Rebecca L. Vincelette

Thermal lensing in ocular media exposed to continuous-wave near-infrared radiation: the 1150–1350-nm region

Ocular damage threshold data remain sparse in the continuous wave (CW), near-infrared (NIR) radiation region save for the 1300-nm area that has been investigated in the past several decades. The 1300-nm ocular damage data have yielded unusual characteristics where CW retinal damage was observed in rabbit models, but never in nonhuman primate models. This paper reviews the existing 1300-nm ocular damage threshold data in terms of the fundamental criteria of an action spectrum to assist in explaining laser-tissue effects from near-infrared radiation in the eye. Reviewing the action spectrum criteria and existing NIR retinal lesion data lend evidence toward the significant presence of thermal lensing in ocular media affecting damage, a relatively unexplored mechanism of laser-tissue interaction.

Trends in retinal damage thresholds from 100-millisecond near-infrared laser radiation exposures: A study at 1,110, 1,130, 1,150, and 1,319 nm

Retinal damage thresholds from 100‐millisecond exposures to laser radiation for wavelengths between 1,100 and 1,350 nm have never previously been established. We sought to determine the retinal damage threshold for 100‐millisecond exposures of near‐infrared (NIR) laser radiation wavelengths at 1,110, 1,130, 1,150, and 1,319 nm. These data were then used to create trends for retinal damage thresholds over the 1,100–1,350 nm NIR region based upon linear absorption of laser radiation in ocular media and chromatic dispersion of the eye.

First-order model of thermal lensing in a virtual eye

An ABCD beam-propagation method was used to build a first-order mathematical model of a thermal lens effect from a near-infrared laser beam in water and ocular media. The model was found to fit experimental z-scan data best when the thermo-optic coefficient dn/dT of liquid water at 292 K was -4.46x10(-5) K(-1). The physiological parameters of the human eye were simulated in a simple eye model using this fitted dn/dT value. Conservative model simulations for 1150 and 1318 nm laser radiation include parameter sets used in experimental ocular exposures performed by Zuclich et al. [Health Phys.92, 15 (2007)] to illustrate the transient response of the thermal lens approaching the limits of the retinal damage thresholds for equivalent laser radiation sources.

Thermal lensing in the ocular media

A tissue phantom of water with an absorbing dye, Allura Red, was used to observe the effects of thermal lensing in a thick sample exposed to a CW 532 nm Verdi laser. A collimated beam was sent through a sample 2.9 cm thick. Results from the collimated beam revealed qualitative information about thermal lensing in a liquid media. The studies presented here argue the relevance of incorporating oscillating factors such as convectional flow into higher order thermal lensing models in a fluid such as water.

Porcine skin ED50 damage thresholds for 1214 nm laser irradiation

A series of experiments were conducted in vivo on porcine skin to determine the ED50 damage thresholds for 1214 nm continuous wave laser irradiation. These results provide new information for refinement of Maximum Permissible Exposure (MPE). The study employed exposure durations of 1 sec, 3 sec, and 10 seconds with nominal spot diameters of 6 mm, 8 mm and 10 mm and as a function of laser power. The effect of each irradiation was evaluated acutely, one hour after exposure, and 24 hours post exposure. Probit analysis was conducted to estimate the dose for 50% probability of laser-induced damage (ED50); Damage was defined as persistent redness at the site of irradiation for the pig skin after 24 hours. The results indicated that Maximum Permissible Exposure (MPE) limits should be lowered for the laser beam diameters larger than 6 mm.

A Comparison of a First-Order Thermal Lensing Model to a Closed Aperture Z-Scan for the Propagation of Light in Ocular Media

The phenomenon of thermal lensing was investigated in water using a Z-scan method and corresponding first-order mathematical models. Data from first-order thermal lensing models and ABCD beam propagation methods were used to simulate the non-linear absorption of water held in a thin sample cuvette for a Z-scan optical set up of CW cases at 1313 nm. The single beam closed aperture Z-scan was then used to determine the non-linear absorption at 1313 nm for water in 10 mm and 2 mm cuvettes at 48.00, 16.80, 9.80 and 2.83 mW then compared to the first-order model data. The results from the closed aperture Z-scan were also used to back calculate the spot size in the far field for comparison to the models prediction of the beams temporal response. Experimental Z-scan data were found not to correlate strongly with our first-order model suggesting the need for higher order models to successfully predict spot size in absorbing media inside the Rayleigh range.

