A Scanning Laser Ophthalmoscopy for Imaging Retina Using a New Liquid Crystal Spatial Light Modulator

Abstract

Purpose: : To test if photoreceptor cells and retinal vasculatures can be visualized in living human eyes with a confocal adaptive optics scanning laser ophthalmoscope (AO-SLO) using a new liquid crystal spatial light modulator (LC-SLM). In addition, to determine if the AO-SLO allow visualization of pathological changes in the photoreceptor layers in eyes with retinal diseases.

Methods: : Recently, a new LC-SLM device based on liquid crystal on silicon technology (LCOS) has become commercially available. Although they have a limitation that only one specific linearly polarized light can be modulated, they are superior to a micro electro mechanical systems deformable mirror (MEMS-DM) in resolution and in precise alignment with respect to a wavefront sensor owing to an electrically variable operating area, both of which are very important in the practical AO application. We developed an AO-SLO system using this LC-SLM device. The light source is 840 nm superluminescent diode for high resolution imaging and 780 nm laser diode for wave-front sensing. The power into the eye is 210 µW and 70 µW, respectiely. It has a badal optics to correct subjects’ defocus. The accumulated astigmatism resulting from the sequence of the off-axis reflection at concave mirrors is cancelled by introducing a counter astigmatism, which enables the RMS wavefront error intrinsic to the optical system to be lower than 0.1 wave at the subject's pupil without AO correction. The optical resolution of this AO-SLO is experimentally confirmed to be around 3 µm. The high resolution retinal imaging is acquired at 10 frames per second and its field of view is 1.5 degrees by 1.5 degrees. Custom software made the images from the analogue output voltage of an avalanche photodiode detector and the images were stored in video files on hard drives. All the images were taken at Kyoto University Hospital.

Results: : Photoreceptors were clearly seen with the AO-SLO using a LC-SLM both in normal and diseased eyes. The AO-SLO allowed detection of the defects of a cluster of photoreceptors in various retinal diseases. The images were comparable to those obtained using a MEMS deformable mirror.

Conclusions: : High-resolution imaging of small structures of the retina in living human eyes is possible with an AO-SLO using this new LC-SLM device as a wavefront corrector.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • photoreceptors • retina