T2 weighted sagittal MR images of the lumbar spine showing central disc extrusion with cranial migration.
A disk is classified as an extrusion if any distance between the edges of the disk material beyond the disk space is greater than the distance between the edges of the base measured in the same plane.
Extruded disk material that has no continuity with the disk of origin may be further characterized as sequestrated. A sequestrated disk is a subtype of extruded disk. By definition, a sequestrated disk can never be classified as a protruded disk. Disk material that is displaced away from the site of extrusion, regardless of the presence or absence of continuity, may be called migrated, a term that is useful for interpreting images because it is often impossible to show continuity on imaging.