Morphometry of the Retrobulbar Human Optic Nerve: Comparison between Conventional Sonography and Ultrafast Magnetic Resonance Sequences

Abstract

To compare different methods for quantification of optic nerve and nerve sheath diameter. To apply a novel magnetic resonance protocol using half-Fourier acquired single-shot turbo spin-echo (HASTE) sequences to analyze optic nerve dimensions. Measurements were taken in right eyes of 33 subjects whose median age was 25 years. A-scan ultrasonography was repeated three times in straight gaze. B-scan ultrasonography was repeated three times in straight gaze and abduction. HASTE sequences were applied in straight gaze, analyzed twice by two different radiologists, and completely repeated in a subset of 10 subjects; 95% confidence intervals and coefficients of variation were calculated. HASTE sequences yielded high contrast between cerebrospinal fluid and optic nerve parenchyma. Acquisition time for each sequence was 1.5 seconds per slice. Optic nerve diameters decreased from 3.23 mm at 5 mm to 2.67 mm at 15 mm behind the eye. Sheath diameters decreased from 5.72 mm to 3.98 mm. A- and B-scan ultrasonography yielded significantly smaller diameters. For HASTE sequences, the coefficients of variation ranged from 2% to 7% and were significantly smaller than those obtained with ultrasonographic measurements (9%-13%). The precision of magnetic resonance imaging exceeds that of ultrasonographic methods for determining optic nerve and nerve sheath diameters. HASTE sequences appear particularly appropriate for investigating the retrobulbar optic nerve complex and may be useful in future studies quantifying axonal loss within the optic nerve.