In the posterior third of the substantia nigra of the midbrain, in Parkinson disease, there is loss of the normal swallow tail sign (loss of bright signal within nigrosome 1).
Axial illustration 1: Showing normal signal intensity of nigrosome 1 which retains a high signal intensity on SWI/T2* sequences (structure highlighted in yellow).
Axial illustration 2: Swallow tail sign is representing the normal appearance of nigrosome 1 within the substantia nigra posterior third. The high signal intensity of nigrosome 1 bordered by hypointense signal of substantia nigra looks like the tail of a swallow as represented.
Axial illustration 3: Nigrosome 1 hyperintense signal is lost on parkinson disease thus there is loss of normal swallow tail sign.
Normal patient MRI SWI sequence with swallow tail sign for comparison.