Improved Optic Nerve Visualization and Surgical Planning through a Novel MRI Protocol

Introduction: The optic nerve and surrounding structures are beset by a wide range of intrinsic and extrinsic pathologies. The ability to differentiate nerve from pathology on imaging—and in which direction the optic nerve may be displaced by extrinsic lesions—is critical for accurate diagnosis and clinical decision making. Orbital MRI sequences combine thin-cut, fat-suppressed, T1 postcontrast, and T2-weighted images to highlight intraorbital contents including the globe, extraocular muscles, optic nerve and nerve sheath, and vasculature. However, standard coronal and sagittal views obtained from orbital MRIs may obscure the extent of cerebrospinal fluid (CSF) surrounding the optic nerve given the oblique anatomic orientation of the nerve relative to these standard imaging planes. We developed an optic nerve MRI protocol tailored for improved visualization of the optic nerve and surrounding CSF sheath along its course to better establish the relationship of adjacent anatomy and pathology. This was accomplished in part by incorporating images orthogonal and parallel to the optic nerve itself, taking into account its oblique angle relative to traditional coronal and sagittal planes.

Methods: We reviewed patients from a single institution who underwent our modified optic nerve MRI protocol for skull base or orbital pathologies. Clinical records including radiology, operative notes, and pathology reports were assessed. For the optic nerve MRI, dedicated fat suppressed T2-weighted and T1 gadolinium-enhanced images were acquired perpendicular and parallel to the longitudinal axis of the optic nerve ([Fig. 1]) to achieve “en-face” and in-line views along the course of the nerve.

Results: Dedicated optic nerve MRI imaging provided superior visualization of the optic nerve, the surrounding CSF within the nerve sheath, and the nerve’s relationship to adjacent pathologies compared with traditional orbital MRI. Clear delineation of the position of the nerve with respect to extrinsic lesions involving the optic canal aided choice of the operative trajectory. Masses that displaced the optic nerve medially were well suited to an ipsilateral surgical approach, while medially based lesions that deviate the optic nerve laterally were more amenable to a contralateral approach.

Discussion: We present a dedicated optic nerve MRI protocol which affords improved cross-sectional visualization of the nerve, the surrounding CSF and nerve sheath, and directionality of extrinsic compression. This improved visualization abets orbital diagnostic radiology and surgical planning and is simple to execute using commonly available MR apparatus.