Recent extensions of AO technology to animal retinal imaging including non-human primates, mice and rats have proven to be very important in vision science research as well as in the development and testing of new imaging methods. The latter is covered in this latest report by Guevara-Torres et al. were an AO scanning laser ophthalmoscopy (AO-SLO) mouse retinal imaging system has been used to test the new detection scheme. AO-based mouse retinal imaging in comparison to human retinal imaging offers higher numerical aperture, (better lateral resolution and confocal sectioning) and reduced eye movement in anesthetized animals. This is why mouse retinal imaging systems can act as the perfect platform for testing new detection schemes that could be in future translated to human imaging. As a starting point in their paper, the authors modified the detection channel in the Rochester mouse AO-SLO by removing the confocal pinhole and instead placing a silver coated knife edge prism diverting the right and left half of the light distribution at the image focal plane into two detectors. Data from the left and right detectors was digitally subtracted to produce a difference image, which revealed the non-common information between the two channels. This allowed visualization of several retinal structures including a mosaic of small and densely packed cellular structures corresponding to distal processes of the photoreceptors, tightly packed multilayered photoreceptor cell bodies (somata) as well as sparse horizontal cells residing in a narrow focal plane near outer plexiform layer.
In summary, the method presented here consisting of split detection and differential contrast imaging allows visualization for the first time of translucent cell bodies of photoreceptors and horizontal cells in the living mouse retina, without the use of any exogenous contrast agents. I agree with the authors that this demonstration opens a new era in contrast-agent-free retinal imaging. Application of similar detection schemes to human AO systems may provide new ways to study horizontal cells and photoreceptor somata structures, and could be useful to study a variety of retinal degenerative diseases without requiring the use of contrast agents.
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