Ultra-High Field Magnetic Resonance Imaging of the Retrobulbar Optic Nerve, Subarachnoid Space, and Optic Nerve Sheath in Emmetropic and Myopic Eyes

Abstract

Purpose: We aimed to image the optic nerve, subarachnoid space and optic nerve sheath in emmetropes and myopes ultra-high field (7-Tesla) magnetic resonance imaging (MRI). We targeted the retrobulbar distance of approximately 3 mm behind the eyeball, an area of clinical interest because of optic nerve sheath distensibility and pressure-related enlargement.

Methods: Eleven emmetropes (+0.75 to -0.50D, aged 20-41 years) and 10 myopes (-4.5 to -12D, aged 21-37 years) participated. Cross-sectional area of the optic nerve, subarachnoid space and optic nerve sheath at approximately 3 mm behind the eye were measured from two-dimensional T2-weighted coronal oblique MRI images obtained through the left optic nerve. Axial length of the left eye was measured from T2-weighted axial MRI images. In nine emmetropes and seven myopes, the optic nerve head was imaged with optical coherence tomography to compare retrobulbar and intraocular measures.

Results: Retrobulbar optic nerve, subarachnoid space and optic nerve sheath dimensions differed between myopes and emmetropes. Myopes tended to have smaller optic nerve and subarachnoid space. Longer MRI-derived axial length was associated with smaller optic nerve area (P = 0.03). Bruch's membrane opening area did not predict retrobulbar optic nerve area (P = 0.48).

Conclusions: This study demonstrates the feasibility of using 7-Tesla MRI to measure optic nerve, subarachnoid space, and optic nerve sheath dimensions behind the eye. In healthy adults, the retrobulbar optic nerve and subarachnoid space size are influenced by the degree of myopia.

Translational relevance: ultra-high field MRI is a practical tool for assessing the morphometry of the optic nerve and surrounding anatomy behind the eye.

Figure 1.

(A) Eye coil setup prior to entering the scanner. The eye coil was placed on a plastic frame for stability. Fixation was aided by an adjustable mirror (blue) connected to the eye coil at a 45° angle. (B) The fixation task consisted of four nonius lines pointing to a central target, which was customized in size (i.e., larger target for myopes, target size = minimum angle of resolution of visual acuity). (C) Example 2D T2-weighted axial image closest to eye equator to determine the MRI-derived axial length measurement, from a straight line manually placed between the posterior cornea and the vitreoretinal interface along the central axis, through the center of the lens and vitreous. (D) Example 2D T2-weighted axial image with superimposed scan plane (green line, in this case, 3 mm behind the eyeball), manually placed perpendicular to the insertion of the left eye optic nerve. (E) Example 2D T2-weighted coronal oblique image (enlarged and cropped), from which the optic nerve, subarachnoid space and optic nerve sheath were delineated manually. The main MRI-derived outcome measures of interest are schematized on the right: cross-sectional area of the (a) optic nerve, (b) subarachnoid space, and (c) optic nerve sheath (see Table 1 for cross-sectional measures); vertical outer diameters of the (d) optic nerve, (e) subarachnoid space, and (f) optic nerve sheath; and horizontal outer diameters of the (g) optic nerve, (h) subarachnoid space, and (i) optic nerve sheath (see Table 2 for diameter measures).

Figure 2.

(A) Individual axial length in 11 emmetropes (filled symbols) and 10 myopes (unfilled symbols) derived from T2-weighted axial MRI images. Group mean ± 95% confidence intervals of the mean are shown. (B) Individual axial length in 10 emmetropes (filled symbols) and seven myopes (unfilled symbols) derived from ultrasound biometry. Group mean ± 95% confidence intervals of the mean are shown. (C) Bland-Altman plot, showing the difference in axial length (MRI − Biometry) as a function of average axial length derived from the two methods. The bias (average difference) was positive (0.5 mm), indicating that, on average, MRI measures of axial length were higher than those obtained by ocular biometry. The 95% limits of agreement were −0.7 to 1.7 mm. Shorter axial lengths tended to be overestimated by MRI compared to ultrasound biometry.

Figure 3.

(A) Individual retrobulbar optic nerve, subarachnoid space and optic nerve sheath cross-sectional area (mm²) in 10 emmetropes (filled symbols) and nine myopes (unfilled symbols) derived from T2-weighted axial MRI images. Group mean ± 95% confidence intervals of the mean are shown. (B) Correlation between MRI-derived axial length (mm) and optic nerve cross-sectional area (mm²) in 10 emmetropes (filled symbols) and nine myopes (unfilled symbols). (C) Correlation between biometry axial length (mm) and optic nerve cross-sectional area (mm²) in nine emmetropes (filled symbols) and six myopes (unfilled symbols).

Figure 4.

Example MRI cross-sectional coronal oblique images illustrating the representative group difference in subarachnoid space area. (A) Emmetropic left eye of a 40 year old female (0.00D spherical equivalent). (B) Myopic left eye of a 26 year old male (−6.50D spherical equivalent). In the bottom panel the same images have been enlarged to show the optic nerve (orange arrow), subarachnoid space (green arrowhead), and optic nerve sheath. The two eyes show the average subarachnoid space area for their respective groups, illustrating the trend in our data; that is, larger subarachnoid space area in (A) the emmetropic eye (11.9 mm²) than (B) the myopic eye (8.7 mm²), with the same total optic nerve sheath complex area (A) 30.7 mm² and (B) 30.9 mm².

Figure 5.

Example MRI cross-sectional coronal oblique images illustrating the representative group difference in optic nerve area. (A) Emmetropic left eye of a 23-year-old male (+0.50D spherical equivalent) (B) Myopic left eye of a 37-year-old male (−6.75D spherical equivalent). In the bottom panel the same images have been enlarged to show the optic nerve (orange arrow), subarachnoid space (green arrowhead), and optic nerve sheath. The two eyes show the average optic nerve area for their respective groups, illustrating the trend in our data; that is, larger optic nerve area in (A) the emmetropic eye (9.7 mm²) than (B) the myopic eye (7.6 mm²), with similar total optic nerve sheath complex area (A) 32.7 mm² and (B) 33.4 mm².

Figure 6.

(A) Individual Bruch's membrane opening area (mm²) in nine emmetropes (filled symbols) and seven myopes (unfilled symbols) derived from spectral-domain OCT in the subset of participants with OCT and MRI images. Group mean ± 95% confidence intervals of the mean are shown. (B) Correlation between OCT-derived Bruch's membrane opening area (mm²) and MRI-derived optic nerve cross-sectional area (mm²).