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Adaptive optics

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  • AO Group

    Jörgen Thaung, jorgen.thaung@oft.gu.se
    Zoran Popovic, zoran@oft.gu.se

    The AO Group performs research in the field of Multi-Conjugate Adaptive Optics (MCAO) applications for retinal imaging.

    This project is supported by the Marcus and Amalia Wallenberg Memorial Foundation, Vinnova - Sweden's innovation agency, and De Blindas Vänner i Göteborg.

    Associated collaborators
    Per Knutsson, per.knutsson@oft.gu.se
    Mette Owner-Petersen, mette.owner@gmail.com

    Wide-field adaptive optics imaging

    Retinal imaging is limited due to optical aberrations caused by imperfections in the optical media of the eye. Consequently, diffraction limited retinal imaging can only be achieved if optical aberrations in the eye are measured and corrected. Information about retinal pathology and structure on a cellular level is thus not available in a clinical setting but only from histological studies of excised retinal tissue. In addition to limitations such as tissue shrinkage and distortion, the main limitation of histological preparations is that longitudinal studies of disease progression and/or results of medical treatment are not possible.

    Adaptive optics (AO) is the science, technology and art of capturing diffraction-limited images in adverse circumstances that would normally lead to strongly degraded image quality and loss of resolution. In non-military applications, it was first proposed and implemented in astronomy [1]. AO technology has since been applied in many disciplines, including vision science, where retinal features down to a few microns can be resolved by correcting the aberrations of ocular optics.

    The general principle of AO is to measure the aberrations introduced by the media between an object of interest and its image with a wavefront sensor, analyze the measurements, and calculate a correction with a control computer. The corrections are applied to a corrective element, e.g. a deformable mirror (DM), positioned in the optical path between the object and its image, thereby enabling high-resolution imaging of the object.

    Modern telescopes with integrated AO systems employ the laser guide star technique [2] to create an artificial reference object above the earth's atmosphere. Analogously, the vast majority of present-day vision research AO systems employ a single point source on the retina as a reference object for aberration measurements, consequently termed guide star (GS). AO correction is accomplished with a single DM in a plane conjugated to the pupil plane. An AO system with one GS and one DM will henceforth be referred to as single-conjugate AO (SCAO) system. Aberrations in such a system are measured for a single field angle and correction is uniformly applied over the entire field of view (FOV). Since the eye's optical aberrations are dependent on the field angle this will result in a small corrected FOV of approximately 2 degrees [3]. The property of non-uniformity is shared by most optical aberrations such as e.g. the well known primary aberrations of coma, astigmatism, field curvature and distortion.

    A method to deal with this limitation of SCAO was first proposed by Dicke [4] and later developed by Beckers [5]. The proposed method is known as multiconjugate AO (MCAO) and uses multiple DMs conjugated to separate turbulent layers of the atmosphere and several GS to increase the corrected FOV. In theory, correcting (in reverse order) for each turbulent layer could yield diffraction limited performance over the entire FOV. However, as is the case for both the atmosphere and the eye, aberrations do not originate solely from a discrete set of thin layers but from a distributed volume. By measuring aberrations in different angular directions using several GSs and correcting aberrations in several layers of the eye using multiple DMs (at least two), it is possible to correct aberrations over a larger FOV than compared to SCAO.

    The concept of MCAO for astronomy has been the studied extensively [6-12], a number of experimental papers have also been published [13-16], and on-sky experiments have recently been launched [17]. However, MCAO for the eye is just emerging, with only a few published theoretical papers [3, 18-21]. Our group recently published the first experimental study [21] and practical application [22] of this technique in the eye, implementing a laboratory demonstrator comprising multiple GSs and two DMs, consequently termed dual-conjugate adaptive optics (DCAO). It enables imaging of retinal features down to a few microns, such as retinal cone photoreceptors and capillaries [22], the smallest blood vessels in the retina, over an imaging area of approximately 7 deg x 7 deg. It is unique in its ability to acquire single images over a retinal area that is up to 50 times larger than most other research based flood illumination AO instruments, thus potentially allowing for clinical use.

