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Akos Kusnyerik, Balazs Rozsa, Janos Nemeth, Pal Maak; Correction aberration and astigmatism of the human eye in two-photon microscopy based on acousto-optic scanners. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2123.
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Our approach intends to correct for aberration and astigmatism using the combination of an aspheric lens and the electronic astigmatic focusing abilities of the acousto-optic scanner. We intend to show that this combination allows for fluorescence imaging with subcellular resolution in a wide range of individual eyes of different parameters (aberrations and astigmatism), taken from measurement results.
We adopted our modeling for clinically assessed eye parameters, measuring the eye parameters of 25 patients. Mean astigmatism was 0.323D (SD 0.17D). We used the parameters measured with LENSTAR (Haag-Streit) device to construct the models. Measurements of the abberated wavefront with WASCA were used to refine aberration modeling reliability. Optical engineering was performed in ZEMAX software (RadiantzemaxR). The astigmatism compensation possibility is in the focusing power of the acousto-optic arrangement that can be set separately for two perpendicular planes aligned parallel to the local x and y axes of the scanner (z being the longitudinal axis). This is performed by chirping the acoustic waves differently in the deflectors performing scanning in the perpendicular x-z and y-z scanning, respectively. We designed a lens with aspheric surfaces in order to compensate residual aberrations.
Properly selecting the chirp parameters focusing and astigmatism compensation can be achieved simultaneously. We examined the spot distribution variation with off axis angles during scanning. Unfortunately, this system introduces residual angular dispersion proportional to the scanning angle, which causes elongation of the focal spot at increasing off axis coordinates. With the optimized interface lens we could obtain diffraction limited focal spot of the 800nm exciting laser beam in the central region of the fovea and maintain lateral resolution within 3 micron range in a circular pattern of 200 micron radius on the retina, centered at the fovea.
Acousto-optic 3D scanning in two-photon microscopy as a new imaging method in ophthalmology offers great potential for compensation of astigmatism. Combining the microscope with carefully optimised aspheric interface lens provides diffraction limited projection of the exciting laser beam. The modelling showed that the true subcellular resolution can be obtained over a wide range of human eyes using the same interface lens.
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