MRI was performed as previously described.
22,23 On the day of the examination, animals were anesthetized by intraperitoneal injection of ketamine (90 mg/kg) and xylazine (10 mg/kg). A contrast agent, gadopentetate dimeglumine (117 mg/mL, Magnevist; Berlex Laboratories, Wayne, NJ), was used as eyedrops to highlight the anterior chamber of the eye (see
Fig. 1S for an MRI image of the mouse eye without this contrast agent). Each mouse was then gently positioned on an MRI-compatible homemade holder. Animals were allowed to breathe spontaneously during the experiment. Rectal temperature was continuously monitored while the animal was inside the magnet. MRI data were acquired on an MRI system (4.7T Avance; Bruker BioSpin MRI GmbH, Ettlingen, Germany) using a two-turn transmit/receive surface coil (0.8-cm diameter) placed over the eye. A preliminary low-resolution scan, in the horizontal plane of the eye, was used to position the high-resolution scan in the sagittal plane of the eye. The plane for the high-resolution scan was positioned to go through the center of the lens, the center of the cornea, and the optic nerve, thus ensuring its proximity to the optical axis of the eye. Five serial low-resolution slices were collected using an adiabatic spin-echo imaging sequence (repetition time, 0.45 seconds; echo time, 13.6 ms; number of acquisitions, 1; matrix size, 256 × 128; slice thickness, 1 mm; field of view, 32 × 32 mm
2; 57 seconds for five slices). This resulted in an in-plane resolution of 125 μm
2. High-resolution images were collected using an adiabatic spin-echo imaging sequence (repetition time, 1 second; echo time, 13.6 ms; number of acquisitions, 4; matrix size, 512 × 512; slice thickness, 0.62 mm; field of view, 12 × 12 mm
2; 35 minutes/image). This resulted in an in-plane resolution of 23.4 μm
2. Virtual sagittal slices through the optical axis of the eye were obtained for each eye, and axial length, equatorial diameter, anterior chamber depth, anterior chamber width, lens thickness, vitreous chamber depth, and circumference of the eye were measured with imaging software (Image; Scion, Frederick, MD) and derived macros, as previously described.
23 Five independent measurements were taken for each parameter, and the mean and SD were calculated for each eye. Axial length was measured as a distance from the posterior surface of the cornea to the anterior surface of the sclera. Equatorial diameter was measured as a distance between the inner surfaces of the sclera at the equator. Anterior chamber depth (ACD) was measured as a distance from the posterior surface of the cornea to the anterior pole of the lens. Anterior chamber width (ACW) was measured as a distance between the inner surfaces of the cornea in the iridocorneal angle, at the level of the anterior pole of the lens. Corneal radius of curvature (CRC) was calculated as CRC = ACD/2 + ACW
2/8ACD. Lens thickness was determined as a distance between the anterior and posterior poles of the lens. Vitreous chamber depth was measured as a distance from the posterior pole of the lens to the anterior surface of the sclera.