    A second-generation Proof-of-Concept (PoC) prototype based on the DCAO laboratory demonstrator featuring several improvements is currently under construction [23].

    References
    [1] Babcock. HW. The Possibility of Compensating Astronomical Seeing. Publications of the Astronomical Society of the Pacific. 1953;65(386):229.
    [2] Foy, Labeyrie. Feasibility of Adaptive Telescope with Laser Probe. Astronomy and Astrophysics. 1985;152(2):L29-L31.
    [3] Dubinin A, Cherezova T, Belyakov A, Kudryashov A. Human Retina Imaging: Widening of High Resolution Area. Journal of Modern Optics. 2008;55(4-5):671-681.
    [4] Dicke RH. Phase-Contrast Detection of Telescope Seeing Errors and Their Correction. Astrophysical Journal. 1975;198(3):605-615.
    [5] Beckers JM. Increasing the Size of the Isoplanatic Patch with Multiconjugate Adaptive Optics. ESO Conference and Workshop on Very Large Telescopes and their Instrumentation; 1988; Garching, Germany: European Southern Observatory (ESO) p. 69.
    [6] Beckers JM. Detailed Compensation of Atmospheric Seeing Using Multiconjugate Adaptive Optics. Roddier FJ, editor1989. p. 215-217.
    [7] Ellerbroek BL. First-Order Performance Evaluation of Adaptive-Optics Systems for Atmospheric-Turbulence Compensation in Extended-Field-of-View Astronomical Telescopes. Journal of the Optical Society of America a-Optics Image Science and Vision. 1994;11(2):783-805.
    [8] Fried DL, Belsher JF. Analysis of Fundamental Limits to Artificial-Guide-Star Adaptive-Optics-System Performance for Astronomical Imaging. Journal of the Optical Society of America a-Optics Image Science and Vision. 1994;11(1):277-287.
    [9] Fusco T, Conan JM, Michau V, Rousset G, Mugnier LM. Isoplanatic Angle and Optimal Guide Star Separation for Multiconjugate Adaptive Optics. In: Wizinowich PL, editor. Adaptive Optical Systems Technology, Pts 1 and 22000. p. 1044-1055.
    [10] Johnston DC, Welsh BM. Analysis of Multiconjugate Adaptive Optics. Journal of the Optical Society of America a-Optics Image Science and Vision. 1994;11(1):394-408.
    [11] Owner-Petersen M, Goncharov A. Multiconjugate Adaptive Optics for Large Telescopes: Analytical Control of the Mirror Shapes. Journal of the Optical Society of America a-Optics Image Science and Vision. 2002;19(3):537-548.
    [12] Rigaut FJ, Ellerbroek BL, Flicker R. Principles, Limitations and Performance of Multi- Conjugate Adaptive Optics. Adaptive Optical Systems Technology, Pts 1 and 2. 2000;4007:1022-1031.
    [13] Berkefeld T, Soltau D, von der Luhe O. Multi-Conjugate Adaptive Optics at the Vacuum Tower Telescope, Tenerife. Adaptive Optical System Technologies Ii, Pts 1 and 2. 2003;4839:544-553.
    [14] Marchetti E, Hubin N, Fedrigo E, Brynnel J, Delabre B, Donaldson R, et al. Mad the Eso Multi-Conjugate Adaptive Optics Demonstrator. Adaptive Optical System Technologies Ii, Pts 1 and 2. 2003;4839:317-328.
    [15] Rimmele T, Hegwer S, Marino J, Richards K, Schmidt D, Waldmann T, et al. Solar Multi-Conjugate Adaptive Optics at the Dunn Solar Telescope. 1st Ao4elt Conference - Adaptive Optics for Extremely Large Telescopes. 2009.
    [16] von der Luhe O, Berkefeld T, Soltau D. Multi-Conjugate Solar Adaptive Optics at the Vacuum Tower. Comptes Rendus Physique. 2005;6(10):1139-1147.
    [17] Rigaut F, Neichel B, Boccas M, d'Orgeville C, Arriagada G, Fesquet V, et al. Gems: First on-Sky Results. Adaptive Optics Systems III; 2012: Proc. SPIE.
    [18] Bedggood P, Daaboul M, Ashman R, Smith G, Metha A. Characteristics of the Human Isoplanatic Patch and Implications for Adaptive Optics Retinal Imaging. J Biomed Opt. 2008;13(2):024008. Epub 2008/05/10.
    [19] Bedggood P, Metha A. System Design Considerations to Improve Isoplanatism for Adaptive Optics Retinal Imaging. Journal of the Optical Society of America a-Optics Image Science and Vision. 2010;27(11):A37-A47.
    [20] Bedggood PA, Ashman R, Smith G, Metha AB. Multiconjugate Adaptive Optics Applied to an Anatomically Accurate Human Eye Model. Optics Express. 2006;14(18): 8019-8030.
    [21] Thaung J, Knutsson P, Popovic Z, Owner-Petersen M. Dual-Conjugate Adaptive Optics for Wide-Field High-Resolution Retinal Imaging. Optics Express. 2009;17(6): 4454-4467.
    [22] Popovic Z, Knutsson P, Thaung J, Owner-Petersen M, Sjostrand J. Noninvasive Imaging of Human Foveal Capillary Network Using Dual-Conjugate Adaptive Optics. Investigative Ophthalmology & Visual Science. 2011;52(5):2649-2655.
    [23] Popovic Z, Thaung J, Knutsson P, Owner-Petersen M. Dual Conjugate Adaptive Optics Prototype for Wide Field High Resolution Retinal Imaging, Adaptive Optics Progress, Dr. Robert Tyson (Ed.), ISBN: 978-953-51-0894-8, InTech, DOI: 10.5772/53640.

