<prompt> Absent swallow tail sign is seen inHuntingtons choreaAlzheimers diseaseParkinsonismProgressive supranuclear palsy  <response>: SWALLOW TAIL SIGN: The Swallow tail sign describes the normal axial imaging appearance of nigrosome-1 within the substantia nigra on high resolution T2SWI weighted MRI. Absence of the sign absent swallow tail sign is repoed to have a diagnostic accuracy of greater than 90 for Parkinson disease. Figure showing normal swallow tail. Figure showing absent swallow tail Figure showing absent swallow tail sign in Parkinsonism. 