CT evaluation of perineural orbital lesions: evaluation of the "tram-track" sign.

Cranial computed tomographic scans of 225 patients with orbital mass lesions were reviewed. In 11 patients, the optic nerve was seen as a linear defect through the orbital mass. These 11 cases comprised three optic nerve sheath meningiomas, three hemangiomas, two cases of pseudotumor, and one case each of Erdheim-Chester disease, optic neuritis, and metastatic disease. Optic nerve sheath meningiomas could be further differentiated from the other mass lesions by showing the entire length of the optic nerve through the lesion, smooth margins, a fusiform shape, and parallel enhancement of the periphery of the optic nerve even greater than that of the surrounding mass. These features may aid in differentiating optic nerve sheath meningiomas from other mass lesions in the orbit.