Section Editor: Sandy Cheng-Yu Chen, M.D.
Taipei Medical University Hospital, Taipei, Taiwan
A dorsal arachnoid web is an abnormal extramedullary band of arachnoid tissue that can compress the spinal cord, usually in the upper thoracic spine. This results in a characteristic appearance called the "scalpel sign." The ventral cord surface is normal. There is a probable spectrum of cord signal intensity changes on MR imaging, from a presyrinx state to the development of syringomyelia. T2WI demonstrates a focal dorsal indentation in the upper thoracic spinal cord (A, arrow). Spinal cord proximal to the web shows syrinx (B, arrow) and widened CSF space posteriorly (C, arrow).