High resolution retinal images for better diagnosis

Abstract

Eye diseases can often be detected at an early stage if cellularscale images of the retina are available. However, it is not easy to obtain high-resolution images of photoreceptors—lightdetecting cells—because of inherent flaws in the eye’s own optics, so-called aberrations. The size of a photoreceptor is around a few microns, and consequently, the required resolution is less than 5μm. Present standard retinal imaging systems cannot provide such a high resolution. ‘Adaptive optics’ (AO) is based on a feedback loop comprising aberration measurement and dynamic wavefront correction to cancel the aberration of the eye. It has recently been introduced for a variety of high-resolution retinal imaging applications1–3 that have revealed microstructures of the retina. AO instruments have also shown a probe-wavelength-dependent contrast in the retinal microstructure.4 Until now, the retinal microstructure has been investigated with visible and near infrared light, up to 840nm,which is still sensible by the human eye despite not being in the visible (380–750nm) spectrum. We are working on highresolution long-wavelength imaging to reveal more properties of the retina. The 1μm wavelength we are using has several advantages. Not only are the eye’s photoreceptors unaffected by this wavelength of light, but water, the principal component of the fluid in the eye, has a local minimum in its absorption of light at 1.05μm.5 In addition, longer wavelengths are less scattered by the retina, and they enable deeper penetration into the eye.6 Further, water has a relatively constant refraction index around 1μm, which minimizes wavelength-dependent blurring of images. We have developed an AO scanning laser ophthalmoscope (AO-SLO) with a 1.04μm probe beam.7 The AO subsystem is a confocal microscope used to correct the retinal images obtained by the SLO subsystem for the eye aberration. An amplified spontaneous emission light source with a center wavelength Figure 1. Schematic of adaptive-optics (AO) scanning laser ophthalmoscope with a probe wavelength of 1μm. The AO setup is made up of a deformable mirror (DM) and a Hartmann-Shack wavefront sensor (HSWS). (Inset) A vertically off-axis pair of spherical mirrors.