On the same day, subjects were investigated with a 3T magnetic resonance scanner (Magnetom Trio; Siemens, Erlangen, Germany) that uses an eight-channel phased-array head coil. Subjects were instructed to fixate on a target inside the scanner, with the right eye in straight gaze. As published previously in more detail,
35 an ultrafast T
2-weighted HASTE sequence was applied with the following characteristics: half-Fourier acquisition in single-shot turbo spin-echo (HASTE); TR, 1500 ms; TE, 146 ms; number of excitations 1; bandwidth, 195 Hz/pixel; fast sync pulses (duration, 1 ms); FOV, 23 × 18 cm
2; matrix, 512 × 367; nominal spatial resolution, 0.45 × 0.49 mm
2; slice thickness, 3 mm. The images were then interpolated to a matrix size of 2048 × 1468, leading to a pixel size of 0.11 × 0.12 mm
2. In these HASTE images, cerebrospinal fluid (CSF) yielded a high, white signal and the optic nerve a low, dark signal. Voxels containing CSF and optic nerve or CSF and adjacent tissue showed a defined gray shade proportional to their fractional contents of CSF and tissue (partial volume effect of MRI
36 ). Hence, quantification of the optic nerve and CSF sheath diameter was facilitated by the high-contrast differences in the nerve compared with its surroundings. As a result, measurement accuracy was then limited by the reproducibility of the region of interest (ROI).
35 Three slices perpendicular to the optic nerve were acquired in each subject. They were placed at 5, 10, and 15 mm behind the globe. The exact imaging time was documented for each image.