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    How adaptive optics will change retinal imaging

    Adaptive optics could allow clinicians to monitor the progression of retinal diseases cell by cell, according to Jacque Duncan, MD.

    “We’re able to identify the source of vision loss when there is a problem with patients’ cones,” said Dr. Duncan, professor of clinical ophthalmology, University of California, San Francisco.

    The technology compensates for optical aberrations allowing any ophthalmoscope modality, including flood-illuminated fundus cameras, scanning laser ophthalmoscopes, and optical coherence tomography (OCT), to produce sharper images. It can yield new understanding of retinal diseases, and could be used to evaluate the effects of treatments on photoreceptors, Dr. Duncan said.

    Astronomers developed adaptive optics to compensate for the aberrations caused by turbulence in the earth’s atmosphere.

    Similarly, the technique can measure and correct for the optical imperfections of the eye. Current systems use the Shack-Hartmann wavefront sensor, an approach pioneered for ophthalmology in the 1990s, Dr. Duncan pointed out.

    How it measures

    The sensor measures the aberrations introduced into light exiting the eye using an array of lenslets, where each lenslet samples a local portion of the incidence wavefront and focuses light on a charge-coupled device. Based on software algorithms, a series of actuators deflect the surface of a deformable mirror to compensate for these aberrations so light exits the surface of the mirror in parallel planes.

    “You can apply this technology to all forms of imaging,” said Dr. Duncan. Already one adaptive optics fundus camera is commercially available: the rtx1 by Imagine Eyes, she said. However, the fundus camera “doesn’t allow you to see individual cones within the central 1º from the fovea,” she added.

    However, scanning laser ophthalmoscopes are suited to adaptive optics, Dr. Duncan said. These systems form images by recording the light scattered from a focused beam as it is scanned across the retina. Continuous scanning in a raster fashion allows it to sample large areas at a faster rate than conventional flash fundus imaging using less exposure to visible light.

    With adaptive optics, scanning laser ophthalmoscopes can distinguish features as small as 2 µm, said Dr. Duncan. “We’re able to identify individual cones within a mosaic that are difficult to see using other methods,” Dr. Duncan added.

    Scanning laser ophthalmoscopes typically use confocal imaging in which a pinhole is positioned close to the detector, optically conjugate to the focused spot on the retina. Light not originating from the focal plane of the retina is excluded. This results in an image with a higher contrast and allows axial sectioning of the retina and visualization of various layers.

    Superimposed images

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