The sergeant stripes sign refers to the oblique direction of the cerebellar tonsillar folia a feature sometimes visible in cases of Chiari I malformation on sagittal imaging. It is unclear which insignia this is meant to represent as the direction of the chevrons depends on nationality. For example, British, Australian and Canadian armed forces have their sergeant's chevrons pointing down whereas the opposite is true of United States, French and Finish armed forces 5. The downward pointing chevrons make more sense, although this distinction is something that seems to be avoided in most publications 1-4.
Furthermore, this is not an easy feature to appreciate in most instances even with good quality routine MRI imaging.