neuronsneurones affected in Parkinson's disease, a condition^1,3. It has been reported that causesin Parkinson disease that there is absence of the high SWI signal within nigrosome-1 to be absent. Absence ofthe substantia nigra, known as the normalabsent swallow tail appearance of nigrosome-1 is reported to be a reliable sign of Parkinson's disease ¹. There are two main possible mechanisms for However, given findings of recent studies, this, increased iron content or decreased neuromelanin content with decreased iron storage capacity leading absence likely correlates to more free iron with paramagnetic properties ²than just nigrosome-1 loss.