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Michael Pircher, Franz Felberer, Matthias Salas, Richard Haindl, Bernhard Baumann, Andreas Wartak, Christoph K Hitzenberger; Adaptive optics SLO/OCT for phase sensitive retinal imaging. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the capabilities of a phase sensitive adaptive optics scanning laser ophthalmoscope / optical coherence tomography (AO-SLO/OCT) instrument for measuring subtle length changes of retinal structures in the nanometer range.
The system is based on our AO SLO/OCT instrument. In order to measure small length changes a second OCT channel was implemented into the instrument. Both OCT channels share in large parts the same sample and reference arm which enables stable phase measurements between the channels. However, the reference arm lengths of each channel can be adjusted independently. Thus the coherence gate (CG) of each OCT channel can be set at different locations within tissue. Using the phase information that is provided by OCT, length changes between these locations can be measured with sub imaging wavelength precision. The system records en-face OCT and SLO images simultaneously at frame rates between 10 and 40 Hz. These high frame rates enable frame averaging which decreases phase noise and further increases the precision of the method.
Images from the photoreceptor mosaic were recorded with the dual channel system. To demonstrate the phase stability of the system, the CG was set to the junction between the inner and outer segments (IS/OS) of cone photoreceptors. The phase difference between both channels was close to zero for all cones and showed very low phase noise. Through frame averaging the phase noise was further reduced which allows the observation of length changes in the nanometer range. In a next step the CG of the first channel was set to the IS/OS while the other was set to the end tips of cone photoreceptors. The corresponding phase difference image showed varying values for each cone due to different lengths of the cone outer segments. The phase difference image did not change significantly between data sets that were recorded within one minute. However, large changes were observed between data sets that were recorded with a separation of 30 minutes (cf. Fig. 1).
The phase sensitivity of the instrument allows for high precision measurements of subtle length changes in the human retina. The instrument might be used to accurately measure changes in the outer segment length of cone photoreceptors.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Fig. 1. A) OCT en-face image of IS/OS, B) measurements taken within a minute, C) measurements taken 30 minutes apart. (Color scale: change of phase difference in radiant)
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