Motion-free volumetric retinal imaging with adaptive optics spectral-domain optical coherence tomography

We report first observations of the three-dimensional morphology of cone photoreceptors in the living human retina. Images were acquired with a high-speed adaptive optics (AO) spectral domain optical coherence tomography (SD-OCT) camera. The AO system consists of a Shack-Hartmann wavefront sensor and bimorph deformable mirror (AOptix) that measure and correct the ocular and system aberrations at a closed-loop rate of 12 Hz. Unlike previous AO-OCT and AOSLO instruments, the bimorph mirror was strategically positioned between the XY mechanical scanners and the subject's eye so as to avoid beam distortion at the pupil plane, which is created when the mirror compensates for the refractive error of the eye. This new configuration is evaluated empirically and with commercial ray tracing software. The SDOCT system consists of a superluminescent diode and a 512 pixel line scan charge-coupled device (CCD) that acquires 75,000 A-scans/sec. This rate is more than two times faster than that previously reported. Retina motion artifiacts were minimized by quickly acquiring small volume images of the retina with and without AO compensation. Camera sensitivity was sufficient to detect reflections from all major retinal layers. The distribution of bright spots observed within C-scans at the inner segment / outer segment (IS/OS) junction and at the posterior tips of the OS were found to be highly correlated with one another and with the expected cone spacing. No correlation was found between the IS/OS junction and either the plexiform layers or the layers immediately behind the OS posterior tips.