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).
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