Specific Imaging Findings

Optic pathway gliomas (OPG) are found in the optic nerve, optic chiasm and optic tracts, with unilateral (most common) or bilateral distribution. They typically show fusiform enlargement of the optic pathway, which is iso- to hypodense on nonenhanced CT, T1 iso- to hypointense, and T2 iso- to hyperintense. Post-contrast enhancement varies from completely absent to intense. Kinking of the affected nerve and enlargement of the optic canal are frequently associated findings. OPG may show extension of neoplastic glial cells into the subarachnoid spaces with thickening and enhancement of the perioptic meninges, called “arachnoidal hyperplasia” or “arachnoidal gliomatosis”. This finding is almost exclusively seen in patients with neurofibromatosis type-1 (NF-1). On the other hand, cystic components seen as focal well-demarcated areas of hypointense T1 and hyperintense T2 signal are much more common in the sporadic OPG. Sporadic OPGs also tend to be larger, extend beyond the optic pathways, and progress over time. Extension of hyperintense T2 signal into adjacent tissue, especially hypothalamus, is suggestive of invasion. The majority of the OPG are relatively benign tumors and spontaneous regression may occur, especially in association with NF-1. The enhancement pattern may change over time, appear and disappear, which is without clear clinical implications in NF-1 patients. Findings of relatively low myo-inositol levels on MR spectroscopy and of increased permeability on perfusion imaging are suggestive of more aggressive neoplasms.