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Abstract
Thresholds for microcavitation of bovine and porcine melanosomes were determined using nanosecond laser pulses in the near-infrared (1000 to 1319 nm) wavelength regime. Isolated melanosomes were irradiated by single pulses (10 or 50 ns) using a Q-switched Spectra Physics Nd:YAG laser coupled with an optical parametric oscillator (1000 to 1200 nm) or a continuum laser at 1319 nm. Time-resolved nanosecond strobe photography after the arrival of the irradiation beam allowed imaging of microcavitation events. Average fluence thresholds for microcavitation increased nonlinearly with increasing wavelength from ∼0.5 J/cm2 at 1000 nm to 2.6 J/cm2 at 1319 nm. Fluence thresholds were also measured for 10-ns pulses at 532 nm and found to be comparable to visible nanosecond pulse values published in previous reports. Calculated melanosome absorption coefficients decreased from 925 cm-1 at 1000 nm to 176 cm-1 at 1319 nm. This trend was found to be comparable to the decrease in retinal pigmented epithelial layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy retinal damage threshold values were determined in order to compare to current and proposed maximum permissible exposure (MPE) safe levels. Results from this study support recently proposed changes to the MPE levels.
Highlights
As a consequence of the widespread use and availability of lasers, laser eye safety has increasingly become a concern due to the possibility of accidental, high-intensity exposures to tissue in the eye
While many studies have been reported for visible nanosecond pulse melanosome microcavitation, NIR nanosecond microcavitation has been neglected
This study reports the first threshold values for NIR nanosecond pulse melanosome microcavitation, as well as the first reported values for NIR melanosome absorption coefficients
Summary
As a consequence of the widespread use and availability of lasers, laser eye safety has increasingly become a concern due to the possibility of accidental, high-intensity exposures to tissue in the eye. In order for committees such as Z136 to monitor and make recommendations for safe exposure levels,[1,2] retinal damage threshold studies must be conducted with varying laser radiation parameters to quantify the magnitude of these hazards.[3] It is imperative to understand the mechanisms of retinal damage, including melanosome microcavitation, as a function of wavelength, spot size, and exposure duration. The proposed revision to the near-infrared (NIR) safe exposure limits is supported by ocular damage threshold data from a number of studies.[3,4] little data exist in the longer-wavelength NIR regime in the nanosecond time scale. An additional barrier to updating the laser safety standards is the lack of understanding of the fundamental damage mechanisms of melanosome microcavitation in the retinal pigment epithelium (RPE) in this time regime. The heat produced by the strongly absorbing melanosomes in the RPE exposed to visible and NIR nanosecond pulses allows for selective damage of RPE cells while preserving overlying photoreceptors.[12,15]
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