The general mouse preparation for high resolution MRI is well established in our laboratory.
29 Briefly, all animals were maintained in darkness for at least 16 hours before and during the dark phase of MRI examination. High-resolution anatomical and ADC data were acquired on a 7-T system (ClinScan; Bruker, Ewing Twp., NJ, USA), using a receive-only surface coil (1.0-cm diameter) centered on the left eye. The end of a fiber optic bundle was attached to a light source (Mark II Light Source; Prescott, Inc., Monument, CO, USA) placed caudal to the eye, projecting at a white screen ∼1 cm from the eye, similar to that previously described.
53 We exposed the eye to 0 (i.e., dark) or ∼500 lux (confirmed outside the magnet by using a traceable dual-range light meter (Control Company, Friendswood, TX, USA) placed against a 1-cm-diameter aperture; measured this way, room lighting is ∼300 lux). Aside from the fiber optic light source, all lights in the MRI room were turned off. In all groups, immediately before the MRI experiment, animals were anesthetized with urethane (36% solution intraperitoneally; 0.083 mL/20 g animal weight, prepared fresh daily; Sigma-Aldrich Corp.) and treated topically with 1% atropine to ensure dilation of the pupil during light exposure followed by 2% lidocaine to minimize responses that could lead to artifact-inducing eye motion. Anatomical and ADC (parallel to the optic nerve, the most sensitive direction for detecting changes at the location of the SRS
50) MRI data sets were collected, first in the dark and then again 15 minutes after turning on the light. Because each ADC data set takes 10 minutes to collect, we refer to the mid-point in the ADC collection as 20 minutes of light exposure. Anatomical images were acquired using a spin-echo sequence (slice thickness of 600 μm, TR 1000 ms, TE 11 ms, matrix size 192 × 320, field of view 8 × 8 mm
2, NA 2). ADC data were collected using a modified diffusion-weighted spin-echo sequence (TR 1000 ms; slice thickness of 600 μm; TE, 33 ms; matrix size, 174 × 288; field of view, 8 × 8 mm
2;
b = 0, 100, 250, 500, 600, 750, or 990 s/mm
2 collected in pseudo-random order; NA 1 per
b value.
53 Images were registered and analyzed (using in-house code) to generate ADC profiles from the central retina as previously described.
53