Imaging Findings in Dorsal Thoracic Arachnoid Web and the Differential Diagnosis of “Scalpel Sign”

A dorsal thoracic arachnoid web represents an intradural extramedullary transverse band of arachnoid tissue that causes mass effect and dorsal cord indentation, and can or cannot be associated with spinal cord altered signal. On sagittal MR imaging, this focal dorsal indentation of the thoracic spinal cord resembles a scalpel with its blade pointing posteriorly (called a “scalpel sign”). Although very suggestive of dorsal thoracic arachnoid web, this sign is not specific and should be differentiated from other ventral cord displacement causes (eg, idiopathic spinal cord herniation and spinal arachnoid cyst). In idiopathic spinal cord herniation, cord tissue protrudes through a ventral dural defect, and the focal deformity can be seen along the ventral aspect of the cord on spinal axial MR imaging and with a characteristic “C sign” on sagittal MR imaging; in spinal arachnoid cysts, the marginated walls and the presence of smooth, wide scalloping of the cord surface can be identified. Recognition of these imaging findings, especially the scalpel sign, can help radiologists and clinicians make a correct diagnosis of ventral cord displacement causes and allow subsequent prompt treatment for the patient.