    Sample images

    Below are sample images obtained with our laboratory demonstrator in normal subjects that show the foveal cone photoreceptor mosaic (Image 1), parafoveal capillaries (Image 2, IOVS paper), the blind spot rim capillaries (Image 3), and major bloodvessels and superficial capillaries of the retinal nerve fiber layer (Image 4). Move your mouse pointer over the images to obtain a close-up. The zoom function is realized with Magic Zoom™ v4.0 from Magic Toolbox.


    Foveal cone mosaic structure (registered sum of two images) covering 5.5 x 5.5 deg.

    7 x 7 deg image parafoveal capillaries, depicting the foveal avascular zone

    7 x 7 deg image of blind spot rim capillaries

    14 x 12 deg photomontage of four images showing major blood vessels and superficial capillaries of the retinal nerve fiber layer (blind spot situated just above to the left of this image)

    Publications

    Thaung J, Knutsson P, Popovic Z, Owner-Petersen M. Dual-conjugate adaptive optics for wide-field high-resolution retinal imaging. Opt Express. 2009 Mar 16;17(6):4454-67.

    Popovic Z, Knutsson P, Thaung J, Owner-Petersen M, Sjöstrand J. Noninvasive imaging of human foveal capillary network using dual-conjugate adaptive optics. Invest. Ophthalmol. Vis. Sci. 2011 April 22; 52(5):2649-55.

    Popovic Z, Thaung J, Knutsson P, Owner-Petersen M. Dual Conjugate Adaptive Optics Prototype for Wide Field High Resolution Retinal Imaging, Adaptive Optics Progress, Dr. Robert Tyson (Ed.), ISBN: 978-953-51-0894-8, InTech, DOI: 10.5772/53640.

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  • Department of Ophthalmology Sahlgrenska University Hospital/MölndalSE-431 80 Mölndal