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| Upper left: En face projection of the cone mosaic, produced by co-adding intensity from the inner segment outer segment junction (ISOS) and outer segment posterior tip (PT) layers, segmented from a single AO-OCT volume. The bright spots correspond to individual cones. Each cone is ~5 μ in diameter. Scale bar 50 μ. Upper right: En face projection of the outer segment referenced phase, created by subtracting the phase at ISOS from the phase at PT. Phase correlation is apparent, at a scale similar to that of the intensity projection. Scale bar 50 μ. Lower left: Autocorrelation of the intensity projection, possessing the stereotypical appearance of a uniformly packed mosaic. The distance between concentric peaks agrees with the predicted cone row spacing. Scale bar 5 μ. Lower right: Autocorrelation of the referenced phase projection, lacking the concentric rings observed in the intensity autocorrelation. Scale bar 5 μ. The similarity between autocorrelations' central peaks suggests that both intensity and phase are correlated among pixels within the cone, while the dissimilarity between the tails suggests that periodicity exists in the intensity image but not in the phase image. (Credit: Ravi Jonnal, Indiana University) |
Bloomington, IN--Information hidden within optical coherence tomography (OCT) data is allowing researchers at Indiana University in Bloomington to track size changes of the "outer segments" of the eye's color-sensing cone cells.1 The so-called outer segment, made up of a series of stacked discs, each about 30 nm thick, is the light-sensing portion of the cone cell; it goes through daily changes in length (visible as changes in optical path length) that could help medical researchers identify potential retinal problems.
To make an OCT scan of the retina, a beam of light is split: one portion is scattered off the retina while the other is the reference beam. The beams are interfered, and the resulting phase information used to procure a precise measurement of a sample’s position. FOr this experiment, the researchers used an adaptive-optics (AO) OCT setup. But since in this case their samples were attached to live people, the researchers had to adapt the to counteract any movements that the subjects’ eyes might insert into the data.
Instead of measuring the phase of a single interference pattern, the researchers measured phase differences between patterns originating from two reference points within the retinal cells: the top and bottom of the outer segment. The team used this hidden phase information to measure microscopic changes in hundreds of cones, over a matter of hours, in two test subjects with normal vision. Researchers found they could resolve the changes in length down to about 45 nm, which is just slightly longer than the thickness of a single one of the stacked discs that make up the outer segment. The work shows that the outer segments of the cone cells grow at a rate of about 150 nm per hour.
REFERENCE:
1. Ravi S. Jonnal et al., Biomedical Optics Express, Vol. 3, Issue 1, pp. 104-124 (2012)
http://dx.doi.org/10.1364/BOE.3.000104