Confocal Imaging of Thermal Lensing Induced by Near-IR Laser Radiation in an Artificial Eye

A custom confocal imaging system was built and used to record a probe beams spatiotemporal response to a thermal lens induced by a near-IR laser radiation source in a water-filled artificial eye. The IR laser radiation input power levels were varied between 150 and 890 mW at wavelengths of 1110, 1130, 1150 and 1318 nm in order to determine the strength of the resulting thermal lens as a function of time, input power, and wavelength. A high-frame-rate camera captured the probe beams logarithmic excitation and exponential decay caused by the thermal lens (supplemental video data are provided). Data showed that for equivalent input powers and beam geometries, thermal lensing was strongest for the 1150-nm laser radiation wavelength followed by 1130, 1318 and 1110 nm.

A first-order model of thermal lensing of laser propagation in the eye and implications for laser safety

Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media’s index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the damage is related to multiple pulse exposures. We present a first-order modeling effort for the thermal lensing effect in the eye along with initial estimates of impact on damage thresholds as predicted through computational biophysics thermal damage models.Many medical studies have investigated the safety threshold of ocular media at various wavelengths. Understanding and determining the damage threshold of laser exposure to the retina is important in order to set safety guidelines. A phenomenon known as thermal lensing where a media’s index of refraction changes as the laser energy is absorbed causes the beam profile and subsequently the position of the focal point of the beam to change as the temperature changes within the media. These changes in ocular media such as vitreous and aqueous humor, the cornea, crystalline lens, and retina all result in a slight change in the beam profile making the prediction of the focal point and laser spot size at the retina difficult to accurately predict. This paper describes how the effects of thermal lensing in the eye can be investigated to aid in determining the damage threshold for pulse durations of a few nanoseconds and continuous wave, CW, laser irradiation delivered to the retina and the relationship of how the ...

Thermal lensing from near-infrared laser radiation in an artificial eye

A confocal imaging system mounted to a micrometer stage was used to image the thermal lens induced into a water filled Cain-cell artificial eye. A dual-beam pump-probe geometry was used to quantify the 633-nm visible wavelength probe beams transient response when exposed to the near-infrared pump-beam source. The infrared laser radiation wavelengths tested were 1110, 1130, 1150 and 1318 nm for 1-s exposures to 450-mW of power. Analysis of video data revealed the amount of refractive shift, induced by the thermal lens, as a function of time. Data demonstrate how the formation and dissipation of the thermal lens follow a logarithmic excitation and exponential decay in time respectively. Confocal imaging showed that thermal lensing was strongest for the 1150-nm wavelength followed by 1130, 1318 and 1110-nm.

Biomarkers and sensitive evaluation of retinal laser lesions

Advanced imaging techniques and proteomic technology continue to push the boundaries for diagnosing and understanding retinal laser lesion exposures. We conducted retinal imaging in the rhesus macaque to compare the appearance of suprathreshold laser lesions in the macula from both photothermal (532 nm, 100 ms) and photomechanical (532 nm, 9 ns) insult using five different imaging systems: three clinically approved systems; Heidelberg Spectralis SD-OCT-SLO, Heidelberg HRT3 cSLO, and Topcon Fundus camera, and two experimental systems; multispectral using a fundus camera and hyperspectral detection using a PSI Inc. LSLO. In addition to imaging, blood plasma samples were acquired before and after (6 and 24 hrs) laser exposure to search for biomarkers occurring from two different laser damage mechanisms. Imaging results should help identify the best potential imaging systems for capturing retinal laser lesions from photothermal or photomechanical injury. The proteomics results will assist in understanding the elicited molecular pathways involved in damage response to the two types of retinal laser insult, and may be used to hallmark approaches for potential treatment options.Advanced imaging techniques and proteomic technology continue to push the boundaries for diagnosing and understanding retinal laser lesion exposures. We conducted retinal imaging in the rhesus macaque to compare the appearance of suprathreshold laser lesions in the macula from both photothermal (532 nm, 100 ms) and photomechanical (532 nm, 9 ns) insult using five different imaging systems: three clinically approved systems; Heidelberg Spectralis SD-OCT-SLO, Heidelberg HRT3 cSLO, and Topcon Fundus camera, and two experimental systems; multispectral using a fundus camera and hyperspectral detection using a PSI Inc. LSLO. In addition to imaging, blood plasma samples were acquired before and after (6 and 24 hrs) laser exposure to search for biomarkers occurring from two different laser damage mechanisms. Imaging results should help identify the best potential imaging systems for capturing retinal laser lesions from photothermal or photomechanical injury. The proteomics results will assist in understanding the...