Cellular in vivo imaging of the human retina with adaptive optics enhanced SLO/OCT

Abstract

Purpose: To demonstrate the capability of a newly developed adaptive optics scanning laser ophthalmoscope / optical coherence tomography (AO-SLO/OCT) instrument for in vivo imaging of cellular structures in the human retina.

Methods: The SLO/OCT instrument can be operated at frame rates between 10 and 40 fps (transverse frames, imaging area ~1° x 1°) and a 3D volume is acquired within a few seconds. The instrument utilizes the en-face OCT technique which reduces transverse motion artifacts because of the fast acquisition of en-face OCT frames. In addition the focal plane can be shifted in depth with the coherence gate which enables recording a sharp OCT volume throughout imaging depth. SLO images are recorded simultaneously and are used in a post processing step to correct for residual transverse motion. Axial eye motion artifacts are eliminated using an active high speed axial eye tracker operating at 1kHz. This enables recording of nearly motion artifact free 3D data of the retina. The adaptive optics correction is performed in closed loop mode. The imaging beam diameter of the instrument is ~8mm at a center wavelength of 840nm which provides ~2-3µm lateral resolution for both imaging modalities.

Results: The instrument was capable of resolving very small cellular structures such as the foveal cone mosaic. In some volunteers a regular pattern could be observed at the level of the RPE which we believe correspond to the RPE cell mosaic. Imaging at some eccentricity from the fovea revealed both cone and rod photoreceptor structures. Thereby the depth resolution provided by OCT was essential to separate between layers of the photoreceptors that revealed completely different structures. We associated these with the posterior tips of cone and rod photoreceptors, respectively.

Conclusions: Initial measurements using the AO-SLO/OCT instrument on healthy volunteers show the potential of the technique to visualize in 3D different cellular structures in the retina such as foveal cones, rods, and RPE cells.

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 648 photoreceptors • 701 retinal pigment epithelium