Nigrosomes

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 millimeters in size have been described with the largest labeled as nigrosome-1 .

Nigrasome-1

Nigrosome-1 within the posterior third of the substantia nigra is able to be visualized on high resolution T2*/SWI weighted MRI. This nigrosome 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 referred to as the swallow tail appearance.

Nigrasome-1 contains the largest proportion of neurons affected in Parkinson's disease, a condition that causes the high SWI signal within nigrosome-1 to be absent. Absence of the normal swallow tail appearance of nigrosome-1 is reported to be a reliable sign of Parkinson's disease . 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 .