## Materials and Methods
### System Design
#### 2.2.2 Corneal Contact  
Many OCT systems, including almost all human retinal imaging systems, are non-contact systems meaning the objective lens does not make contact with the subject's cornea. This is particularly helpful for human subjects who may be told to hold still for several seconds while an OCT image is captured. Patient discomfort is thus minimized by removing the need for an index matching medium between the objective and the patient's cornea. Mice are typically anesthetized when their retinas are imaged using OCT, thus allowing for a corneal contact system.  
One issue with non-contact OCT retinal imagers is that the beam wavefront is subject to the natural aberrations of the eye--the air-cornea and aqueous-lens interfaces providing most of the optical power. The mouse eye introduces significant amounts of spherical aberration and coma, with astigmatism [26]. In conflicting reports, the murine eye has been reported as both myopic and hyperopic [15; 26]. Significant longitudinal chromatic aberrations exist and the mouse eye has a numerical aperture of 0.5, as opposed to a human eye's 0.2 [15].  
A murine retinal imager must account for these aberrations to obtain quality images. Generally, there are two approaches to limiting the effect of aberrations in murine retinal imaging.  
First, wavefront sensing and adaptive optics can improve the spot size of the beam on the retina and account for aberrations in the eye. Such systems have been reported for human retinal OCT and scanning laser ophthalmoscope systems [27; 28] and for murine retinal OCT systems [29; 30]. Classically, these OCT systems employ a wavefront sensor (either with a Shack-Hartmann wavefront sensor or computationally). Based on the sensed wavefront characteristics, an adaptive optic element (e.g., deformable mirror, spatial light modulator) will adaptively alter the incident beam wavefront until the reflected beam has a high-quality wavefront. This intervention ensures that the beam spot size on the retina is small and thus improves spatial resolution. Although these systems are powerful tools, they are more costly and may be slow to make their way to a clinical setting.  
The second approach is to design a corneal contact system to limit the effect of aberrations from the air-cornea interface [31]. This interface has the most optical power and contributes significantly to optical aberrations in the mouse eye [16]. For a more cost-effective approach to limiting aberrations, a corneal contact lens can be index matched to the cornea [32].  
For non-contact systems, the incident beam is collimated (or its wavefront adjusted with adaptive optics) and relies on the natural optics of the eye to focus on the retina. By removing the optical power of the cornea with a contact lens, light should have some positive (convergent) power upon entering the eye to focus on the retina. Figure 2 presents non-contact and contact designs.  
A custom contact lens was fabricated (Optics Technology, Inc.) by etching down a BK7 window with an anti-reflective (AR) coating. The AR coating was maintained on the non-contact side, and a concave surface was formed at the opposite side (radius = 2 mm). The AR coating limits any unwanted reflections from being coupled back into the fiber interferometer. The concave surface is index-matched to the cornea using methyl cellulose.