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M. Agopov, O. La Schiazza, M.F. Cordeiro, L. Guo, A. Maass, S. Schmitz–Valckenberg, F.W. Fitzke, J.F. Bille, F.G. Holz, M. Niemz; Infrared Femtosecond Laser Coupled HRT for Retinal Imaging – A Safety Study . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4067.
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© ARVO (1962-2015); The Authors (2016-present)
Recently, nonlinear microscopic imaging methods based on two–photon excited fluorescence (TPEF) and second harmonic generation (SHG) have been successfully implemented to reveal cell structures in cornea and retina. It is proposed that femtosecond (fs) lasers may be valuable for high resolution imaging of the human retina in vivo . The purpose of this study is to determine the damage threshold of fs lasers to the Dark Agouti rat retina both in vitro and in vivo and its dependence on the laser average power, exposure times, and scanning angles.
We coupled a Nd:glass femtosecond laser (wavelength = 1054 nm, pulse length = 400 fs, repetition rate = 76 MHz) to a modified Heidelberg Retina Tomograph (HRT) for imaging the retina. Eyes of anesthetized Dark Agouti rats were exposed to a Nd:glass fs laser with different average powers (ranging between 1–40 mW), exposure times (between 1–10 s) and scanning angles (1,5 and 10 degrees). The laser–induced damage on the retina was evaluated by ophthalmological and histological studies to determine the damage threshold. For comparison, a Ti:Sapphire –femtosecond laser (wavelength = 750–920 nm, pulse length = 100 fs, repetition rate = 100 MHz) was coupled to an HRT to estimate the damage threshold in fixated retina samples.
Based on both ophthalmological and histological evaluations, the Nd:glass femtosecond laser is safe for the living retina up to a maximum average power of 4 mW. The signal power from the SHG from the retinal axons and TPEF from the RPE cells as functions of the average excitation power was also monitored in living and post mortem eyes.
According to our study, near infrared fs lasers can be used for imaging of the retina in a living eye, if the average excitation power is kept under 4 mW. This technique may be used in in vivo TPEF and SHG imaging of the retina. Compared to Nd:glass fs laser, which is appropriate for SHG imaging, the Ti:Sapphire fs laser is more suitable for TPEF imaging of the retina due to its large tunable wavelength range.
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