Phase-sensitive imaging of the outer retina using optical coherence tomography and adaptive optics
Spotlight summary: The development of high-resolution imaging techniques to examine the living retina has yielded increased understanding of cellular structure and function in normal and diseased eyes. One exciting area of research being explored through the use of high-resolution retinal imaging is the examination of dynamic changes in the reflectance of individual cone photoreceptors. Changes in cone photoreceptor reflectance could be due to several potential factors, including cone photoreceptor outer segment disc shedding, and have been observed over periods of seconds, minutes and hours using several imaging techniques (including adaptive optics [AO] flood illuminated systems, adaptive optics scanning laser ophthalmoscopes [AOSLOs] and adaptive optics-optical coherence tomography [AO-OCT]). Even though these observations have provided insights into the optical waveguide properties of the cone photoreceptor, several of these techniques (including AO flood illuminated and AOSLO imaging) have been limited in their ability to localize the origin of the reflected signal in depth. While reflections can be localized and the size of the 3-dimensional point spread function can be reduced to ~3 x 3 x 3 um using AO-OCT, this technique still does not provide sufficient resolution to visualize subcellular changes. Therefore, a more sensitive technique is required to detect finer scale changes (such as outer segment disc shedding) that could potentially explain temporal changes in cone reflectance.
In this paper, Jonnal et al. report an exciting new methodology to detect changes in the cone outer segment on a scale that is smaller than the axial resolution afforded with their combined ultra-high resolution spectral domain OCT (UHR-SD-OCT) and adaptive optics system. Their technique, termed referenced phase imaging, capitalizes on being able to quantify phase differences between different reflective layers in the retina. By examining the phase information inherent in UHR-SD-OCT volume images, the authors are able to improve their mean sensitivity in detecting a change in cone outer segment length to 45 nm, a value that is over 60 times smaller than their axial imaging resolution. The authors use this technique to show that their phase sensitivity is highest in cones nearest the fovea and decreases with increasing eccentricity (possibly indicating that the smaller diameter foveal cones support a fewer number of modes than the larger diameter cones located further from the foveal center). Moreover, the authors examined changes in the outer segments in 2 normal individuals over the course of a few hours using this technique and calculated an average elongation rate of 150 nm/hr.
When coupled with UHR-SD-OCT and AO, referenced phase imaging represents an important step forward in being able to quantify subcellular dynamics in the living retina. The tools developed by the authors to segment, register, identify and track features automatically on a cellular level have potentially broad application to imaging retinal disease and performing psychophysical experiments. The authors’ demonstrated use of this technique for examining subcellular changes in retinal layers outside of the cone outer segment shows its potential use for examining multiple cell types in the retina. It will be exciting to see whether this technique can yield increased understanding about physiological mechanisms associated with normal cone photoreceptors (such as disc shedding), as well as with diseased photoreceptors (such as alterations in disc shedding that occur in retinitis pigmentosa).
-- Jason Porter
Technical Division: Information Acquisition, Processing, and Display
ToC Category: Optical Coherence Tomography
|OCIS Codes:||(010.1080) Atmospheric and oceanic optics : Active or adaptive optics|
|(100.5070) Image processing : Phase retrieval|
|(170.0180) Medical optics and biotechnology : Microscopy|
|(170.4500) Medical optics and biotechnology : Optical coherence tomography|
|(330.5310) Vision, color, and visual optics : Vision - photoreceptors|
|(100.3175) Image processing : Interferometric imaging|
|(100.5088) Image processing : Phase unwrapping|
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